write-unit-tests

Write Unit Tests

Guide for writing effective unit tests that verify behavior and catch regressions.

When to Use

• Testing pure logic and business rules
• Verifying individual components in isolation
• Testing edge cases and error conditions
• TDD red-green-refactor cycle

Test Structure (Arrange-Act-Assert)

[Fact]
public void MethodName_Scenario_ExpectedBehavior()
{
    // Arrange - Set up test data and dependencies
    var dependency = Substitute.For<IDependency>();
    dependency.GetData().Returns(expectedData);
    var sut = new SystemUnderTest(dependency);
    
    // Act - Execute the behavior being tested
    var result = sut.MethodUnderTest(input);
    
    // Assert - Verify expected outcomes
    result.Should().Be(expectedValue);
    dependency.Received(1).GetData();
}

Test Naming

Use descriptive names that explain what's being tested:

• MethodName_Scenario_ExpectedBehavior
• MethodName_WhenCondition_ShouldDoSomething
• MethodName_GivenInput_ReturnsExpectedOutput

Examples:

• Calculate_WithNegativeNumber_ThrowsArgumentException
• ProcessOrder_WhenInventoryInsufficient_ReturnsFailureResult
• FormatDate_GivenNull_ReturnsEmptyString

Test Coverage

1. Happy Path

• Normal, expected inputs
• Successful execution
• Expected outputs

2. Edge Cases

• Boundary values (min, max, zero, empty)
• Null inputs
• Empty collections
• Special characters

3. Error Scenarios

• Invalid inputs
• Exceptions
• Validation failures
• Constraint violations

Mocking & Isolation

• Use test doubles for dependencies
• Mock external services, databases, APIs
• Keep tests fast and deterministic
• Don't mock what you don't own (value objects, DTOs)

// Good - Mock interface
var repository = Substitute.For<IRepository>();

// Bad - Don't mock concrete classes or value objects
var dto = Substitute.For<DataTransferObject>(); // ❌

Assertions

Use fluent assertions for clarity:

// Good - Fluent and readable
result.Should().Be(expected);
result.Should().BeEquivalentTo(expected);
collection.Should().HaveCount(3);
action.Should().Throw<ArgumentException>()
    .WithMessage("*invalid*");

// Avoid - Less clear
Assert.Equal(expected, result);
Assert.True(result != null);