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ASP.NET Core Web Host

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ASP.NET Core apps configure and launch a host. The host is responsible for app startup and lifetime management. At a minimum, the host configures a server and a request processing pipeline. The host can also set up logging, dependency injection, and configuration.

This article covers the Web Host, which remains available only for backward compatibility. The ASP.NET Core templates create a WebApplicationBuilder and WebApplication, which is recommended for web apps. For more information on WebApplicationBuilder and WebApplication, see Migrate from ASP.NET Core 5.0 to 6.0

Set up a host

Create a host using an instance of IWebHostBuilder. This is typically performed in the app's entry point, the Main method in Program.cs. A typical app calls CreateDefaultBuilder to start setting up a host:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

The code that calls CreateDefaultBuilder is in a method named CreateWebHostBuilder, which separates it from the code in Main that calls Run on the builder object. This separation is required if you use Entity Framework Core tools. The tools expect to find a CreateWebHostBuilder method that they can call at design time to configure the host without running the app. An alternative is to implement IDesignTimeDbContextFactory. For more information, see Design-time DbContext Creation.

CreateDefaultBuilder performs the following tasks:

The configuration defined by CreateDefaultBuilder can be overridden and augmented by ConfigureAppConfiguration, ConfigureLogging, and other methods and extension methods of IWebHostBuilder. A few examples follow:

  • ConfigureAppConfiguration is used to specify additional IConfiguration for the app. The following ConfigureAppConfiguration call adds a delegate to include app configuration in the appsettings.xml file. ConfigureAppConfiguration may be called multiple times. Note that this configuration doesn't apply to the host (for example, server URLs or environment). See the Host configuration values section.

    WebHost.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((hostingContext, config) =>
    {
        config.AddXmlFile("appsettings.xml", optional: true, reloadOnChange: true);
    })
    ...

  • The following ConfigureLogging call adds a delegate to configure the minimum logging level (SetMinimumLevel) to LogLevel.Warning. This setting overrides the settings in appsettings.Development.json (LogLevel.Debug) and appsettings.Production.json (LogLevel.Error) configured by CreateDefaultBuilder. ConfigureLogging may be called multiple times.

    WebHost.CreateDefaultBuilder(args)
    .ConfigureLogging(logging => 
    {
        logging.SetMinimumLevel(LogLevel.Warning);
    })
    ...

  • The following call to ConfigureKestrel overrides the default Limits.MaxRequestBodySize of 30,000,000 bytes established when Kestrel was configured by CreateDefaultBuilder:

    WebHost.CreateDefaultBuilder(args)
    .ConfigureKestrel((context, options) =>
    {
        options.Limits.MaxRequestBodySize = 20000000;
    });

The content root determines where the host searches for content files, such as MVC view files. When the app is started from the project's root folder, the project's root folder is used as the content root. This is the default used in Visual Studio and the dotnet new templates.

For more information on app configuration, see Configuration in ASP.NET Core.

Note

As an alternative to using the static CreateDefaultBuilder method, creating a host from WebHostBuilder is a supported approach with ASP.NET Core 2.x.

When setting up a host, Configure and ConfigureServices methods can be provided. If a Startup class is specified, it must define a Configure method. For more information, see App startup in ASP.NET Core. Multiple calls to ConfigureServices append to one another. Multiple calls to Configure or UseStartup on the WebHostBuilder replace previous settings.

Host configuration values

WebHostBuilder relies on the following approaches to set the host configuration values:

  • Host builder configuration, which includes environment variables with the format ASPNETCORE_{configurationKey}. For example, ASPNETCORE_ENVIRONMENT.
  • Extensions such as UseContentRoot and UseConfiguration (see the Override configuration section).
  • UseSetting and the associated key. When setting a value with UseSetting, the value is set as a string regardless of the type.

The host uses whichever option sets a value last. For more information, see Override configuration in the next section.

Application Key (Name)

The IWebHostEnvironment.ApplicationName property is automatically set when UseStartup or Configure is called during host construction. The value is set to the name of the assembly containing the app's entry point. To set the value explicitly, use the WebHostDefaults.ApplicationKey:

Key: applicationName
Type: string
Default: The name of the assembly containing the app's entry point.
Set using: UseSetting
Environment variable: ASPNETCORE_APPLICATIONNAME

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.ApplicationKey, "CustomApplicationName")

Capture Startup Errors

This setting controls the capture of startup errors.

Key: captureStartupErrors
Type: bool (true or 1)
Default: Defaults to false unless the app runs with Kestrel behind IIS, where the default is true.
Set using: CaptureStartupErrors
Environment variable: ASPNETCORE_CAPTURESTARTUPERRORS

When false, errors during startup result in the host exiting. When true, the host captures exceptions during startup and attempts to start the server.

WebHost.CreateDefaultBuilder(args)
    .CaptureStartupErrors(true)

Content root

This setting determines where ASP.NET Core begins searching for content files.

Key: contentRoot
Type: string
Default: Defaults to the folder where the app assembly resides.
Set using: UseContentRoot
Environment variable: ASPNETCORE_CONTENTROOT

The content root is also used as the base path for the web root. If the content root path doesn't exist, the host fails to start.

WebHost.CreateDefaultBuilder(args)
    .UseContentRoot("c:\\<content-root>")

For more information, see:

Detailed Errors

Determines if detailed errors should be captured.

Key: detailedErrors
Type: bool (true or 1)
Default: false
Set using: UseSetting
Environment variable: ASPNETCORE_DETAILEDERRORS

When enabled (or when the Environment is set to Development), the app captures detailed exceptions.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.DetailedErrorsKey, "true")

Environment

Sets the app's environment.

Key: environment
Type: string
Default: Production
Set using: UseEnvironment
Environment variable: ASPNETCORE_ENVIRONMENT

The environment can be set to any value. Framework-defined values include Development, Staging, and Production. Values aren't case sensitive. By default, the Environment is read from the ASPNETCORE_ENVIRONMENT environment variable. When using Visual Studio, environment variables may be set in the launchSettings.json file. For more information, see Use multiple environments in ASP.NET Core.

WebHost.CreateDefaultBuilder(args)
    .UseEnvironment(EnvironmentName.Development)

Hosting Startup Assemblies

Sets the app's hosting startup assemblies.

Key: hostingStartupAssemblies
Type: string
Default: Empty string
Set using: UseSetting
Environment variable: ASPNETCORE_HOSTINGSTARTUPASSEMBLIES

A semicolon-delimited string of hosting startup assemblies to load on startup.

Although the configuration value defaults to an empty string, the hosting startup assemblies always include the app's assembly. When hosting startup assemblies are provided, they're added to the app's assembly for loading when the app builds its common services during startup.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupAssembliesKey, "assembly1;assembly2")

HTTPS Port

Set the HTTPS redirect port. Used in enforcing HTTPS.

Key: https_port
Type: string
Default: A default value isn't set.
Set using: UseSetting
Environment variable: ASPNETCORE_HTTPS_PORTS

WebHost.CreateDefaultBuilder(args)
    .UseSetting("https_port", "8080")

Hosting Startup Exclude Assemblies

A semicolon-delimited string of hosting startup assemblies to exclude on startup.

