---
name: csharp-testing-74e63cf3ffcd
description: C# and .NET testing patterns with xUnit, FluentAssertions, mocking, integration tests, and test organization best practices.
origin: ecc
---

# C# Testing Patterns

Comprehensive testing patterns for .NET applications using xUnit, FluentAssertions, and modern testing practices.

## When to Activate

- Writing new tests for C# code
- Reviewing test quality and coverage
- Setting up test infrastructure for .NET projects
- Debugging flaky or slow tests

## Test Framework Stack

| Tool | Purpose |
|---|---|
| **xUnit** | Test framework (preferred for .NET) |
| **FluentAssertions** | Readable assertion syntax |
| **NSubstitute** or **Moq** | Mocking dependencies |
| **Testcontainers** | Real infrastructure in integration tests |
| **WebApplicationFactory** | ASP.NET Core integration tests |
| **Bogus** | Realistic test data generation |

## Unit Test Structure

### Arrange-Act-Assert

```csharp
public sealed class OrderServiceTests
{
    private readonly IOrderRepository _repository = Substitute.For<IOrderRepository>();
    private readonly ILogger<OrderService> _logger = Substitute.For<ILogger<OrderService>>();
    private readonly OrderService _sut;

    public OrderServiceTests()
    {
        _sut = new OrderService(_repository, _logger);
    }

    [Fact]
    public async Task PlaceOrderAsync_ReturnsSuccess_WhenRequestIsValid()
    {
        // Arrange
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-123",
            Items = [new OrderItem("SKU-001", 2, 29.99m)]
        };

        // Act
        var result = await _sut.PlaceOrderAsync(request, CancellationToken.None);

        // Assert
        result.IsSuccess.Should().BeTrue();
        result.Value.Should().NotBeNull();
        result.Value!.CustomerId.Should().Be("cust-123");
    }

    [Fact]
    public async Task PlaceOrderAsync_ReturnsFailure_WhenNoItems()
    {
        // Arrange
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-123",
            Items = []
        };

        // Act
        var result = await _sut.PlaceOrderAsync(request, CancellationToken.None);

        // Assert
        result.IsSuccess.Should().BeFalse();
        result.Error.Should().Contain("at least one item");
    }
}
```

### Parameterized Tests with Theory

```csharp
[Theory]
[InlineData("", false)]
[InlineData("a", false)]
[InlineData("ab@c.d", false)]
[InlineData("user@example.com", true)]
[InlineData("user+tag@example.co.uk", true)]
public void IsValidEmail_ReturnsExpected(string email, bool expected)
{
    EmailValidator.IsValid(email).Should().Be(expected);
}

[Theory]
[MemberData(nameof(InvalidOrderCases))]
public async Task PlaceOrderAsync_RejectsInvalidOrders(CreateOrderRequest request, string expectedError)
{
    var result = await _sut.PlaceOrderAsync(request, CancellationToken.None);

    result.IsSuccess.Should().BeFalse();
    result.Error.Should().Contain(expectedError);
}

public static TheoryData<CreateOrderRequest, string> InvalidOrderCases => new()
{
    { new() { CustomerId = "", Items = [ValidItem()] }, "CustomerId" },
    { new() { CustomerId = "c1", Items = [] }, "at least one item" },
    { new() { CustomerId = "c1", Items = [new("", 1, 10m)] }, "SKU" },
};
```

## Mocking with NSubstitute

```csharp
[Fact]
public async Task GetOrderAsync_ReturnsNull_WhenNotFound()
{
    // Arrange
    var orderId = Guid.NewGuid();
    _repository.FindByIdAsync(orderId, Arg.Any<CancellationToken>())
        .Returns((Order?)null);

    // Act
    var result = await _sut.GetOrderAsync(orderId, CancellationToken.None);

    // Assert
    result.Should().BeNull();
}

[Fact]
public async Task PlaceOrderAsync_PersistsOrder()
{
    // Arrange
    var request = ValidOrderRequest();

    // Act
    await _sut.PlaceOrderAsync(request, CancellationToken.None);

    // Assert — verify the repository was called
    await _repository.Received(1).AddAsync(
        Arg.Is<Order>(o => o.CustomerId == request.CustomerId),
        Arg.Any<CancellationToken>());
}
```

## ASP.NET Core Integration Tests

### WebApplicationFactory Setup

```csharp
public sealed class OrderApiTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly HttpClient _client;

    public OrderApiTests(WebApplicationFactory<Program> factory)
    {
        _client = factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureServices(services =>
            {
                // Replace real DB with in-memory for tests
                services.RemoveAll<DbContextOptions<AppDbContext>>();
                services.AddDbContext<AppDbContext>(options =>
                    options.UseInMemoryDatabase("TestDb"));
            });
        }).CreateClient();
    }

    [Fact]
    public async Task GetOrder_Returns404_WhenNotFound()
    {
        var response = await _client.GetAsync($"/api/orders/{Guid.NewGuid()}");

        response.StatusCode.Should().Be(HttpStatusCode.NotFound);
    }

    [Fact]
    public async Task CreateOrder_Returns201_WithValidRequest()
    {
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-1",
            Items = [new("SKU-001", 1, 19.99m)]
        };

        var response = await _client.PostAsJsonAsync("/api/orders", request);

        response.StatusCode.Should().Be(HttpStatusCode.Created);
        response.Headers.Location.Should().NotBeNull();
    }
}
```

