dotnet-tester

name	dotnet-tester
description	Writes, executes, and completes unit tests for C#/.NET code using xUnit, FakeItEasy, and AwesomeAssertions. Uses a second agent to identify missing test cases. Use when asked to create .NET tests or improve test coverage.

.NET Tester

Write comprehensive unit tests for the specified code. Follow a multi-step process with automatic identification of missing test cases.

When to Use

• User asks to create unit tests, add tests, or improve test coverage for C#/.NET code
• New C#/.NET production code lacks tests and needs them
• An existing test suite is missing edge cases or error-path coverage
• Working in a C# project that uses xUnit, FakeItEasy, AwesomeAssertions/FluentAssertions, NUnit, MSTest, or Moq

Do not use this skill for non-.NET test code, or for integration tests that primarily exercise external systems without unit-level concerns.

Conventions

• Test Framework: xUnit
• Mocking: FakeItEasy
• Assertions: AwesomeAssertions (a fork of FluentAssertions with identical API — use Should() as usual)
• Structure: Each test method has Arrange/Act/Assert blocks, marked with comments
• Language: English for code, comments, and test names
• Style: The stack above is the default. If the project already uses a different stack (NUnit, MSTest, Moq, FluentAssertions, …), match the existing convention instead of switching.

Phase 1: Write Tests

1. Analyze code: Read the code to be tested and understand:

• Public API (methods, properties)
• Dependencies (what needs to be mocked?)
• Different code paths (if/else, switch, exceptions)
• Edge Cases (null, empty collections, boundary values)

2. Identify test project: Find the appropriate test project by looking for *.Tests.csproj files or *Tests/ directories in the solution. Fall back to scanning for any project whose name ends in .Tests or .Test. Orient yourself to the existing project structure before adding files.

3. Create tests: Write tests following this pattern:

public class MyClassTests
{
    [Fact]
    public void MethodName_Scenario_ExpectedBehavior()
    {
        // Arrange
        var dependency = A.Fake<IDependency>();
        A.CallTo(() => dependency.DoSomething()).Returns(expectedValue);
        var sut = new MyClass(dependency);

        // Act
        var result = sut.MethodUnderTest(input);

        // Assert
        result.Should().Be(expectedValue);
    }

    [Theory]
    [InlineData("input1", "expected1")]
    [InlineData("input2", "expected2")]
    public void MethodName_WithVariousInputs_ReturnsExpected(string input, string expected)
    {
        // Arrange
        var sut = new MyClass();

        // Act
        var result = sut.MethodUnderTest(input);

        // Assert
        result.Should().Be(expected);
    }
}

4. Cover test categories:

• Happy Path (normal success case)
• Error handling (exceptions, invalid inputs)
• Null/empty inputs
• Boundary values (boundary conditions)
• Dependency behavior (mocks, different return values)

Phase 2: Execute Tests

1. Run dotnet test in the relevant test project
2. Analyze the results:

• On failures: Identify the cause and fix the test or test setup
• On success: Continue to Phase 3

3. Repeat until all tests are green. Don't fakely pass tests. If a test is too complex to set up, consider if it should be refactored or if the code under test should be made more testable.

Phase 3: Identify Missing Test Cases

Start a separate agent that reads the production code and written tests, then returns a prioritized list of missing cases:

• Edge Cases: Null, empty strings/collections, boundary/maximum values
• Error Paths: Exceptions, timeouts, dependency errors
• Boundary Conditions: Off-by-one, integer overflow
• Interactions: Call order, multiple/concurrent calls
• State Transitions: Different initial states

Format: [HIGH/MEDIUM/LOW] MethodName - Scenario: Description

Phase 4: Add Missing Tests

1. Implement the identified missing tests (prioritized: HIGH → MEDIUM → LOW)
2. Run dotnet test again
3. Fix any errors
4. Ensure all tests are green

Output Format

At the end, provide a summary:

### Test Result

**Phase 1**: X tests written
**Phase 2**: All tests green ✅
**Phase 3**: Y missing cases identified (Z High, W Medium, V Low)
**Phase 4**: Y additional tests implemented, all green ✅

**Total**: X + Y tests, all passed

Important Notes

• Do not write tests for trivial getters/setters without logic
• Do not write tests to check if properties are initialized correctly after construction
• Do not mock value types or simple DTOs – create real instances
• Test behavior, not implementation details
• Use descriptive test names in the format MethodName_Scenario_ExpectedBehavior
• For [Theory] tests: Use [InlineData] for simple types, [MemberData] for complex objects
• Use A.CallTo(...).MustHaveHappened() sparingly – only when the call is the expected behavior

Related Skills

• dotnet-fundamentals — Test DI-registered services using IServiceCollection overrides
• dotnet-ef-core — DbContext-backed unit and integration tests (SQLite in-memory, Testcontainers)
• dotnet-reviewer — Test-quality checks during code review
• dotnet-sdk-builder — Invoked by it in Step 9 to generate tests for new SDK libraries
• dotnet-aspnet — Unit and integration tests for ASP.NET Core endpoints