Key: hostingStartupExcludeAssemblies
Type: string
Default: Empty string
Set using: UseSetting
Environment variable: ASPNETCORE_HOSTINGSTARTUPEXCLUDEASSEMBLIES

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupExcludeAssembliesKey, "assembly1;assembly2")

Prefer Hosting URLs

Indicates whether the host should listen on the URLs configured with the WebHostBuilder instead of those configured with the IServer implementation.

Key: preferHostingUrls
Type: bool (true or 1)
Default: false
Set using: PreferHostingUrls
Environment variable: ASPNETCORE_PREFERHOSTINGURLS

WebHost.CreateDefaultBuilder(args)
    .PreferHostingUrls(true)

Prevent Hosting Startup

Prevents the automatic loading of hosting startup assemblies, including hosting startup assemblies configured by the app's assembly. For more information, see Use hosting startup assemblies in ASP.NET Core.

Key: preventHostingStartup
Type: bool (true or 1)
Default: false
Set using: UseSetting
Environment variable: ASPNETCORE_PREVENTHOSTINGSTARTUP

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.PreventHostingStartupKey, "true")

Server URLs

Indicates the IP addresses or host addresses with ports and protocols that the server should listen on for requests.

Key: urls
Type: string
Default: http://localhost:5000
Set using: UseUrls
Environment variable: ASPNETCORE_URLS

Set to a semicolon-separated (;) list of URL prefixes to which the server should respond. For example, http://localhost:123. Use "*" to indicate that the server should listen for requests on any IP address or hostname using the specified port and protocol (for example, http://*:5000). The protocol (http:// or https://) must be included with each URL. Supported formats vary among servers.

WebHost.CreateDefaultBuilder(args)
    .UseUrls("http://*:5000;http://localhost:5001;https://hostname:5002")

Kestrel has its own endpoint configuration API. For more information, see Configure endpoints for the ASP.NET Core Kestrel web server.

Shutdown Timeout

Specifies the amount of time to wait for Web Host to shut down.

Key: shutdownTimeoutSeconds
Type: int
Default: 5
Set using: UseShutdownTimeout
Environment variable: ASPNETCORE_SHUTDOWNTIMEOUTSECONDS

Although the key accepts an int with UseSetting (for example, .UseSetting(WebHostDefaults.ShutdownTimeoutKey, "10")), the UseShutdownTimeout extension method takes a TimeSpan.

During the timeout period, hosting:

If the timeout period expires before all of the hosted services stop, any remaining active services are stopped when the app shuts down. The services stop even if they haven't finished processing. If services require additional time to stop, increase the timeout.

WebHost.CreateDefaultBuilder(args)
    .UseShutdownTimeout(TimeSpan.FromSeconds(10))

Startup Assembly

Determines the assembly to search for the Startup class.

Key: startupAssembly
Type: string
Default: The app's assembly
Set using: UseStartup
Environment variable: ASPNETCORE_STARTUPASSEMBLY

The assembly by name (string) or type (TStartup) can be referenced. If multiple UseStartup methods are called, the last one takes precedence.

WebHost.CreateDefaultBuilder(args)
    .UseStartup("StartupAssemblyName")

WebHost.CreateDefaultBuilder(args)
    .UseStartup<TStartup>()

Web root

Sets the relative path to the app's static assets.

Key: webroot
Type: string
Default: The default is wwwroot. The path to {content root}/wwwroot must exist. If the path doesn't exist, a no-op file provider is used.
Set using: UseWebRoot
Environment variable: ASPNETCORE_WEBROOT

WebHost.CreateDefaultBuilder(args)
    .UseWebRoot("public")

For more information, see:

Override configuration

Use Configuration to configure Web Host. In the following example, host configuration is optionally specified in a hostsettings.json file. Any configuration loaded from the hostsettings.json file may be overridden by command-line arguments. The built configuration (in config) is used to configure the host with UseConfiguration. IWebHostBuilder configuration is added to the app's configuration, but the converse isn't true—ConfigureAppConfiguration doesn't affect the IWebHostBuilder configuration.

Overriding the configuration provided by UseUrls with hostsettings.json config first, command-line argument config second:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
    {
        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("hostsettings.json", optional: true)
            .AddCommandLine(args)
            .Build();

        return WebHost.CreateDefaultBuilder(args)
            .UseUrls("http://*:5000")
            .UseConfiguration(config)
            .Configure(app =>
            {
                app.Run(context => 
                    context.Response.WriteAsync("Hello, World!"));
            });
    }
}

hostsettings.json:

{
    urls: "http://*:5005"
}

Note

UseConfiguration only copies keys from the provided IConfiguration to the host builder configuration. Therefore, setting reloadOnChange: true for JSON, INI, and XML settings files has no effect.

To specify the host run on a particular URL, the desired value can be passed in from a command prompt when executing dotnet run. The command-line argument overrides the urls value from the hostsettings.json file, and the server listens on port 8080:

dotnet run --urls "http://*:8080"

Manage the host

Run

The Run method starts the web app and blocks the calling thread until the host is shut down:

host.Run();

Start

Run the host in a non-blocking manner by calling its Start method:

using (host)
{
    host.Start();
    Console.ReadLine();
}

If a list of URLs is passed to the Start method, it listens on the URLs specified:

var urls = new List<string>()
{
    "http://*:5000",
    "http://localhost:5001"
};

var host = new WebHostBuilder()
    .UseKestrel()
    .UseStartup<Startup>()
    .Start(urls.ToArray());

using (host)
{
    Console.ReadLine();
}

The app can initialize and start a new host using the pre-configured defaults of CreateDefaultBuilder using a static convenience method. These methods start the server without console output and with WaitForShutdown wait for a break (Ctrl-C/SIGINT or SIGTERM):

Start(RequestDelegate app)

Start with a RequestDelegate:

using (var host = WebHost.Start(app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, RequestDelegate app)

Start with a URL and RequestDelegate:

using (var host = WebHost.Start("http://localhost:8080", app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(RequestDelegate app), except the app responds on http://localhost:8080.

Start(Action<IRouteBuilder> routeBuilder)

Use an instance of IRouteBuilder (Microsoft.AspNetCore.Routing) to use routing middleware:

using (var host = WebHost.Start(router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Use the following browser requests with the example:

Request Response
http://localhost:5000/hello/Martin Hello, Martin!
http://localhost:5000/buenosdias/Catrina Buenos dias, Catrina!
http://localhost:5000/throw/ooops! Throws an exception with string "ooops!"
http://localhost:5000/throw Throws an exception with string "Uh oh!"
http://localhost:5000/Sante/Kevin Sante, Kevin!
http://localhost:5000 Hello World!

WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, Action<IRouteBuilder> routeBuilder)

Use a URL and an instance of IRouteBuilder:

using (var host = WebHost.Start("http://localhost:8080", router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(Action<IRouteBuilder> routeBuilder), except the app responds at http://localhost:8080.

StartWith(Action<IApplicationBuilder> app)

Provide a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith(app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

StartWith(string url, Action<IApplicationBuilder> app)

Provide a URL and a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith("http://localhost:8080", app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Produces the same result as StartWith(Action<IApplicationBuilder> app), except the app responds on http://localhost:8080.

