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Tutorial: Test a .NET class library using Visual Studio Code

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This tutorial shows how to automate unit testing by adding a test project to a solution.

Prerequisites

Create a unit test project

Unit tests provide automated software testing during your development and publishing. The testing framework that you use in this tutorial is MSTest. MSTest is one of three test frameworks you can choose from. The others are xUnit and nUnit.

  1. Start Visual Studio Code.

  2. Open the ClassLibraryProjects solution you created in Create a .NET class library using Visual Studio Code.

  3. Create a unit test project named "StringLibraryTest".

    dotnet new mstest -o StringLibraryTest

    The project template creates a UnitTest1.cs file with the following code:

    namespace StringLibraryTest;

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
    }
}

    The source code created by the unit test template does the following:

    Each method tagged with [TestMethod] in a test class tagged with [TestClass] is run automatically when the unit test is invoked.

  4. Add the test project to the solution.

    dotnet sln add StringLibraryTest/StringLibraryTest.csproj

Add a project reference

For the test project to work with the StringLibrary class, add a reference in the StringLibraryTest project to the StringLibrary project.

  1. Run the following command:

    dotnet add StringLibraryTest/StringLibraryTest.csproj reference StringLibrary/StringLibrary.csproj

Add and run unit test methods

When Visual Studio invokes a unit test, it runs each method that is marked with the TestMethodAttribute attribute in a class that is marked with the TestClassAttribute attribute. A test method ends when the first failure is found or when all tests contained in the method have succeeded.

The most common tests call members of the Assert class. Many assert methods include at least two parameters, one of which is the expected test result and the other of which is the actual test result. Some of the Assert class's most frequently called methods are shown in the following table:

Assert methods Function
Assert.AreEqual Verifies that two values or objects are equal. The assert fails if the values or objects aren't equal.
Assert.AreSame Verifies that two object variables refer to the same object. The assert fails if the variables refer to different objects.
Assert.IsFalse Verifies that a condition is false. The assert fails if the condition is true.
Assert.IsNotNull Verifies that an object isn't null. The assert fails if the object is null.

You can also use the Assert.ThrowsException method in a test method to indicate the type of exception it's expected to throw. The test fails if the specified exception isn't thrown.

In testing the StringLibrary.StartsWithUpper method, you want to provide a number of strings that begin with an uppercase character. You expect the method to return true in these cases, so you can call the Assert.IsTrue method. Similarly, you want to provide a number of strings that begin with something other than an uppercase character. You expect the method to return false in these cases, so you can call the Assert.IsFalse method.

Since your library method handles strings, you also want to make sure that it successfully handles an empty string (String.Empty) and a null string. An empty string is one that has no characters and whose Length is 0. A null string is one that hasn't been initialized. You can call StartsWithUpper directly as a static method and pass a single String argument. Or you can call StartsWithUpper as an extension method on a string variable assigned to null.

You'll define three methods, each of which calls an Assert method for each element in a string array. You'll call a method overload that lets you specify an error message to be displayed in case of test failure. The message identifies the string that caused the failure.

To create the test methods:

  1. Open StringLibraryTest/UnitTest1.cs and replace all of the code with the following code.

    using Microsoft.VisualStudio.TestTools.UnitTesting;
using UtilityLibraries;

namespace StringLibraryTest;

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestStartsWithUpper()
    {
        // Tests that we expect to return true.
        string[] words = { "Alphabet", "Zebra", "ABC", "Αθήνα", "Москва" };
        foreach (var word in words)
        {
            bool result = word.StartsWithUpper();
            Assert.IsTrue(result,
                   string.Format("Expected for '{0}': true; Actual: {1}",
                                 word, result));
        }
    }

    [TestMethod]
    public void TestDoesNotStartWithUpper()
    {
        // Tests that we expect to return false.
        string[] words = { "alphabet", "zebra", "abc", "αυτοκινητοβιομηχανία", "государство",
                               "1234", ".", ";", " " };
        foreach (var word in words)
        {
            bool result = word.StartsWithUpper();
            Assert.IsFalse(result,
                   string.Format("Expected for '{0}': false; Actual: {1}",
                                 word, result));
        }
    }

