githubcopilot-csharp-tunit

name	githubcopilot-csharp-tunit
description	Get best practices for TUnit unit testing, including data-driven tests

TUnit Best Practices

Your goal is to help me write effective unit tests with TUnit, covering both standard and data-driven testing approaches.

Project Setup

• Use a separate test project with naming convention [ProjectName].Tests
• Reference TUnit package and TUnit.Assertions for fluent assertions
• Create test classes that match the classes being tested (e.g., CalculatorTests for Calculator)
• Use .NET SDK test commands: dotnet test for running tests
• TUnit requires .NET 8.0 or higher

Test Structure

• No test class attributes required (like xUnit/NUnit)
• Use [Test] attribute for test methods (not [Fact] like xUnit)
• Follow the Arrange-Act-Assert (AAA) pattern
• Name tests using the pattern MethodName_Scenario_ExpectedBehavior
• Use lifecycle hooks: [Before(Test)] for setup and [After(Test)] for teardown
• Use [Before(Class)] and [After(Class)] for shared context between tests in a class
• Use [Before(Assembly)] and [After(Assembly)] for shared context across test classes
• TUnit supports advanced lifecycle hooks like [Before(TestSession)] and [After(TestSession)]

Standard Tests

• Keep tests focused on a single behavior
• Avoid testing multiple behaviors in one test method
• Use TUnit's fluent assertion syntax with await Assert.That()
• Include only the assertions needed to verify the test case
• Make tests independent and idempotent (can run in any order)
• Avoid test interdependencies (use [DependsOn] attribute if needed)

Data-Driven Tests

• Use [Arguments] attribute for inline test data (equivalent to xUnit's [InlineData])
• Use [MethodData] for method-based test data (equivalent to xUnit's [MemberData])
• Use [ClassData] for class-based test data
• Create custom data sources by implementing ITestDataSource
• Use meaningful parameter names in data-driven tests
• Multiple [Arguments] attributes can be applied to the same test method

Assertions

• Use await Assert.That(value).IsEqualTo(expected) for value equality
• Use await Assert.That(value).IsSameReferenceAs(expected) for reference equality
• Use await Assert.That(value).IsTrue() or await Assert.That(value).IsFalse() for boolean conditions
• Use await Assert.That(collection).Contains(item) or await Assert.That(collection).DoesNotContain(item) for collections
• Use await Assert.That(value).Matches(pattern) for regex pattern matching
• Use await Assert.That(action).Throws<TException>() or await Assert.That(asyncAction).ThrowsAsync<TException>() to test exceptions
• Chain assertions with .And operator: await Assert.That(value).IsNotNull().And.IsEqualTo(expected)
• Use .Or operator for alternative conditions: await Assert.That(value).IsEqualTo(1).Or.IsEqualTo(2)
• Use .Within(tolerance) for DateTime and numeric comparisons with tolerance
• All assertions are asynchronous and must be awaited

Advanced Features

• Use [Repeat(n)] to repeat tests multiple times
• Use [Retry(n)] for automatic retry on failure
• Use [ParallelLimit<T>] to control parallel execution limits
• Use [Skip("reason")] to skip tests conditionally
• Use [DependsOn(nameof(OtherTest))] to create test dependencies
• Use [Timeout(milliseconds)] to set test timeouts
• Create custom attributes by extending TUnit's base attributes

Test Organization

• Group tests by feature or component
• Use [Category("CategoryName")] for test categorization
• Use [DisplayName("Custom Test Name")] for custom test names
• Consider using TestContext for test diagnostics and information
• Use conditional attributes like custom [WindowsOnly] for platform-specific tests

Performance and Parallel Execution

• TUnit runs tests in parallel by default (unlike xUnit which requires explicit configuration)
• Use [NotInParallel] to disable parallel execution for specific tests
• Use [ParallelLimit<T>] with custom limit classes to control concurrency
• Tests within the same class run sequentially by default
• Use [Repeat(n)] with [ParallelLimit<T>] for load testing scenarios

Migration from xUnit

• Replace [Fact] with [Test]
• Replace [Theory] with [Test] and use [Arguments] for data
• Replace [InlineData] with [Arguments]
• Replace [MemberData] with [MethodData]
• Replace Assert.Equal with await Assert.That(actual).IsEqualTo(expected)
• Replace Assert.True with await Assert.That(condition).IsTrue()
• Replace Assert.Throws<T> with await Assert.That(action).Throws<T>()
• Replace constructor/IDisposable with [Before(Test)]/[After(Test)]
• Replace IClassFixture<T> with [Before(Class)]/[After(Class)]

Why TUnit over xUnit?

TUnit offers a modern, fast, and flexible testing experience with advanced features not present in xUnit, such as asynchronous assertions, more refined lifecycle hooks, and improved data-driven testing capabilities. TUnit's fluent assertions provide clearer and more expressive test validation, making it especially suitable for complex .NET projects.