IWebHostEnvironment interface

The IWebHostEnvironment interface provides information about the app's web hosting environment. Use constructor injection to obtain the IWebHostEnvironment in order to use its properties and extension methods:

public class CustomFileReader
{
    private readonly IWebHostEnvironment _env;

    public CustomFileReader(IWebHostEnvironment env)
    {
        _env = env;
    }

    public string ReadFile(string filePath)
    {
        var fileProvider = _env.WebRootFileProvider;
        // Process the file here
    }
}

A convention-based approach can be used to configure the app at startup based on the environment. Alternatively, inject the IWebHostEnvironment into the Startup constructor for use in ConfigureServices:

public class Startup
{
    public Startup(IWebHostEnvironment env)
    {
        HostingEnvironment = env;
    }

    public IWebHostEnvironment HostingEnvironment { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        if (HostingEnvironment.IsDevelopment())
        {
            // Development configuration
        }
        else
        {
            // Staging/Production configuration
        }

        var contentRootPath = HostingEnvironment.ContentRootPath;
    }
}

Note

In addition to the IsDevelopment extension method, IWebHostEnvironment offers IsStaging, IsProduction, and IsEnvironment(string environmentName) methods. For more information, see Use multiple environments in ASP.NET Core.

The IWebHostEnvironment service can also be injected directly into the Configure method for setting up the processing pipeline:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        // In Development, use the Developer Exception Page
        app.UseDeveloperExceptionPage();
    }
    else
    {
        // In Staging/Production, route exceptions to /error
        app.UseExceptionHandler("/error");
    }

    var contentRootPath = env.ContentRootPath;
}

IWebHostEnvironment can be injected into the Invoke method when creating custom middleware:

public async Task Invoke(HttpContext context, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        // Configure middleware for Development
    }
    else
    {
        // Configure middleware for Staging/Production
    }

    var contentRootPath = env.ContentRootPath;
}

IHostApplicationLifetime interface

IHostApplicationLifetime allows for post-startup and shutdown activities. Three properties on the interface are cancellation tokens used to register Action methods that define startup and shutdown events.

Cancellation Token Triggered when…
ApplicationStarted The host has fully started.
ApplicationStopped The host is completing a graceful shutdown. All requests should be processed. Shutdown blocks until this event completes.
ApplicationStopping The host is performing a graceful shutdown. Requests may still be processing. Shutdown blocks until this event completes. 
public class Startup
{
    public void Configure(IApplicationBuilder app, IHostApplicationLifetime appLifetime)
    {
        appLifetime.ApplicationStarted.Register(OnStarted);
        appLifetime.ApplicationStopping.Register(OnStopping);
        appLifetime.ApplicationStopped.Register(OnStopped);

        Console.CancelKeyPress += (sender, eventArgs) =>
        {
            appLifetime.StopApplication();
            // Don't terminate the process immediately, wait for the Main thread to exit gracefully.
            eventArgs.Cancel = true;
        };
    }

    private void OnStarted()
    {
        // Perform post-startup activities here
    }

    private void OnStopping()
    {
        // Perform on-stopping activities here
    }

    private void OnStopped()
    {
        // Perform post-stopped activities here
    }
}

StopApplication requests termination of the app. The following class uses StopApplication to gracefully shut down an app when the class's Shutdown method is called:

public class MyClass
{
    private readonly IHostApplicationLifetime _appLifetime;

    public MyClass(IHostApplicationLifetime appLifetime)
    {
        _appLifetime = appLifetime;
    }

    public void Shutdown()
    {
        _appLifetime.StopApplication();
    }
}

Scope validation

CreateDefaultBuilder sets ServiceProviderOptions.ValidateScopes to true if the app's environment is Development.

When ValidateScopes is set to true, the default service provider performs checks to verify that:

  • Scoped services aren't directly or indirectly resolved from the root service provider.
  • Scoped services aren't directly or indirectly injected into singletons.

The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.

Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations when BuildServiceProvider is called.

To always validate scopes, including in the Production environment, configure the ServiceProviderOptions with UseDefaultServiceProvider on the host builder:

WebHost.CreateDefaultBuilder(args)
    .UseDefaultServiceProvider((context, options) => {
        options.ValidateScopes = true;
    })

Additional resources

ASP.NET Core apps configure and launch a host. The host is responsible for app startup and lifetime management. At a minimum, the host configures a server and a request processing pipeline. The host can also set up logging, dependency injection, and configuration.

This article covers the Web Host, which remains available only for backward compatibility. The ASP.NET Core templates create a .NET Generic Host, which is recommended for all app types.

Set up a host

Create a host using an instance of IWebHostBuilder. This is typically performed in the app's entry point, the Main method.

In the project templates, Main is located in Program.cs. A typical app calls CreateDefaultBuilder to start setting up a host:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

The code that calls CreateDefaultBuilder is in a method named CreateWebHostBuilder, which separates it from the code in Main that calls Run on the builder object. This separation is required if you use Entity Framework Core tools. The tools expect to find a CreateWebHostBuilder method that they can call at design time to configure the host without running the app. An alternative is to implement IDesignTimeDbContextFactory. For more information, see Design-time DbContext Creation.

CreateDefaultBuilder performs the following tasks:

The configuration defined by CreateDefaultBuilder can be overridden and augmented by ConfigureAppConfiguration, ConfigureLogging, and other methods and extension methods of IWebHostBuilder. A few examples follow:

  • ConfigureAppConfiguration is used to specify additional IConfiguration for the app. The following ConfigureAppConfiguration call adds a delegate to include app configuration in the appsettings.xml file. ConfigureAppConfiguration may be called multiple times. Note that this configuration doesn't apply to the host (for example, server URLs or environment). See the Host configuration values section.

    WebHost.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((hostingContext, config) =>
    {
        config.AddXmlFile("appsettings.xml", optional: true, reloadOnChange: true);
    })
    ...

  • The following ConfigureLogging call adds a delegate to configure the minimum logging level (SetMinimumLevel) to LogLevel.Warning. This setting overrides the settings in appsettings.Development.json (LogLevel.Debug) and appsettings.Production.json (LogLevel.Error) configured by CreateDefaultBuilder. ConfigureLogging may be called multiple times.

    WebHost.CreateDefaultBuilder(args)
    .ConfigureLogging(logging => 
    {
        logging.SetMinimumLevel(LogLevel.Warning);
    })
    ...

  • The following call to ConfigureKestrel overrides the default Limits.MaxRequestBodySize of 30,000,000 bytes established when Kestrel was configured by CreateDefaultBuilder:

    WebHost.CreateDefaultBuilder(args)
    .ConfigureKestrel((context, options) =>
    {
        options.Limits.MaxRequestBodySize = 20000000;
    });

The content root determines where the host searches for content files, such as MVC view files. When the app is started from the project's root folder, the project's root folder is used as the content root. This is the default used in Visual Studio and the dotnet new templates.

For more information on app configuration, see Configuration in ASP.NET Core.