    [TestMethod]
    public void DirectCallWithNullOrEmpty()
    {
        // Tests that we expect to return false.
        string?[] words = { string.Empty, null };
        foreach (var word in words)
        {
            bool result = StringLibrary.StartsWithUpper(word);
            Assert.IsFalse(result,
                   string.Format("Expected for '{0}': false; Actual: {1}",
                                 word == null ? "<null>" : word, result));
        }
    }
}

    The test of uppercase characters in the TestStartsWithUpper method includes the Greek capital letter alpha (U+0391) and the Cyrillic capital letter EM (U+041C). The test of lowercase characters in the TestDoesNotStartWithUpper method includes the Greek small letter alpha (U+03B1) and the Cyrillic small letter Ghe (U+0433).

  2. Save your changes.

  3. Run the tests:

    dotnet test StringLibraryTest/StringLibraryTest.csproj

    The terminal output shows that all tests passed.

    Starting test execution, please wait...
A total of 1 test files matched the specified pattern.

Passed!  - Failed:     0, Passed:     3, Skipped:     0, Total:     3, Duration: 3 ms - StringLibraryTest.dll (net8.0)

Handle test failures

If you're doing test-driven development (TDD), you write tests first and they fail the first time you run them. Then you add code to the app that makes the test succeed. For this tutorial, you created the test after writing the app code that it validates, so you haven't seen the test fail. To validate that a test fails when you expect it to fail, add an invalid value to the test input.

  1. Modify the words array in the TestDoesNotStartWithUpper method to include the string "Error".

    string[] words = { "alphabet", "Error", "zebra", "abc", "αυτοκινητοβιομηχανία", "государство",
                   "1234", ".", ";", " " };

  2. Run the tests:

    dotnet test StringLibraryTest/StringLibraryTest.csproj

    The terminal output shows that one test fails, and it provides an error message for the failed test: "Assert.IsFalse failed. Expected for 'Error': false; actual: True". Because of the failure, no strings in the array after "Error" were tested.

    Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
  Failed TestDoesNotStartWithUpper [28 ms]
  Error Message:
   Assert.IsFalse failed. Expected for 'Error': false; Actual: True
  Stack Trace:
     at StringLibraryTest.UnitTest1.TestDoesNotStartWithUpper() in C:\ClassLibraryProjects\StringLibraryTest\UnitTest1.cs:line 33

Failed!  - Failed:     1, Passed:     2, Skipped:     0, Total:     3, Duration: 31 ms - StringLibraryTest.dll (net5.0)

  3. Remove the string "Error" that you added in step 1. Rerun the test and the tests pass.

Test the Release version of the library

Now that the tests have all passed when running the Debug build of the library, run the tests an additional time against the Release build of the library. A number of factors, including compiler optimizations, can sometimes produce different behavior between Debug and Release builds.

  1. Run the tests with the Release build configuration:

    dotnet test StringLibraryTest/StringLibraryTest.csproj --configuration Release

    The tests pass.

Debug tests

If you're using Visual Studio Code as your IDE, you can use the same process shown in Debug a .NET console application using Visual Studio Code to debug code using your unit test project. Instead of starting the ShowCase app project, open StringLibraryTest/UnitTest1.cs, and select Debug All Tests between lines 7 and 8. If you're unable to find it, press Ctrl+Shift+P to open the command palette and enter Reload Window.

Visual Studio Code starts the test project with the debugger attached. Execution will stop at any breakpoint you've added to the test project or the underlying library code.

Additional resources

Next steps

In this tutorial, you unit tested a class library. You can make the library available to others by publishing it to NuGet as a package. To learn how, follow a NuGet tutorial:

Create and publish a package using the dotnet CLI

If you publish a library as a NuGet package, others can install and use it. To learn how, follow a NuGet tutorial:

Install and use a package using the dotnet CLI

A library doesn't have to be distributed as a package. It can be bundled with a console app that uses it. To learn how to publish a console app, see the earlier tutorial in this series:

Publish a .NET console application using Visual Studio Code

The Visual Studio Code extension C# Dev Kit provides more tools for developing C# apps and libraries:

C# Dev Kit

This tutorial shows how to automate unit testing by adding a test project to a solution.

