What is the process for adding mock services to WebApplicationFactory in C# using injection?

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Here are the steps to add mock services to WebApplicationFactory in C# using injection:

1. Create an interface for the service that you want to mock. For example, if you want to mock a UserService, create an IUserService interface.

2. Create a mock implementation of the interface. For example, create a MockUserService that implements the IUserService interface.

3. In your WebApplicationFactory, add the mock implementation to the service collection. This can be done in the ConfigureWebHost method using the ConfigureServices method.

4. Register the mock implementation with the IoC container using the AddScoped or AddSingleton method.

5. In your test class, inject the mock implementation into the system under test. You can do this by adding a constructor parameter that accepts the mock implementation and storing it in a private field.

6. Use the mock implementation to set up test scenarios and verify behavior.

Example code:

public interface IUserService
{
    User GetUser(int id);
}

public class MockUserService : IUserService
{
    public User GetUser(int id)
    {
        return new User { Id = id, Name = "Test User" };
    }
}

public class MyTests : IDisposable
{
    private readonly WebApplicationFactory<Startup> _factory;
    private readonly IUserService _userService;

    public MyTests()
    {
        _factory = new WebApplicationFactory<Startup>()
            .WithWebHostBuilder(builder =>
            {
                builder.ConfigureServices(services =>
                {
                    services.AddScoped<IUserService, MockUserService>();
                });
            });

        _userService = _factory.Services.GetRequiredService<IUserService>();
    }

    [Fact]
    public void GetUser_ReturnsCorrectUser()
    {
        // Arrange
        var userId = 1;

        // Act
        var user = _userService.GetUser(userId);

        // Assert
        Assert.NotNull(user);
        Assert.Equal(userId, user.Id);
        Assert.Equal("Test User", user.Name);
    }

    public void Dispose()
    {
        _factory.Dispose();
    }
}