Note

As an alternative to using the static CreateDefaultBuilder method, creating a host from WebHostBuilder is a supported approach with ASP.NET Core 2.x.

When setting up a host, Configure and ConfigureServices methods can be provided. If a Startup class is specified, it must define a Configure method. For more information, see App startup in ASP.NET Core. Multiple calls to ConfigureServices append to one another. Multiple calls to Configure or UseStartup on the WebHostBuilder replace previous settings.

Host configuration values

WebHostBuilder relies on the following approaches to set the host configuration values:

  • Host builder configuration, which includes environment variables with the format ASPNETCORE_{configurationKey}. For example, ASPNETCORE_ENVIRONMENT.
  • Extensions such as UseContentRoot and UseConfiguration (see the Override configuration section).
  • UseSetting and the associated key. When setting a value with UseSetting, the value is set as a string regardless of the type.

The host uses whichever option sets a value last. For more information, see Override configuration in the next section.

Application Key (Name)

The IWebHostEnvironment.ApplicationName property is automatically set when UseStartup or Configure is called during host construction. The value is set to the name of the assembly containing the app's entry point. To set the value explicitly, use the WebHostDefaults.ApplicationKey:

Key: applicationName
Type: string
Default: The name of the assembly containing the app's entry point.
Set using: UseSetting
Environment variable: ASPNETCORE_APPLICATIONNAME

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.ApplicationKey, "CustomApplicationName")

Capture Startup Errors

This setting controls the capture of startup errors.

Key: captureStartupErrors
Type: bool (true or 1)
Default: Defaults to false unless the app runs with Kestrel behind IIS, where the default is true.
Set using: CaptureStartupErrors
Environment variable: ASPNETCORE_CAPTURESTARTUPERRORS

When false, errors during startup result in the host exiting. When true, the host captures exceptions during startup and attempts to start the server.

WebHost.CreateDefaultBuilder(args)
    .CaptureStartupErrors(true)

Content root

This setting determines where ASP.NET Core begins searching for content files.

Key: contentRoot
Type: string
Default: Defaults to the folder where the app assembly resides.
Set using: UseContentRoot
Environment variable: ASPNETCORE_CONTENTROOT

The content root is also used as the base path for the web root. If the content root path doesn't exist, the host fails to start.

WebHost.CreateDefaultBuilder(args)
    .UseContentRoot("c:\\<content-root>")

For more information, see:

Detailed Errors

Determines if detailed errors should be captured.

Key: detailedErrors
Type: bool (true or 1)
Default: false
Set using: UseSetting
Environment variable: ASPNETCORE_DETAILEDERRORS

When enabled (or when the Environment is set to Development), the app captures detailed exceptions.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.DetailedErrorsKey, "true")

Environment

Sets the app's environment.

Key: environment
Type: string
Default: Production
Set using: UseEnvironment
Environment variable: ASPNETCORE_ENVIRONMENT

The environment can be set to any value. Framework-defined values include Development, Staging, and Production. Values aren't case sensitive. By default, the Environment is read from the ASPNETCORE_ENVIRONMENT environment variable. When using Visual Studio, environment variables may be set in the launchSettings.json file. For more information, see Use multiple environments in ASP.NET Core.

WebHost.CreateDefaultBuilder(args)
    .UseEnvironment(EnvironmentName.Development)

Hosting Startup Assemblies

Sets the app's hosting startup assemblies.

Key: hostingStartupAssemblies
Type: string
Default: Empty string
Set using: UseSetting
Environment variable: ASPNETCORE_HOSTINGSTARTUPASSEMBLIES

A semicolon-delimited string of hosting startup assemblies to load on startup.

Although the configuration value defaults to an empty string, the hosting startup assemblies always include the app's assembly. When hosting startup assemblies are provided, they're added to the app's assembly for loading when the app builds its common services during startup.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupAssembliesKey, "assembly1;assembly2")

HTTPS Port

Set the HTTPS redirect port. Used in enforcing HTTPS.

Key: https_port
Type: string
Default: A default value isn't set.
Set using: UseSetting
Environment variable: ASPNETCORE_HTTPS_PORTS

WebHost.CreateDefaultBuilder(args)
    .UseSetting("https_port", "8080")

Hosting Startup Exclude Assemblies

A semicolon-delimited string of hosting startup assemblies to exclude on startup.

Key: hostingStartupExcludeAssemblies
Type: string
Default: Empty string
Set using: UseSetting
Environment variable: ASPNETCORE_HOSTINGSTARTUPEXCLUDEASSEMBLIES

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupExcludeAssembliesKey, "assembly1;assembly2")

Prefer Hosting URLs

Indicates whether the host should listen on the URLs configured with the WebHostBuilder instead of those configured with the IServer implementation.

Key: preferHostingUrls
Type: bool (true or 1)
Default: false
Set using: PreferHostingUrls
Environment variable: ASPNETCORE_PREFERHOSTINGURLS

WebHost.CreateDefaultBuilder(args)
    .PreferHostingUrls(true)

Prevent Hosting Startup

Prevents the automatic loading of hosting startup assemblies, including hosting startup assemblies configured by the app's assembly. For more information, see Use hosting startup assemblies in ASP.NET Core.

Key: preventHostingStartup
Type: bool (true or 1)
Default: false
Set using: UseSetting
Environment variable: ASPNETCORE_PREVENTHOSTINGSTARTUP

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.PreventHostingStartupKey, "true")

Server URLs

Indicates the IP addresses or host addresses with ports and protocols that the server should listen on for requests.

Key: urls
Type: string
Default: http://localhost:5000
Set using: UseUrls
Environment variable: ASPNETCORE_URLS

Set to a semicolon-separated (;) list of URL prefixes to which the server should respond. For example, http://localhost:123. Use "*" to indicate that the server should listen for requests on any IP address or hostname using the specified port and protocol (for example, http://*:5000). The protocol (http:// or https://) must be included with each URL. Supported formats vary among servers.

WebHost.CreateDefaultBuilder(args)
    .UseUrls("http://*:5000;http://localhost:5001;https://hostname:5002")

Kestrel has its own endpoint configuration API. For more information, see Configure endpoints for the ASP.NET Core Kestrel web server.

Shutdown Timeout

Specifies the amount of time to wait for Web Host to shut down.

Key: shutdownTimeoutSeconds
Type: int
Default: 5
Set using: UseShutdownTimeout
Environment variable: ASPNETCORE_SHUTDOWNTIMEOUTSECONDS

Although the key accepts an int with UseSetting (for example, .UseSetting(WebHostDefaults.ShutdownTimeoutKey, "10")), the UseShutdownTimeout extension method takes a TimeSpan.

During the timeout period, hosting:

If the timeout period expires before all of the hosted services stop, any remaining active services are stopped when the app shuts down. The services stop even if they haven't finished processing. If services require additional time to stop, increase the timeout.

WebHost.CreateDefaultBuilder(args)
    .UseShutdownTimeout(TimeSpan.FromSeconds(10))

Startup Assembly

Determines the assembly to search for the Startup class.

