How NUnit or MSTest find the tests in test dll

When NUnit or MSTest frameworks scan a compiled DLL (such as MyProject.Tests.dll) to discover and execute tests, they use reflection and specific conventions to identify classes and methods that are marked as tests. Here’s a more detailed explanation of how this scanning process works:

1. Reflection in .NET

Reflection is a powerful feature in .NET that allows code to inspect itself at runtime. It provides a way to discover information about types (classes, interfaces), methods, properties, and other members of assemblies (DLLs) loaded into the application domain.

2. Test Discovery Process

When you run tests using NUnit or MSTest, the test runner (like dotnet test, Visual Studio Test Explorer, or ReSharper) follows these steps to discover tests within a DLL:

a. Loading the Assembly

• Assembly Loading: The test runner first loads the compiled DLL (MyProject.Tests.dll) into the application domain. This is typically done dynamically at runtime.

b. Finding Test Fixtures (NUnit) or Test Classes (MSTest)

• NUnit ([TestFixture]):

  • For NUnit, the test runner looks for classes decorated with the [TestFixture] attribute. This attribute indicates that the class contains one or more test methods.

• MSTest ([TestClass]):

  • For MSTest, the test runner looks for classes decorated with the [TestClass] attribute. Similarly, this attribute indicates that the class contains test methods.

c. Discovering Test Methods

• Identifying Test Methods:
  • Once a test fixture (NUnit) or test class (MSTest) is identified, the test runner inspects the methods within these classes.
  • Methods marked with [Test] (NUnit) or [TestMethod] (MSTest) attributes are recognized as test methods.

d. Execution

• Executing Tests:
  • After discovering all test methods, the test runner executes each test method sequentially.
  • It captures the results (pass/fail) and any output generated during test execution.

3. Execution Context

• Isolation: Each test method typically runs in isolation from others, ensuring that one test does not affect the outcome of another.

• Reporting: Test runners provide detailed reports on test results, including summaries and logs, which help in identifying issues and tracking test coverage.

4. Integration with Build Systems and IDEs

• Integration: Test runners are integrated into build systems (dotnet test), IDEs (Visual Studio, JetBrains Rider), and continuous integration (CI) pipelines (Azure DevOps, Jenkins). This integration automates the test execution process and facilitates continuous testing practices.