DivertR

DivertR is a .NET proxy framework that can be used to create test doubles. It is similar to existing mocking frameworks like Moq but provides additional features for integration testing of wired-up systems.

Installing

Install DivertR as a NuGet package:

Install-Package DivertR

Or via the .NET command line interface:

dotnet add package DivertR

Example Usage

DivertR works by decorating dependency injection services with proxies that behave the same as the originals but that can be manipulated and reset dynamically. This facilitate a style of testing where you start with the integrated, wired-up system and only mock out specific parts required per test.

For example, it can significantly speed up integration tests running against a WebApplicationFactory (TestServer) instance by altering and mocking dependencies between tests without requiring reinitialisation like this:

[Fact]
public async Task GivenBookExists_WhenGetBookById_ThenReturnsBook()
{
    // ARRANGE
    var mockedBook = new Book { Id = Guid.NewGuid(), Name = "Test Book" };
    
    // Configure IBookService to return a mocked result
    _diverter
        .Redirect<IBookService>() // Redirect IFooService calls 
        .To(x => x.GetBookAsync(mockedBook.Id)) // matching this method and argument value
        .Via(() => Task.FromResult(mockedBook)); // via this delegate
    
    // ACT
    var bookResult = await _httpClient.GetFromJsonAsync<Book>($"/books/{mockedBook.Id}");
    
    // ASSERT
    Assert.NotNull(bookResult);
    Assert.Equal(mockedBook.Id, bookResult.Id);
    Assert.Equal(mockedBook.Name, bookResult.Name);
}

[Fact]
public async Task GivenBookServiceError_WhenGetBookById_ThenReturns500InternalServerError()
{
    // ARRANGE
    var bookId = Guid.NewGuid();
    
    // Configure IBookService to throw an exception
    _diverter
        .Redirect<IBookService>()
        .To(x => x.GetBookAsync(bookId)) // match on bookId value only
        .Via(() => throw new Exception("Test"));
    
    // ACT
    var controlResponse = await _httpClient.GetAsync($"/books/{Guid.NewGuid()}");
    var testResponse = await _httpClient.GetAsync($"/books/{bookId}");
    
    
    // ASSERT
    Assert.Equal(HttpStatusCode.NotFound, controlResponse.StatusCode);
    Assert.Equal(HttpStatusCode.InternalServerError, testResponse.StatusCode);
}