Key: startupAssembly
Type: string
Default: The app's assembly
Set using: UseStartup
Environment variable: ASPNETCORE_STARTUPASSEMBLY

The assembly by name (string) or type (TStartup) can be referenced. If multiple UseStartup methods are called, the last one takes precedence.

WebHost.CreateDefaultBuilder(args)
    .UseStartup("StartupAssemblyName")

WebHost.CreateDefaultBuilder(args)
    .UseStartup<TStartup>()

Web root

Sets the relative path to the app's static assets.

Key: webroot
Type: string
Default: The default is wwwroot. The path to {content root}/wwwroot must exist. If the path doesn't exist, a no-op file provider is used.
Set using: UseWebRoot
Environment variable: ASPNETCORE_WEBROOT

WebHost.CreateDefaultBuilder(args)
    .UseWebRoot("public")

For more information, see:

Override configuration

Use Configuration to configure Web Host. In the following example, host configuration is optionally specified in a hostsettings.json file. Any configuration loaded from the hostsettings.json file may be overridden by command-line arguments. The built configuration (in config) is used to configure the host with UseConfiguration. IWebHostBuilder configuration is added to the app's configuration, but the converse isn't true—ConfigureAppConfiguration doesn't affect the IWebHostBuilder configuration.

Overriding the configuration provided by UseUrls with hostsettings.json config first, command-line argument config second:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
    {
        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("hostsettings.json", optional: true)
            .AddCommandLine(args)
            .Build();

        return WebHost.CreateDefaultBuilder(args)
            .UseUrls("http://*:5000")
            .UseConfiguration(config)
            .Configure(app =>
            {
                app.Run(context => 
                    context.Response.WriteAsync("Hello, World!"));
            });
    }
}

hostsettings.json:

{
    urls: "http://*:5005"
}

Note

UseConfiguration only copies keys from the provided IConfiguration to the host builder configuration. Therefore, setting reloadOnChange: true for JSON, INI, and XML settings files has no effect.

To specify the host run on a particular URL, the desired value can be passed in from a command prompt when executing dotnet run. The command-line argument overrides the urls value from the hostsettings.json file, and the server listens on port 8080:

dotnet run --urls "http://*:8080"

Manage the host

Run

The Run method starts the web app and blocks the calling thread until the host is shut down:

host.Run();

Start

Run the host in a non-blocking manner by calling its Start method:

using (host)
{
    host.Start();
    Console.ReadLine();
}

If a list of URLs is passed to the Start method, it listens on the URLs specified:

var urls = new List<string>()
{
    "http://*:5000",
    "http://localhost:5001"
};

var host = new WebHostBuilder()
    .UseKestrel()
    .UseStartup<Startup>()
    .Start(urls.ToArray());

using (host)
{
    Console.ReadLine();
}

The app can initialize and start a new host using the pre-configured defaults of CreateDefaultBuilder using a static convenience method. These methods start the server without console output and with WaitForShutdown wait for a break (Ctrl-C/SIGINT or SIGTERM):

Start(RequestDelegate app)

Start with a RequestDelegate:

using (var host = WebHost.Start(app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, RequestDelegate app)

Start with a URL and RequestDelegate:

using (var host = WebHost.Start("http://localhost:8080", app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(RequestDelegate app), except the app responds on http://localhost:8080.

Start(Action<IRouteBuilder> routeBuilder)

Use an instance of IRouteBuilder (Microsoft.AspNetCore.Routing) to use routing middleware:

using (var host = WebHost.Start(router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Use the following browser requests with the example:

Request Response
http://localhost:5000/hello/Martin Hello, Martin!
http://localhost:5000/buenosdias/Catrina Buenos dias, Catrina!
http://localhost:5000/throw/ooops! Throws an exception with string "ooops!"
http://localhost:5000/throw Throws an exception with string "Uh oh!"
http://localhost:5000/Sante/Kevin Sante, Kevin!
http://localhost:5000 Hello World!

WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, Action<IRouteBuilder> routeBuilder)

Use a URL and an instance of IRouteBuilder:

using (var host = WebHost.Start("http://localhost:8080", router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(Action<IRouteBuilder> routeBuilder), except the app responds at http://localhost:8080.

StartWith(Action<IApplicationBuilder> app)

Provide a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith(app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

StartWith(string url, Action<IApplicationBuilder> app)

Provide a URL and a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith("http://localhost:8080", app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Produces the same result as StartWith(Action<IApplicationBuilder> app), except the app responds on http://localhost:8080.

IWebHostEnvironment interface

The IWebHostEnvironment interface provides information about the app's web hosting environment. Use constructor injection to obtain the IWebHostEnvironment in order to use its properties and extension methods:

public class CustomFileReader
{
    private readonly IWebHostEnvironment _env;

    public CustomFileReader(IWebHostEnvironment env)
    {
        _env = env;
    }

    public string ReadFile(string filePath)
    {
        var fileProvider = _env.WebRootFileProvider;
        // Process the file here
    }
}

A convention-based approach can be used to configure the app at startup based on the environment. Alternatively, inject the IWebHostEnvironment into the Startup constructor for use in ConfigureServices:

public class Startup
{
    public Startup(IWebHostEnvironment env)
    {
        HostingEnvironment = env;
    }

    public IWebHostEnvironment HostingEnvironment { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        if (HostingEnvironment.IsDevelopment())
        {
            // Development configuration
        }
        else
        {
            // Staging/Production configuration
        }

        var contentRootPath = HostingEnvironment.ContentRootPath;
    }
}

Note

In addition to the IsDevelopment extension method, IWebHostEnvironment offers IsStaging, IsProduction, and IsEnvironment(string environmentName) methods. For more information, see Use multiple environments in ASP.NET Core.

The IWebHostEnvironment service can also be injected directly into the Configure method for setting up the processing pipeline:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        // In Development, use the Developer Exception Page
        app.UseDeveloperExceptionPage();
    }
    else
    {
        // In Staging/Production, route exceptions to /error
        app.UseExceptionHandler("/error");
    }

    var contentRootPath = env.ContentRootPath;
}

IWebHostEnvironment can be injected into the Invoke method when creating custom middleware:

public async Task Invoke(HttpContext context, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        // Configure middleware for Development
    }
    else
    {
        // Configure middleware for Staging/Production
    }

    var contentRootPath = env.ContentRootPath;
}

IHostApplicationLifetime interface

IHostApplicationLifetime allows for post-startup and shutdown activities. Three properties on the interface are cancellation tokens used to register Action methods that define startup and shutdown events.

