Source: kevintsengtw/dotnet-testing-agent-skills (MIT). Ported into dotnet-agent-harness.

.NET Unit Test Fundamentals Guide

Applicable Scenarios

Use this skill when asked to perform the following tasks:

• Create unit tests for .NET classes or methods
• Review or improve existing test quality
• Design test cases following FIRST principles
• Explain test naming conventions and best practices
• Write tests using xUnit

FIRST Principles

Every unit test must conform to the following principles:

F - Fast

Test execution time should be in milliseconds, not dependent on external resources.

[Fact] // Fast: no external dependencies, executes quickly
public void Add_WithInput1And2_ShouldReturn3()
{
    // Pure memory operations, no I/O or network latency
    var calculator = new Calculator();
    var result = calculator.Add(1, 2);
    Assert.Equal(3, result);
}
```text

### I - Independent

Tests should not have dependencies, each test creates a new instance.

```csharp
[Fact] // Independent: each test creates a new instance
public void Increment_StartingFrom0_ShouldReturn1()
{
    var counter = new Counter(); // Each test creates a new instance, not affected by other tests
    counter.Increment();
    Assert.Equal(1, counter.Value);
}
```text

### R - Repeatable

Should produce the same results in any environment, not dependent on external state.

```csharp
[Fact] // Repeatable: same results every execution
public void Increment_ExecutedMultipleTimes_ShouldProduceConsistentResults()
{
    var counter = new Counter();
    counter.Increment();
    counter.Increment();
    counter.Increment();

    // This test produces the same result every execution
    Assert.Equal(3, counter.Value);
}
```text

### S - Self-Validating

Test results should be clearly pass or fail, using clear assertions.

```csharp
[Fact] // Self-Validating: clear validation
public void IsValidEmail_WithValidInput_ShouldReturnTrue()
{
    var emailHelper = new EmailHelper();
    var result = emailHelper.IsValidEmail("test@example.com");

    Assert.True(result); // Clear pass or fail
}
```text

### T - Timely

Tests should be written before or simultaneously with production code, ensuring code testability.

## 3A Pattern Structure

Every test method **must** follow the Arrange-Act-Assert pattern:

```csharp
[Fact]
public void Add_WithNegativeAndPositiveNumbers_ShouldReturnCorrectResult()
{
    // Arrange - prepare test data and dependencies
    var calculator = new Calculator();
    const int a = -5;
    const int b = 3;
    const int expected = -2;

    // Act - execute the method under test
    var result = calculator.Add(a, b);

    // Assert - verify results match expectations
    Assert.Equal(expected, result);
}
```text

### Block Responsibilities

| Block        | Responsibility                           | Notes                            |
| ----------- | ------------------------------ | ----------------------------------- |
| **Arrange** | Prepare objects, data, and Mocks needed for testing | Use `const` to declare constant values, improve readability |
| **Act**     | Execute the method under test               | Usually only one line, calling the method under test          |
| **Assert**  | Verify results                       | Each test only validates one behavior              |

## Test Naming Conventions

Use the following format to name test methods:

```text
[MethodUnderTest]_[TestScenario]_[ExpectedBehavior]
```text

### Naming Examples

| Method Name                                       | Description         |
| ---------------------------------------------- | ------------ |
| `Add_WithInput1And2_ShouldReturn3`                         | Test normal input |
| `Add_WithNegativeAndPositiveNumbers_ShouldReturnCorrectResult`            | Test boundary conditions |
| `Divide_WithInput10And0_ShouldThrowDivideByZeroException` | Test exception case |
| `IsValidEmail_WithNullInput_ShouldReturnFalse`          | Test invalid input |
| `GetDomain_WithValidEmail_ShouldReturnDomainName`       | Test return value   |

> 💡 **Tip**: Using Chinese naming can make test reports more readable, especially during team communication.

## xUnit Test Attributes

### [Fact] - Single Test Case

Used for testing a single scenario:

```csharp
[Fact]
public void Add_WithInput0And0_ShouldReturn0()
{
    var calculator = new Calculator();
    var result = calculator.Add(0, 0);
    Assert.Equal(0, result);
}
```text

### [Theory] + [InlineData] - Parameterized Tests

Used for testing multiple input combinations:

```csharp
[Theory]
[InlineData(1, 2, 3)]
[InlineData(-1, 1, 0)]
[InlineData(0, 0, 0)]
[InlineData(100, -50, 50)]
public void Add_WithVariousNumberCombinations_ShouldReturnCorrectResult(int a, int b, int expected)
{
    var calculator = new Calculator();
    var result = calculator.Add(a, b);
    Assert.Equal(expected, result);
}
```text

### Testing Multiple Invalid Inputs

```csharp
[Theory]
[InlineData("invalid-email")]
[InlineData("@example.com")]
[InlineData("test@")]
[InlineData("test.example.com")]
public void IsValidEmail_WithInvalidEmailFormats_ShouldReturnFalse(string invalidEmail)
{
    var emailHelper = new EmailHelper();
    var result = emailHelper.IsValidEmail(invalidEmail);
    Assert.False(result);
}
```text

## Exception Testing

Test expected exception throwing scenarios:

```csharp
[Fact]
public void Divide_WithInput10And0_ShouldThrowDivideByZeroException()
{
    // Arrange
    var calculator = new Calculator();
    const decimal dividend = 10m;
    const decimal divisor = 0m;

    // Act & Assert
    var exception = Assert.Throws<DivideByZeroException>(
        () => calculator.Divide(dividend, divisor)
    );

    // Verify exception message
    Assert.Equal("Divisor cannot be zero", exception.Message);
}
```text

## Test Project Structure

Recommended project structure:

```text
Solution/
├── src/
│   └── MyProject/
│       ├── Calculator.cs
│       └── MyProject.csproj
└── tests/
    └── MyProject.Tests/
        ├── CalculatorTests.cs
        └── MyProject.Tests.csproj
```text

## Test Project Template (.csproj)

```xml
<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <TargetFramework>net9.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <IsPackable>false</IsPackable>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="coverlet.collector" Version="6.0.4">
            <PrivateAssets>all</PrivateAssets>
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
        </PackageReference>
        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
        <PackageReference Include="xunit" Version="2.9.3" />
        <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
            <PrivateAssets>all</PrivateAssets>
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
        </PackageReference>
    </ItemGroup>

    <ItemGroup>
        <Using Include="Xunit" />
    </ItemGroup>

    <ItemGroup>
        <ProjectReference Include="..\..\src\MyProject\MyProject.csproj" />
    </ItemGroup>

</Project>
```text

## Common Assertion Methods

| Assertion Method                            | Purpose             |
| ----------------------------------- | ---------------- |
| `Assert.Equal(expected, actual)`    | Verify equality         |
| `Assert.NotEqual(expected, actual)` | Verify inequality       |
| `Assert.True(condition)`            | Verify condition is true     |
| `Assert.False(condition)`           | Verify condition is false     |
| `Assert.Null(object)`               | Verify is null      |
| `Assert.NotNull(object)`            | Verify is not null    |
| `Assert.Throws<T>(action)`          | Verify throws specific exception |
| `Assert.Empty(collection)`          | Verify collection is empty     |
| `Assert.Contains(item, collection)` | Verify collection contains item |

## Test Generation Checklist

When generating tests for a method, ensure coverage of:

- [ ] **Happy Path** - Standard input produces expected output
- [ ] **Boundary Conditions** - Minimum, maximum values, zero, empty strings
- [ ] **Invalid Input** - null, negative numbers, wrong formats
- [ ] **Exception Cases** - Scenarios expected to throw exceptions

## Reference Resources

### Original Articles

This skill content is extracted from "Old School Software Engineer's Testing Practice - 30 Day Challenge" series:

- **Day 01 - Old School Engineer's Testing Enlightenment**
  - Ironman Article: https://ithelp.ithome.com.tw/articles/10373888
  - Sample Code: https://github.com/kevintsengtw/30Days_in_Testing_Samples/tree/main/day01