Prerequisites

Create a unit test project

Unit tests provide automated software testing during your development and publishing. The testing framework that you use in this tutorial is MSTest. MSTest is one of three test frameworks you can choose from. The others are xUnit and nUnit.

  1. Start Visual Studio Code.

  2. Open the ClassLibraryProjects solution you created in Create a .NET class library using Visual Studio Code.

  3. Create a unit test project named "StringLibraryTest".

    dotnet new mstest -o StringLibraryTest

    The project template creates a UnitTest1.cs file with the following code:

    using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace StringLibraryTest
{
    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMethod1()
        {
        }
    }
}

    The source code created by the unit test template does the following:

    Each method tagged with [TestMethod] in a test class tagged with [TestClass] is run automatically when the unit test is invoked.

  4. Add the test project to the solution.

    dotnet sln add StringLibraryTest/StringLibraryTest.csproj

Add a project reference

For the test project to work with the StringLibrary class, add a reference in the StringLibraryTest project to the StringLibrary project.

  1. Run the following command:

    dotnet add StringLibraryTest/StringLibraryTest.csproj reference StringLibrary/StringLibrary.csproj

Add and run unit test methods

When Visual Studio invokes a unit test, it runs each method that is marked with the TestMethodAttribute attribute in a class that is marked with the TestClassAttribute attribute. A test method ends when the first failure is found or when all tests contained in the method have succeeded.

The most common tests call members of the Assert class. Many assert methods include at least two parameters, one of which is the expected test result and the other of which is the actual test result. Some of the Assert class's most frequently called methods are shown in the following table:

Assert methods Function
Assert.AreEqual Verifies that two values or objects are equal. The assert fails if the values or objects aren't equal.
Assert.AreSame Verifies that two object variables refer to the same object. The assert fails if the variables refer to different objects.
Assert.IsFalse Verifies that a condition is false. The assert fails if the condition is true.
Assert.IsNotNull Verifies that an object isn't null. The assert fails if the object is null.

You can also use the Assert.ThrowsException method in a test method to indicate the type of exception it's expected to throw. The test fails if the specified exception isn't thrown.

In testing the StringLibrary.StartsWithUpper method, you want to provide a number of strings that begin with an uppercase character. You expect the method to return true in these cases, so you can call the Assert.IsTrue method. Similarly, you want to provide a number of strings that begin with something other than an uppercase character. You expect the method to return false in these cases, so you can call the Assert.IsFalse method.

Since your library method handles strings, you also want to make sure that it successfully handles an empty string (String.Empty) and a null string. An empty string is one that has no characters and whose Length is 0. A null string is one that hasn't been initialized. You can call StartsWithUpper directly as a static method and pass a single String argument. Or you can call StartsWithUpper as an extension method on a string variable assigned to null.

You'll define three methods, each of which calls an Assert method for each element in a string array. You'll call a method overload that lets you specify an error message to be displayed in case of test failure. The message identifies the string that caused the failure.

To create the test methods:

  1. Open StringLibraryTest/UnitTest1.cs and replace all of the code with the following code.

    using Microsoft.VisualStudio.TestTools.UnitTesting;
using UtilityLibraries;

namespace StringLibraryTest;

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestStartsWithUpper()
    {
        // Tests that we expect to return true.
        string[] words = { "Alphabet", "Zebra", "ABC", "Αθήνα", "Москва" };
        foreach (var word in words)
        {
            bool result = word.StartsWithUpper();
            Assert.IsTrue(result,
                   string.Format("Expected for '{0}': true; Actual: {1}",
                                 word, result));
        }
    }

    [TestMethod]
    public void TestDoesNotStartWithUpper()
    {
        // Tests that we expect to return false.
        string[] words = { "alphabet", "zebra", "abc", "αυτοκινητοβιομηχανία", "государство",
                               "1234", ".", ";", " " };
        foreach (var word in words)
        {
            bool result = word.StartsWithUpper();
            Assert.IsFalse(result,
                   string.Format("Expected for '{0}': false; Actual: {1}",
                                 word, result));
        }
    }