Cancellation Token Triggered when…
ApplicationStarted The host has fully started.
ApplicationStopped The host is completing a graceful shutdown. All requests should be processed. Shutdown blocks until this event completes.
ApplicationStopping The host is performing a graceful shutdown. Requests may still be processing. Shutdown blocks until this event completes. 
public class Startup
{
    public void Configure(IApplicationBuilder app, IHostApplicationLifetime appLifetime)
    {
        appLifetime.ApplicationStarted.Register(OnStarted);
        appLifetime.ApplicationStopping.Register(OnStopping);
        appLifetime.ApplicationStopped.Register(OnStopped);

        Console.CancelKeyPress += (sender, eventArgs) =>
        {
            appLifetime.StopApplication();
            // Don't terminate the process immediately, wait for the Main thread to exit gracefully.
            eventArgs.Cancel = true;
        };
    }

    private void OnStarted()
    {
        // Perform post-startup activities here
    }

    private void OnStopping()
    {
        // Perform on-stopping activities here
    }

    private void OnStopped()
    {
        // Perform post-stopped activities here
    }
}

StopApplication requests termination of the app. The following class uses StopApplication to gracefully shut down an app when the class's Shutdown method is called:

public class MyClass
{
    private readonly IHostApplicationLifetime _appLifetime;

    public MyClass(IHostApplicationLifetime appLifetime)
    {
        _appLifetime = appLifetime;
    }

    public void Shutdown()
    {
        _appLifetime.StopApplication();
    }
}

Scope validation

CreateDefaultBuilder sets ServiceProviderOptions.ValidateScopes to true if the app's environment is Development.

When ValidateScopes is set to true, the default service provider performs checks to verify that:

  • Scoped services aren't directly or indirectly resolved from the root service provider.
  • Scoped services aren't directly or indirectly injected into singletons.

The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.

Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations when BuildServiceProvider is called.

To always validate scopes, including in the Production environment, configure the ServiceProviderOptions with UseDefaultServiceProvider on the host builder:

WebHost.CreateDefaultBuilder(args)
    .UseDefaultServiceProvider((context, options) => {
        options.ValidateScopes = true;
    })

Additional resources

ASP.NET Core apps configure and launch a host. The host is responsible for app startup and lifetime management. At a minimum, the host configures a server and a request processing pipeline. The host can also set up logging, dependency injection, and configuration.

This article covers the Web Host, which remains available only for backward compatibility. The ASP.NET Core templates create a .NET Generic Host, which is recommended for all app types.

Set up a host

Create a host using an instance of IWebHostBuilder. This is typically performed in the app's entry point, the Main method.

In the project templates, Main is located in Program.cs. A typical app calls CreateDefaultBuilder to start setting up a host:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

The code that calls CreateDefaultBuilder is in a method named CreateWebHostBuilder, which separates it from the code in Main that calls Run on the builder object. This separation is required if you use Entity Framework Core tools. The tools expect to find a CreateWebHostBuilder method that they can call at design time to configure the host without running the app. An alternative is to implement IDesignTimeDbContextFactory. For more information, see Design-time DbContext Creation.

CreateDefaultBuilder performs the following tasks:

The configuration defined by CreateDefaultBuilder can be overridden and augmented by ConfigureAppConfiguration, ConfigureLogging, and other methods and extension methods of IWebHostBuilder. A few examples follow:

  • ConfigureAppConfiguration is used to specify additional IConfiguration for the app. The following ConfigureAppConfiguration call adds a delegate to include app configuration in the appsettings.xml file. ConfigureAppConfiguration may be called multiple times. Note that this configuration doesn't apply to the host (for example, server URLs or environment). See the Host configuration values section.

    WebHost.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((hostingContext, config) =>
    {
        config.AddXmlFile("appsettings.xml", optional: true, reloadOnChange: true);
    })
    ...

  • The following ConfigureLogging call adds a delegate to configure the minimum logging level (SetMinimumLevel) to LogLevel.Warning. This setting overrides the settings in appsettings.Development.json (LogLevel.Debug) and appsettings.Production.json (LogLevel.Error) configured by CreateDefaultBuilder. ConfigureLogging may be called multiple times.

    WebHost.CreateDefaultBuilder(args)
    .ConfigureLogging(logging => 
    {
        logging.SetMinimumLevel(LogLevel.Warning);
    })
    ...

  • The following call to ConfigureKestrel overrides the default Limits.MaxRequestBodySize of 30,000,000 bytes established when Kestrel was configured by CreateDefaultBuilder:

    WebHost.CreateDefaultBuilder(args)
    .ConfigureKestrel((context, options) =>
    {
        options.Limits.MaxRequestBodySize = 20000000;
    });

The content root determines where the host searches for content files, such as MVC view files. When the app is started from the project's root folder, the project's root folder is used as the content root. This is the default used in Visual Studio and the dotnet new templates.

For more information on app configuration, see Configuration in ASP.NET Core.

Note

As an alternative to using the static CreateDefaultBuilder method, creating a host from WebHostBuilder is a supported approach with ASP.NET Core 2.x.

When setting up a host, Configure and ConfigureServices methods can be provided. If a Startup class is specified, it must define a Configure method. For more information, see App startup in ASP.NET Core. Multiple calls to ConfigureServices append to one another. Multiple calls to Configure or UseStartup on the WebHostBuilder replace previous settings.

Host configuration values

WebHostBuilder relies on the following approaches to set the host configuration values:

  • Host builder configuration, which includes environment variables with the format ASPNETCORE_{configurationKey}. For example, ASPNETCORE_ENVIRONMENT.
  • Extensions such as UseContentRoot and UseConfiguration (see the Override configuration section).
  • UseSetting and the associated key. When setting a value with UseSetting, the value is set as a string regardless of the type.

The host uses whichever option sets a value last. For more information, see Override configuration in the next section.

Application Key (Name)

The IWebHostEnvironment.ApplicationName property is automatically set when UseStartup or Configure is called during host construction. The value is set to the name of the assembly containing the app's entry point. To set the value explicitly, use the WebHostDefaults.ApplicationKey:

Key: applicationName
Type: string
Default: The name of the assembly containing the app's entry point.
Set using: UseSetting
Environment variable: ASPNETCORE_APPLICATIONNAME

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.ApplicationKey, "CustomApplicationName")

Capture Startup Errors

This setting controls the capture of startup errors.

Key: captureStartupErrors
Type: bool (true or 1)
Default: Defaults to false unless the app runs with Kestrel behind IIS, where the default is true.
Set using: CaptureStartupErrors
Environment variable: ASPNETCORE_CAPTURESTARTUPERRORS

When false, errors during startup result in the host exiting. When true, the host captures exceptions during startup and attempts to start the server.

WebHost.CreateDefaultBuilder(args)
    .CaptureStartupErrors(true)

Content root

This setting determines where ASP.NET Core begins searching for content files.

Key: contentRoot
Type: string
Default: Defaults to the folder where the app assembly resides.
Set using: UseContentRoot
Environment variable: ASPNETCORE_CONTENTROOT

The content root is also used as the base path for the web root. If the content root path doesn't exist, the host fails to start.

WebHost.CreateDefaultBuilder(args)
    .UseContentRoot("c:\\<content-root>")

For more information, see:

Detailed Errors

Determines if detailed errors should be captured.

Key: detailedErrors
Type: bool (true or 1)
Default: false
Set using: UseSetting
Environment variable: ASPNETCORE_DETAILEDERRORS

When enabled (or when the Environment is set to Development), the app captures detailed exceptions.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.DetailedErrorsKey, "true")

Environment

Sets the app's environment.

