using directive

The using directive allows you to use types defined in a namespace without specifying the fully qualified namespace of that type. In its basic form, the using directive imports all the types from a single namespace, as shown in the following example:

using System.Text;

You can apply two modifiers to a using directive:

• The global modifier has the same effect as adding the same using directive to every source file in your project. This modifier was introduced in C# 10.
• The static modifier imports the static members and nested types from a single type rather than importing all the types in a namespace.

You can combine both modifiers to import the static members from a type in all source files in your project.

You can also create an alias for a namespace or a type with a using alias directive.

using Project = PC.MyCompany.Project;

You can use the global modifier on a using alias directive.

The scope of a using directive without the global modifier is the file in which it appears.

The using directive can appear:

• At the beginning of a source code file, before any namespace or type declarations.
• In any namespace, but before any namespaces or types declared in that namespace, unless the global modifier is used, in which case the directive must appear before all namespace and type declarations.

Otherwise, compiler error CS1529 is generated.

Create a using directive to use the types in a namespace without having to specify the namespace. A using directive doesn't give you access to any namespaces that are nested in the namespace you specify. Namespaces come in two categories: user-defined and system-defined. User-defined namespaces are namespaces defined in your code. For a list of the system-defined namespaces, see .NET API Browser.

global modifier

Adding the global modifier to a using directive means that using is applied to all files in the compilation (typically a project). The global using directive was added in C# 10. Its syntax is:

global using <fully-qualified-namespace>;

where fully-qualified-namespace is the fully qualified name of the namespace whose types can be referenced without specifying the namespace.

A global using directive can appear at the beginning of any source code file. All global using directives in a single file must appear before:

• All using directives without the global modifier.
• All namespace and type declarations in the file.

You may add global using directives to any source file. Typically, you'll want to keep them in a single location. The order of global using directives doesn't matter, either in a single file, or between files.

The global modifier may be combined with the static modifier. The global modifier may be applied to a using alias directive. In both cases, the directive's scope is all files in the current compilation. The following example enables using all the methods declared in the System.Math in all files in your project:

global using static System.Math;

You can also globally include a namespace by adding a <Using> item to your project file, for example, <Using Include="My.Awesome.Namespace" />. For more information, see <Using> item.

static modifier

The using static directive names a type whose static members and nested types you can access without specifying a type name. Its syntax is:

using static <fully-qualified-type-name>;

The <fully-qualified-type-name> is the name of the type whose static members and nested types can be referenced without specifying a type name. If you don't provide a fully qualified type name (the full namespace name along with the type name), C# generates compiler error CS0246: "The type or namespace name 'type/namespace' couldn't be found (are you missing a using directive or an assembly reference?)".

The using static directive applies to any type that has static members (or nested types), even if it also has instance members. However, instance members can only be invoked through the type instance.

You can access static members of a type without having to qualify the access with the type name:

using static System.Console;
using static System.Math;
class Program
{
    static void Main()
    {
        WriteLine(Sqrt(3*3 + 4*4));
    }
}

Ordinarily, when you call a static member, you provide the type name along with the member name. Repeatedly entering the same type name to invoke members of the type can result in verbose, obscure code. For example, the following definition of a Circle class references many members of the Math class.

using System;

public class Circle
{
   public Circle(double radius)
   {
      Radius = radius;
   }

   public double Radius { get; set; }

   public double Diameter
   {
      get { return 2 * Radius; }
   }

   public double Circumference
   {
      get { return 2 * Radius * Math.PI; }
   }

   public double Area
   {
      get { return Math.PI * Math.Pow(Radius, 2); }
   }
}

By eliminating the need to explicitly reference the Math class each time a member is referenced, the using static directive produces cleaner code:

using System;
using static System.Math;

public class Circle
{
   public Circle(double radius)
   {
      Radius = radius;
   }

   public double Radius { get; set; }

   public double Diameter
   {
      get { return 2 * Radius; }
   }

   public double Circumference
   {
      get { return 2 * Radius * PI; }
   }

   public double Area
   {
      get { return PI * Pow(Radius, 2); }
   }
}

using static imports only accessible static members and nested types declared in the specified type. Inherited members aren't imported. You can import from any named type with a using static directive, including Visual Basic modules. If F# top-level functions appear in metadata as static members of a named type whose name is a valid C# identifier, then the F# functions can be imported.

using static makes extension methods declared in the specified type available for extension method lookup. However, the names of the extension methods aren't imported into scope for unqualified reference in code.