    [TestMethod]
    public void DirectCallWithNullOrEmpty()
    {
        // Tests that we expect to return false.
        string?[] words = { string.Empty, null };
        foreach (var word in words)
        {
            bool result = StringLibrary.StartsWithUpper(word);
            Assert.IsFalse(result,
                   string.Format("Expected for '{0}': false; Actual: {1}",
                                 word == null ? "<null>" : word, result));
        }
    }
}

    The test of uppercase characters in the TestStartsWithUpper method includes the Greek capital letter alpha (U+0391) and the Cyrillic capital letter EM (U+041C). The test of lowercase characters in the TestDoesNotStartWithUpper method includes the Greek small letter alpha (U+03B1) and the Cyrillic small letter Ghe (U+0433).

  2. Save your changes.

  3. Run the tests:

    dotnet test StringLibraryTest/StringLibraryTest.csproj

    The terminal output shows that all tests passed.

    Starting test execution, please wait...
A total of 1 test files matched the specified pattern.

Passed!  - Failed:     0, Passed:     3, Skipped:     0, Total:     3, Duration: 3 ms - StringLibraryTest.dll (net7.0)

Handle test failures

If you're doing test-driven development (TDD), you write tests first and they fail the first time you run them. Then you add code to the app that makes the test succeed. For this tutorial, you created the test after writing the app code that it validates, so you haven't seen the test fail. To validate that a test fails when you expect it to fail, add an invalid value to the test input.

  1. Modify the words array in the TestDoesNotStartWithUpper method to include the string "Error".

    string[] words = { "alphabet", "Error", "zebra", "abc", "αυτοκινητοβιομηχανία", "государство",
                   "1234", ".", ";", " " };

  2. Run the tests:

    dotnet test StringLibraryTest/StringLibraryTest.csproj

    The terminal output shows that one test fails, and it provides an error message for the failed test: "Assert.IsFalse failed. Expected for 'Error': false; actual: True". Because of the failure, no strings in the array after "Error" were tested.

    Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
  Failed TestDoesNotStartWithUpper [28 ms]
  Error Message:
   Assert.IsFalse failed. Expected for 'Error': false; Actual: True
  Stack Trace:
     at StringLibraryTest.UnitTest1.TestDoesNotStartWithUpper() in C:\ClassLibraryProjects\StringLibraryTest\UnitTest1.cs:line 33

Failed!  - Failed:     1, Passed:     2, Skipped:     0, Total:     3, Duration: 31 ms - StringLibraryTest.dll (net5.0)

  3. Remove the string "Error" that you added in step 1. Rerun the test and the tests pass.

Test the Release version of the library

Now that the tests have all passed when running the Debug build of the library, run the tests an additional time against the Release build of the library. A number of factors, including compiler optimizations, can sometimes produce different behavior between Debug and Release builds.

  1. Run the tests with the Release build configuration:

    dotnet test StringLibraryTest/StringLibraryTest.csproj --configuration Release

    The tests pass.

Debug tests

If you're using Visual Studio Code as your IDE, you can use the same process shown in Debug a .NET console application using Visual Studio Code to debug code using your unit test project. Instead of starting the ShowCase app project, open StringLibraryTest/UnitTest1.cs, and select Debug All Tests between lines 7 and 8. If you're unable to find it, press Ctrl+Shift+P to open the command palette and enter Reload Window.

Visual Studio Code starts the test project with the debugger attached. Execution will stop at any breakpoint you've added to the test project or the underlying library code.

Additional resources

Next steps

In this tutorial, you unit tested a class library. You can make the library available to others by publishing it to NuGet as a package. To learn how, follow a NuGet tutorial:

Create and publish a package using the dotnet CLI

If you publish a library as a NuGet package, others can install and use it. To learn how, follow a NuGet tutorial:

Install and use a package using the dotnet CLI

A library doesn't have to be distributed as a package. It can be bundled with a console app that uses it. To learn how to publish a console app, see the earlier tutorial in this series:

Publish a .NET console application using Visual Studio Code

This tutorial shows how to automate unit testing by adding a test project to a solution.