Key: environment
Type: string
Default: Production
Set using: UseEnvironment
Environment variable: ASPNETCORE_ENVIRONMENT

The environment can be set to any value. Framework-defined values include Development, Staging, and Production. Values aren't case sensitive. By default, the Environment is read from the ASPNETCORE_ENVIRONMENT environment variable. When using Visual Studio, environment variables may be set in the launchSettings.json file. For more information, see Use multiple environments in ASP.NET Core.

WebHost.CreateDefaultBuilder(args)
    .UseEnvironment(EnvironmentName.Development)

Hosting Startup Assemblies

Sets the app's hosting startup assemblies.

Key: hostingStartupAssemblies
Type: string
Default: Empty string
Set using: UseSetting
Environment variable: ASPNETCORE_HOSTINGSTARTUPASSEMBLIES

A semicolon-delimited string of hosting startup assemblies to load on startup.

Although the configuration value defaults to an empty string, the hosting startup assemblies always include the app's assembly. When hosting startup assemblies are provided, they're added to the app's assembly for loading when the app builds its common services during startup.

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupAssembliesKey, "assembly1;assembly2")

HTTPS Port

Set the HTTPS redirect port. Used in enforcing HTTPS.

Key: https_port
Type: string
Default: A default value isn't set.
Set using: UseSetting
Environment variable: ASPNETCORE_HTTPS_PORTS

WebHost.CreateDefaultBuilder(args)
    .UseSetting("https_port", "8080")

Hosting Startup Exclude Assemblies

A semicolon-delimited string of hosting startup assemblies to exclude on startup.

Key: hostingStartupExcludeAssemblies
Type: string
Default: Empty string
Set using: UseSetting
Environment variable: ASPNETCORE_HOSTINGSTARTUPEXCLUDEASSEMBLIES

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.HostingStartupExcludeAssembliesKey, "assembly1;assembly2")

Prefer Hosting URLs

Indicates whether the host should listen on the URLs configured with the WebHostBuilder instead of those configured with the IServer implementation.

Key: preferHostingUrls
Type: bool (true or 1)
Default: false
Set using: PreferHostingUrls
Environment variable: ASPNETCORE_PREFERHOSTINGURLS

WebHost.CreateDefaultBuilder(args)
    .PreferHostingUrls(true)

Prevent Hosting Startup

Prevents the automatic loading of hosting startup assemblies, including hosting startup assemblies configured by the app's assembly. For more information, see Use hosting startup assemblies in ASP.NET Core.

Key: preventHostingStartup
Type: bool (true or 1)
Default: false
Set using: UseSetting
Environment variable: ASPNETCORE_PREVENTHOSTINGSTARTUP

WebHost.CreateDefaultBuilder(args)
    .UseSetting(WebHostDefaults.PreventHostingStartupKey, "true")

Server URLs

Indicates the IP addresses or host addresses with ports and protocols that the server should listen on for requests.

Key: urls
Type: string
Default: http://localhost:5000
Set using: UseUrls
Environment variable: ASPNETCORE_URLS

Set to a semicolon-separated (;) list of URL prefixes to which the server should respond. For example, http://localhost:123. Use "*" to indicate that the server should listen for requests on any IP address or hostname using the specified port and protocol (for example, http://*:5000). The protocol (http:// or https://) must be included with each URL. Supported formats vary among servers.

WebHost.CreateDefaultBuilder(args)
    .UseUrls("http://*:5000;http://localhost:5001;https://hostname:5002")

Kestrel has its own endpoint configuration API. For more information, see Kestrel web server in ASP.NET Core.

Shutdown Timeout

Specifies the amount of time to wait for Web Host to shut down.

Key: shutdownTimeoutSeconds
Type: int
Default: 5
Set using: UseShutdownTimeout
Environment variable: ASPNETCORE_SHUTDOWNTIMEOUTSECONDS

Although the key accepts an int with UseSetting (for example, .UseSetting(WebHostDefaults.ShutdownTimeoutKey, "10")), the UseShutdownTimeout extension method takes a TimeSpan.

During the timeout period, hosting:

If the timeout period expires before all of the hosted services stop, any remaining active services are stopped when the app shuts down. The services stop even if they haven't finished processing. If services require additional time to stop, increase the timeout.

WebHost.CreateDefaultBuilder(args)
    .UseShutdownTimeout(TimeSpan.FromSeconds(10))

Startup Assembly

Determines the assembly to search for the Startup class.

Key: startupAssembly
Type: string
Default: The app's assembly
Set using: UseStartup
Environment variable: ASPNETCORE_STARTUPASSEMBLY

The assembly by name (string) or type (TStartup) can be referenced. If multiple UseStartup methods are called, the last one takes precedence.

WebHost.CreateDefaultBuilder(args)
    .UseStartup("StartupAssemblyName")

WebHost.CreateDefaultBuilder(args)
    .UseStartup<TStartup>()

Web root

Sets the relative path to the app's static assets.

Key: webroot
Type: string
Default: The default is wwwroot. The path to {content root}/wwwroot must exist. If the path doesn't exist, a no-op file provider is used.
Set using: UseWebRoot
Environment variable: ASPNETCORE_WEBROOT

WebHost.CreateDefaultBuilder(args)
    .UseWebRoot("public")

For more information, see:

Override configuration

Use Configuration to configure Web Host. In the following example, host configuration is optionally specified in a hostsettings.json file. Any configuration loaded from the hostsettings.json file may be overridden by command-line arguments. The built configuration (in config) is used to configure the host with UseConfiguration. IWebHostBuilder configuration is added to the app's configuration, but the converse isn't true—ConfigureAppConfiguration doesn't affect the IWebHostBuilder configuration.

Overriding the configuration provided by UseUrls with hostsettings.json config first, command-line argument config second:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
    {
        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("hostsettings.json", optional: true)
            .AddCommandLine(args)
            .Build();

        return WebHost.CreateDefaultBuilder(args)
            .UseUrls("http://*:5000")
            .UseConfiguration(config)
            .Configure(app =>
            {
                app.Run(context => 
                    context.Response.WriteAsync("Hello, World!"));
            });
    }
}

hostsettings.json:

{
    urls: "http://*:5005"
}

Note

UseConfiguration only copies keys from the provided IConfiguration to the host builder configuration. Therefore, setting reloadOnChange: true for JSON, INI, and XML settings files has no effect.

To specify the host run on a particular URL, the desired value can be passed in from a command prompt when executing dotnet run. The command-line argument overrides the urls value from the hostsettings.json file, and the server listens on port 8080:

dotnet run --urls "http://*:8080"

Manage the host

Run

The Run method starts the web app and blocks the calling thread until the host is shut down:

host.Run();

Start

Run the host in a non-blocking manner by calling its Start method:

using (host)
{
    host.Start();
    Console.ReadLine();
}

If a list of URLs is passed to the Start method, it listens on the URLs specified:

var urls = new List<string>()
{
    "http://*:5000",
    "http://localhost:5001"
};

var host = new WebHostBuilder()
    .UseKestrel()
    .UseStartup<Startup>()
    .Start(urls.ToArray());

using (host)
{
    Console.ReadLine();
}

The app can initialize and start a new host using the pre-configured defaults of CreateDefaultBuilder using a static convenience method. These methods start the server without console output and with WaitForShutdown wait for a break (Ctrl-C/SIGINT or SIGTERM):

Start(RequestDelegate app)

Start with a RequestDelegate:

using (var host = WebHost.Start(app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, RequestDelegate app)

Start with a URL and RequestDelegate:

using (var host = WebHost.Start("http://localhost:8080", app => app.Response.WriteAsync("Hello, World!")))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(RequestDelegate app), except the app responds on http://localhost:8080.

