Skill: Writing C# Unit Tests

Overview

This skill covers writing unit tests for the backend using MSTest SDK. The project uses a lean, zero-config approach that leverages Microsoft's official SDK-style testing.

Project Structure

backend/
├── WebApp.sln
├── WebApp.Api/
│   ├── Models/           # DTOs and request/response models
│   ├── Services/         # Business logic and agent integration
│   └── Program.cs        # Minimal API endpoints
└── WebApp.Api.Tests/
    ├── WebApp.Api.Tests.csproj
    └── [TestClass].cs    # Test files organized by class under test

Test Project Configuration

The test project uses MSTest SDK which eliminates the need for explicit package references:

See backend/WebApp.Api.Tests/WebApp.Api.Tests.csproj for current configuration. The project uses MSTest SDK which eliminates the need for explicit package references — just set Sdk="MSTest.Sdk/{version}" in the project element.

Test Anatomy

using WebApp.Api.Models;

namespace WebApp.Api.Tests;

[TestClass]
public class ErrorResponseFactoryTests
{
    [TestMethod]
    public void CreateFromException_ReturnsCorrectStatusCode()
    {
        // Arrange
        var exception = new InvalidOperationException("Test");
        
        // Act
        var result = ErrorResponseFactory.CreateFromException(exception, 500, isDevelopment: false);
        
        // Assert
        Assert.AreEqual(500, result.Status);
    }
    
    [TestMethod]
    [DataRow(400, "Bad Request")]
    [DataRow(401, "Unauthorized")]
    [DataRow(500, "Internal Server Error")]
    public void CreateFromException_MapsStatusToTitle(int status, string expectedTitle)
    {
        // Parameterized test using DataRow
    }
}

Running Tests

# Run all backend tests
cd backend
dotnet test

# Run with verbose output
dotnet test --verbosity normal

# Run specific test class
dotnet test --filter "FullyQualifiedName~ErrorResponseFactoryTests"

# Run with coverage (requires coverlet)
dotnet test --collect:"XPlat Code Coverage"

Testable Units in This Project

Models (Pure, Easy to Test)

Services (Require Integration Testing)

Test Naming Convention

Use descriptive names following: MethodName_Scenario_ExpectedBehavior

[TestMethod]
public void CreateFromException_WhenDevelopmentMode_IncludesStackTrace() { }

[TestMethod]
public void CreateFromException_WhenProductionMode_HidesStackTrace() { }

Assertions Reference

MSTest provides these assertion methods:

// Equality
Assert.AreEqual(expected, actual);
Assert.AreNotEqual(notExpected, actual);

// Nullability
Assert.IsNull(value);
Assert.IsNotNull(value);

// Boolean
Assert.IsTrue(condition);
Assert.IsFalse(condition);

// Type
Assert.IsInstanceOfType(obj, typeof(ExpectedType));

// Collections
CollectionAssert.Contains(collection, element);
CollectionAssert.AreEqual(expected, actual);

// Exceptions
Assert.ThrowsException<InvalidOperationException>(() => MethodThatThrows());

When Unit Tests Aren't Enough

Use the validating-ui-features skill and Playwright when:

• Testing requires a running backend server
• Testing SSE streaming behavior
• Testing authentication flows
• Testing the full chat request/response cycle

Quick Reference