Prerequisites

Create a unit test project

Unit tests provide automated software testing during your development and publishing. The testing framework that you use in this tutorial is MSTest. MSTest is one of three test frameworks you can choose from. The others are xUnit and nUnit.

  1. Start Visual Studio Code.

  2. Open the ClassLibraryProjects solution you created in Create a .NET class library using Visual Studio Code.

  3. Create a unit test project named "StringLibraryTest".

    dotnet new mstest -f net6.0 -o StringLibraryTest

    The -f net6.0 command changes the default target framework to net6.0 version.

    The -o or --output command specifies the location to place the generated output.

    The project template creates a UnitTest1.cs file with the following code:

    using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace StringLibraryTest
{
    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMethod1()
        {
        }
    }
}

    The source code created by the unit test template does the following:

    Each method tagged with [TestMethod] in a test class tagged with [TestClass] is run automatically when the unit test is invoked.

  4. Add the test project to the solution.

    dotnet sln add StringLibraryTest/StringLibraryTest.csproj

Add a project reference

For the test project to work with the StringLibrary class, add a reference in the StringLibraryTest project to the StringLibrary project.

  1. Run the following command:

    dotnet add StringLibraryTest/StringLibraryTest.csproj reference StringLibrary/StringLibrary.csproj

Add and run unit test methods

When Visual Studio invokes a unit test, it runs each method that is marked with the TestMethodAttribute attribute in a class that is marked with the TestClassAttribute attribute. A test method ends when the first failure is found or when all tests contained in the method have succeeded.

The most common tests call members of the Assert class. Many assert methods include at least two parameters, one of which is the expected test result and the other of which is the actual test result. Some of the Assert class's most frequently called methods are shown in the following table:

Assert methods Function
Assert.AreEqual Verifies that two values or objects are equal. The assert fails if the values or objects aren't equal.
Assert.AreSame Verifies that two object variables refer to the same object. The assert fails if the variables refer to different objects.
Assert.IsFalse Verifies that a condition is false. The assert fails if the condition is true.
Assert.IsNotNull Verifies that an object isn't null. The assert fails if the object is null.

You can also use the Assert.ThrowsException method in a test method to indicate the type of exception it's expected to throw. The test fails if the specified exception isn't thrown.

In testing the StringLibrary.StartsWithUpper method, you want to provide a number of strings that begin with an uppercase character. You expect the method to return true in these cases, so you can call the Assert.IsTrue method. Similarly, you want to provide a number of strings that begin with something other than an uppercase character. You expect the method to return false in these cases, so you can call the Assert.IsFalse method.

Since your library method handles strings, you also want to make sure that it successfully handles an empty string (String.Empty) and a null string. An empty string is one that has no characters and whose Length is 0. A null string is one that hasn't been initialized. You can call StartsWithUpper directly as a static method and pass a single String argument. Or you can call StartsWithUpper as an extension method on a string variable assigned to null.

You'll define three methods, each of which calls an Assert method for each element in a string array. You'll call a method overload that lets you specify an error message to be displayed in case of test failure. The message identifies the string that caused the failure.

To create the test methods:

  1. Open StringLibraryTest/UnitTest1.cs and replace all of the code with the following code.

    using Microsoft.VisualStudio.TestTools.UnitTesting;
using UtilityLibraries;

namespace StringLibraryTest;

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestStartsWithUpper()
    {
        // Tests that we expect to return true.
        string[] words = { "Alphabet", "Zebra", "ABC", "Αθήνα", "Москва" };
        foreach (var word in words)
        {
            bool result = word.StartsWithUpper();
            Assert.IsTrue(result,
                   string.Format("Expected for '{0}': true; Actual: {1}",
                                 word, result));
        }
    }

    [TestMethod]
    public void TestDoesNotStartWithUpper()
    {
        // Tests that we expect to return false.
        string[] words = { "alphabet", "zebra", "abc", "αυτοκινητοβιομηχανία", "государство",
                               "1234", ".", ";", " " };
        foreach (var word in words)
        {
            bool result = word.StartsWithUpper();
            Assert.IsFalse(result,
                   string.Format("Expected for '{0}': false; Actual: {1}",
                                 word, result));
        }
    }