Start(Action<IRouteBuilder> routeBuilder)

Use an instance of IRouteBuilder (Microsoft.AspNetCore.Routing) to use routing middleware:

using (var host = WebHost.Start(router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shutdown the host...");
    host.WaitForShutdown();
}

Use the following browser requests with the example:

Request Response
http://localhost:5000/hello/Martin Hello, Martin!
http://localhost:5000/buenosdias/Catrina Buenos dias, Catrina!
http://localhost:5000/throw/ooops! Throws an exception with string "ooops!"
http://localhost:5000/throw Throws an exception with string "Uh oh!"
http://localhost:5000/Sante/Kevin Sante, Kevin!
http://localhost:5000 Hello World!

WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

Start(string url, Action<IRouteBuilder> routeBuilder)

Use a URL and an instance of IRouteBuilder:

using (var host = WebHost.Start("http://localhost:8080", router => router
    .MapGet("hello/{name}", (req, res, data) => 
        res.WriteAsync($"Hello, {data.Values["name"]}!"))
    .MapGet("buenosdias/{name}", (req, res, data) => 
        res.WriteAsync($"Buenos dias, {data.Values["name"]}!"))
    .MapGet("throw/{message?}", (req, res, data) => 
        throw new Exception((string)data.Values["message"] ?? "Uh oh!"))
    .MapGet("{greeting}/{name}", (req, res, data) => 
        res.WriteAsync($"{data.Values["greeting"]}, {data.Values["name"]}!"))
    .MapGet("", (req, res, data) => res.WriteAsync("Hello, World!"))))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Produces the same result as Start(Action<IRouteBuilder> routeBuilder), except the app responds at http://localhost:8080.

StartWith(Action<IApplicationBuilder> app)

Provide a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith(app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Make a request in the browser to http://localhost:5000 to receive the response "Hello World!" WaitForShutdown blocks until a break (Ctrl-C/SIGINT or SIGTERM) is issued. The app displays the Console.WriteLine message and waits for a keypress to exit.

StartWith(string url, Action<IApplicationBuilder> app)

Provide a URL and a delegate to configure an IApplicationBuilder:

using (var host = WebHost.StartWith("http://localhost:8080", app => 
    app.Use(next => 
    {
        return async context => 
        {
            await context.Response.WriteAsync("Hello World!");
        };
    })))
{
    Console.WriteLine("Use Ctrl-C to shut down the host...");
    host.WaitForShutdown();
}

Produces the same result as StartWith(Action<IApplicationBuilder> app), except the app responds on http://localhost:8080.

IWebHostEnvironment interface

The IWebHostEnvironment interface provides information about the app's web hosting environment. Use constructor injection to obtain the IWebHostEnvironment in order to use its properties and extension methods:

public class CustomFileReader
{
    private readonly IWebHostEnvironment _env;

    public CustomFileReader(IWebHostEnvironment env)
    {
        _env = env;
    }

    public string ReadFile(string filePath)
    {
        var fileProvider = _env.WebRootFileProvider;
        // Process the file here
    }
}

A convention-based approach can be used to configure the app at startup based on the environment. Alternatively, inject the IWebHostEnvironment into the Startup constructor for use in ConfigureServices:

public class Startup
{
    public Startup(IWebHostEnvironment env)
    {
        HostingEnvironment = env;
    }

    public IWebHostEnvironment HostingEnvironment { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        if (HostingEnvironment.IsDevelopment())
        {
            // Development configuration
        }
        else
        {
            // Staging/Production configuration
        }

        var contentRootPath = HostingEnvironment.ContentRootPath;
    }
}

Note

In addition to the IsDevelopment extension method, IWebHostEnvironment offers IsStaging, IsProduction, and IsEnvironment(string environmentName) methods. For more information, see Use multiple environments in ASP.NET Core.

The IWebHostEnvironment service can also be injected directly into the Configure method for setting up the processing pipeline:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        // In Development, use the Developer Exception Page
        app.UseDeveloperExceptionPage();
    }
    else
    {
        // In Staging/Production, route exceptions to /error
        app.UseExceptionHandler("/error");
    }

    var contentRootPath = env.ContentRootPath;
}

IWebHostEnvironment can be injected into the Invoke method when creating custom middleware:

public async Task Invoke(HttpContext context, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        // Configure middleware for Development
    }
    else
    {
        // Configure middleware for Staging/Production
    }

    var contentRootPath = env.ContentRootPath;
}

IHostApplicationLifetime interface

IHostApplicationLifetime allows for post-startup and shutdown activities. Three properties on the interface are cancellation tokens used to register Action methods that define startup and shutdown events.

Cancellation Token Triggered when…
ApplicationStarted The host has fully started.
ApplicationStopped The host is completing a graceful shutdown. All requests should be processed. Shutdown blocks until this event completes.
ApplicationStopping The host is performing a graceful shutdown. Requests may still be processing. Shutdown blocks until this event completes. 
public class Startup
{
    public void Configure(IApplicationBuilder app, IHostApplicationLifetime appLifetime)
    {
        appLifetime.ApplicationStarted.Register(OnStarted);
        appLifetime.ApplicationStopping.Register(OnStopping);
        appLifetime.ApplicationStopped.Register(OnStopped);

        Console.CancelKeyPress += (sender, eventArgs) =>
        {
            appLifetime.StopApplication();
            // Don't terminate the process immediately, wait for the Main thread to exit gracefully.
            eventArgs.Cancel = true;
        };
    }

    private void OnStarted()
    {
        // Perform post-startup activities here
    }

    private void OnStopping()
    {
        // Perform on-stopping activities here
    }

    private void OnStopped()
    {
        // Perform post-stopped activities here
    }
}

StopApplication requests termination of the app. The following class uses StopApplication to gracefully shut down an app when the class's Shutdown method is called:

public class MyClass
{
    private readonly IHostApplicationLifetime _appLifetime;

    public MyClass(IHostApplicationLifetime appLifetime)
    {
        _appLifetime = appLifetime;
    }

    public void Shutdown()
    {
        _appLifetime.StopApplication();
    }
}

Scope validation

CreateDefaultBuilder sets ServiceProviderOptions.ValidateScopes to true if the app's environment is Development.

When ValidateScopes is set to true, the default service provider performs checks to verify that:

  • Scoped services aren't directly or indirectly resolved from the root service provider.
  • Scoped services aren't directly or indirectly injected into singletons.

The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.

Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations when BuildServiceProvider is called.

To always validate scopes, including in the Production environment, configure the ServiceProviderOptions with UseDefaultServiceProvider on the host builder:

WebHost.CreateDefaultBuilder(args)
    .UseDefaultServiceProvider((context, options) => {
        options.ValidateScopes = true;
    })

Additional resources