### Testing with Testcontainers

```csharp
public sealed class PostgresOrderRepositoryTests : IAsyncLifetime
{
    private readonly PostgreSqlContainer _postgres = new PostgreSqlBuilder()
        .WithImage("postgres:16-alpine")
        .Build();

    private AppDbContext _db = null!;

    public async Task InitializeAsync()
    {
        await _postgres.StartAsync();
        var options = new DbContextOptionsBuilder<AppDbContext>()
            .UseNpgsql(_postgres.GetConnectionString())
            .Options;
        _db = new AppDbContext(options);
        await _db.Database.MigrateAsync();
    }

    public async Task DisposeAsync()
    {
        await _db.DisposeAsync();
        await _postgres.DisposeAsync();
    }

    [Fact]
    public async Task AddAsync_PersistsOrder()
    {
        var repo = new SqlOrderRepository(_db);
        var order = Order.Create("cust-1", [new OrderItem("SKU-001", 2, 10m)]);

        await repo.AddAsync(order, CancellationToken.None);

        var found = await repo.FindByIdAsync(order.Id, CancellationToken.None);
        found.Should().NotBeNull();
        found!.Items.Should().HaveCount(1);
    }
}
```

## Test Organization

```
tests/
  MyApp.UnitTests/
    Services/
      OrderServiceTests.cs
      PaymentServiceTests.cs
    Validators/
      EmailValidatorTests.cs
  MyApp.IntegrationTests/
    Api/
      OrderApiTests.cs
    Repositories/
      OrderRepositoryTests.cs
  MyApp.TestHelpers/
    Builders/
      OrderBuilder.cs
    Fixtures/
      DatabaseFixture.cs
```

## Test Data Builders

```csharp
public sealed class OrderBuilder
{
    private string _customerId = "cust-default";
    private readonly List<OrderItem> _items = [new("SKU-001", 1, 10m)];

    public OrderBuilder WithCustomer(string customerId)
    {
        _customerId = customerId;
        return this;
    }

    public OrderBuilder WithItem(string sku, int quantity, decimal price)
    {
        _items.Add(new OrderItem(sku, quantity, price));
        return this;
    }

    public Order Build() => Order.Create(_customerId, _items);
}

// Usage in tests
var order = new OrderBuilder()
    .WithCustomer("cust-vip")
    .WithItem("SKU-PREMIUM", 3, 99.99m)
    .Build();
```

## Common Anti-Patterns

| Anti-Pattern | Fix |
|---|---|
| Testing implementation details | Test behavior and outcomes |
| Shared mutable test state | Fresh instance per test (xUnit does this via constructors) |
| `Thread.Sleep` in async tests | Use `Task.Delay` with timeout, or polling helpers |
| Asserting on `ToString()` output | Assert on typed properties |
| One giant assertion per test | One logical assertion per test |
| Test names describing implementation | Name by behavior: `Method_ExpectedResult_WhenCondition` |
| Ignoring `CancellationToken` | Always pass and verify cancellation |

## Running Tests

```bash
# Run all tests
dotnet test

# Run with coverage
dotnet test --collect:"XPlat Code Coverage"

# Run specific project
dotnet test tests/MyApp.UnitTests/

# Filter by test name
dotnet test --filter "FullyQualifiedName~OrderService"

# Watch mode during development
dotnet watch test --project tests/MyApp.UnitTests/
```


---
Source attribution: mturac
Original project: https://github.com/mturac/everything-openai-codex/blob/b5057da5f42ed6b12cd3a59e89af0ccd12cff7c2/skills/csharp-testing/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Dave AI.

## Upstream license notice

MIT License

Copyright (c) 2026 Mehmet Turac

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.


## Additional upstream notices

csharp-testing-74e63cf3ffcd/DAVEAI-UPSTREAM-NOTICE.md

# Notices and Acknowledgements

Everything OpenAI Codex is a Codex-focused adaptation inspired by the public
[`affaan-m/everything-claude-code`](https://github.com/affaan-m/everything-claude-code)
project.

Respect to Affaan Mustafa and the contributors to that project for making the
multi-harness agent operating-system pattern visible in the open.

This repository adapts the public idea and workflow surface for OpenAI Codex:
names, defaults, install targets, docs, validators, and cross-harness guidance
are maintained here for the Codex-oriented project.


# Dave AI resource metadata

Creator / publisher: mturac
Original project: https://github.com/mturac/everything-openai-codex/blob/b5057da5f42ed6b12cd3a59e89af0ccd12cff7c2/skills/csharp-testing/SKILL.md
Attribution basis: Source repository record

Fields of work: Development & Engineering, Product Management
Roles: Backend Developer, Product Manager, QA / Test Engineer
Specialties: Backend, Testing
Tools: Bogus, Fluentassertions, Moq, Nsubstitute, Testcontainers, Webapplicationfactory, Xunit
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: 7ab8baff27589f097129d7af6f0fbcccf8a49efa9490d3467a499ed8fe1ef93b