    [TestMethod]
    public void DirectCallWithNullOrEmpty()
    {
        // Tests that we expect to return false.
        string?[] words = { string.Empty, null };
        foreach (var word in words)
        {
            bool result = StringLibrary.StartsWithUpper(word);
            Assert.IsFalse(result,
                   string.Format("Expected for '{0}': false; Actual: {1}",
                                 word == null ? "<null>" : word, result));
        }
    }
}

    The test of uppercase characters in the TestStartsWithUpper method includes the Greek capital letter alpha (U+0391) and the Cyrillic capital letter EM (U+041C). The test of lowercase characters in the TestDoesNotStartWithUpper method includes the Greek small letter alpha (U+03B1) and the Cyrillic small letter Ghe (U+0433).

  2. Save your changes.

  3. Run the tests:

    dotnet test StringLibraryTest/StringLibraryTest.csproj

    The terminal output shows that all tests passed.

    Starting test execution, please wait...
A total of 1 test files matched the specified pattern.

Passed!  - Failed:     0, Passed:     3, Skipped:     0, Total:     3, Duration: 3 ms - StringLibraryTest.dll (net6.0)

Handle test failures

If you're doing test-driven development (TDD), you write tests first and they fail the first time you run them. Then you add code to the app that makes the test succeed. For this tutorial, you created the test after writing the app code that it validates, so you haven't seen the test fail. To validate that a test fails when you expect it to fail, add an invalid value to the test input.

  1. Modify the words array in the TestDoesNotStartWithUpper method to include the string "Error".

    string[] words = { "alphabet", "Error", "zebra", "abc", "αυτοκινητοβιομηχανία", "государство",
                   "1234", ".", ";", " " };

  2. Run the tests:

    dotnet test StringLibraryTest/StringLibraryTest.csproj

    The terminal output shows that one test fails, and it provides an error message for the failed test: "Assert.IsFalse failed. Expected for 'Error': false; actual: True". Because of the failure, no strings in the array after "Error" were tested.

    Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
  Failed TestDoesNotStartWithUpper [28 ms]
  Error Message:
   Assert.IsFalse failed. Expected for 'Error': false; Actual: True
  Stack Trace:
     at StringLibraryTest.UnitTest1.TestDoesNotStartWithUpper() in C:\ClassLibraryProjects\StringLibraryTest\UnitTest1.cs:line 33

Failed!  - Failed:     1, Passed:     2, Skipped:     0, Total:     3, Duration: 31 ms - StringLibraryTest.dll (net5.0)

  3. Remove the string "Error" that you added in step 1. Rerun the test and the tests pass.

Test the Release version of the library

Now that the tests have all passed when running the Debug build of the library, run the tests an additional time against the Release build of the library. A number of factors, including compiler optimizations, can sometimes produce different behavior between Debug and Release builds.

  1. Run the tests with the Release build configuration:

    dotnet test StringLibraryTest/StringLibraryTest.csproj --configuration Release

    The tests pass.

Debug tests

If you're using Visual Studio Code as your IDE, you can use the same process shown in Debug a .NET console application using Visual Studio Code to debug code using your unit test project. Instead of starting the ShowCase app project, open StringLibraryTest/UnitTest1.cs, and select Debug All Tests between lines 7 and 8. If you're unable to find it, press Ctrl+Shift+P to open the command palette and enter Reload Window.

Visual Studio Code starts the test project with the debugger attached. Execution will stop at any breakpoint you've added to the test project or the underlying library code.

Additional resources

Next steps

In this tutorial, you unit tested a class library. You can make the library available to others by publishing it to NuGet as a package. To learn how, follow a NuGet tutorial:

Create and publish a package using the dotnet CLI

If you publish a library as a NuGet package, others can install and use it. To learn how, follow a NuGet tutorial:

Install and use a package using the dotnet CLI

A library doesn't have to be distributed as a package. It can be bundled with a console app that uses it. To learn how to publish a console app, see the earlier tutorial in this series:

Publish a .NET console application using Visual Studio Code