Methods with the same name imported from different types by different using static directives in the same compilation unit or namespace form a method group. Overload resolution within these method groups follows normal C# rules.

The following example uses the using static directive to make the static members of the Console, Math, and String classes available without having to specify their type name.

using System;
using static System.Console;
using static System.Math;
using static System.String;

class Program
{
   static void Main()
   {
      Write("Enter a circle's radius: ");
      var input = ReadLine();
      if (!IsNullOrEmpty(input) && double.TryParse(input, out var radius)) {
         var c = new Circle(radius);

         string s = "\nInformation about the circle:\n";
         s = s + Format("   Radius: {0:N2}\n", c.Radius);
         s = s + Format("   Diameter: {0:N2}\n", c.Diameter);
         s = s + Format("   Circumference: {0:N2}\n", c.Circumference);
         s = s + Format("   Area: {0:N2}\n", c.Area);
         WriteLine(s);
      }
      else {
         WriteLine("Invalid input...");
      }
   }
}

public class Circle
{
   public Circle(double radius)
   {
      Radius = radius;
   }

   public double Radius { get; set; }

   public double Diameter
   {
      get { return 2 * Radius; }
   }

   public double Circumference
   {
      get { return 2 * Radius * PI; }
   }

   public double Area
   {
      get { return PI * Pow(Radius, 2); }
   }
}
// The example displays the following output:
//       Enter a circle's radius: 12.45
//
//       Information about the circle:
//          Radius: 12.45
//          Diameter: 24.90
//          Circumference: 78.23
//          Area: 486.95

In the example, the using static directive could also have been applied to the Double type. Adding that directive would make it possible to call the TryParse(String, Double) method without specifying a type name. However, using TryParse without a type name creates less readable code, since it becomes necessary to check the using static directives to determine which numeric type's TryParse method is called.

using static also applies to enum types. By adding using static with the enum, the type is no longer required to use the enum members.

using static Color;

enum Color
{
    Red,
    Green,
    Blue
}

class Program
{
    public static void Main()
    {
        Color color = Green;
    }
}

using alias

Create a using alias directive to make it easier to qualify an identifier to a namespace or type. In any using directive, the fully qualified namespace or type must be used regardless of the using directives that come before it. No using alias can be used in the declaration of a using directive. For example, the following example generates a compiler error:

using s = System.Text;
using s.RegularExpressions; // Generates a compiler error.

The following example shows how to define and use a using alias for a namespace:

namespace PC
{
    // Define an alias for the nested namespace.
    using Project = PC.MyCompany.Project;
    class A
    {
        void M()
        {
            // Use the alias
            var mc = new Project.MyClass();
        }
    }
    namespace MyCompany
    {
        namespace Project
        {
            public class MyClass { }
        }
    }
}

A using alias directive can't have an open generic type on the right-hand side. For example, you can't create a using alias for a List<T>, but you can create one for a List<int>.

The following example shows how to define a using directive and a using alias for a class:

using System;

// Using alias directive for a class.
using AliasToMyClass = NameSpace1.MyClass;

// Using alias directive for a generic class.
using UsingAlias = NameSpace2.MyClass<int>;

namespace NameSpace1
{
    public class MyClass
    {
        public override string ToString()
        {
            return "You are in NameSpace1.MyClass.";
        }
    }
}

namespace NameSpace2
{
    class MyClass<T>
    {
        public override string ToString()
        {
            return "You are in NameSpace2.MyClass.";
        }
    }
}

namespace NameSpace3
{
    class MainClass
    {
        static void Main()
        {
            var instance1 = new AliasToMyClass();
            Console.WriteLine(instance1);

            var instance2 = new UsingAlias();
            Console.WriteLine(instance2);
        }
    }
}
// Output:
//    You are in NameSpace1.MyClass.
//    You are in NameSpace2.MyClass.

Beginning with C# 12, you can create aliases for types that were previously restricted, including tuple types, pointer types, and other unsafe types. For more information on the updated rules, see the feature spec.

C# language specification

For more information, see Using directives in the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

For more information on the global using modifier, see the global usings feature specification - C# 10.

See also

• C# keywords
• Namespaces
• Style rule IDE0005 - Remove unnecessary 'using' directives
• Style rule IDE0065 - 'using' directive placement
• using statement