FakeHttpMessageHandler

FakeHttpMessageHandler seamlessly fakes http request sent from System.Net.Http.HttpClient for purpose of unit testing code that previously could only be integration tested.

Examples

If FakeHttpMessageHandler is initialized with no parameters, it will create an instance of T, serialize it as JSON and return it in a content of HttpResponse.

using(var httpMessageHandler = new FakeHttpMessageHandler<OutputType>())
using(var httpClient = new HttpClient(httpMessageHandler))
{
    var response = await httpClient.GetAsync("http://zheludov.com");
}

You may also choose to override returned object by either passing an instance of returned object to a constructor or a function that will retrieve it. In both cases an object will be serialized into JSON unless serializer is overwritten or T is string;

// Passing an instance of output object
var output = new OutputType();
using(var httpMessageHandler = new FakeHttpMessageHandler<OutputType>(output))
using(var httpClient = new HttpClient(httpMessageHandler))
{
    var response = await httpClient.GetAsync("http://zheludov.com");
}


// Passing a function that will retrieve output object.
var outputRetrieverMock = new Mock<IResponceObjectRetriever>();
outputRetrieverMock.Setup(retriever => retriever.RetrieveResponseObject()).Returns(new FakeOutput());

using (var httpMessageHandler = new FakeHttpMessageHandler<FakeOutput>(() => outputRetrieverMock.Object.RetrieveResponseObject()))
using (var httpClient = new HttpClient(httpMessageHandler))
{
    var response = await httpClient.GetAsync("http://zheludov.com");
}


// Passing an asynchronous function that will retrieve output object.
var outputRetrieverMock = new Mock<IResponceObjectRetriever>();
outputRetrieverMock.Setup(retriever => retriever.RetrieveResponseObjectAsync()).ReturnsAsync(new FakeOutput());

using (var httpMessageHandler = new FakeHttpMessageHandler<FakeOutput>(() => outputRetrieverMock.Object.RetrieveResponseObject()))
using (var httpClient = new HttpClient(httpMessageHandler))
{
    var response = await httpClient.GetAsync("http://zheludov.com");
}

Additionally you can override serializer function that is called to serilize response content.

var responseString = "zheludov.com";
var serializerMock = new Mock<ISerializer>();
var responseObjectRetrieverMock = new Mock<IResponceObjectRetriever>();
serializerMock.Setup(serializer => serializer.Serialize(It.IsAny<object>())).Returns(responseString);

var handler = new FakeHttpMessageHandler<FakeOutput>(() => responseObjectRetrieverMock.Object.RetrieveResponseObjectAsync(), 
    (FakeOutput output) => serializerMock.Object.Serialize(output));
using (var httpClient = new HttpClient(httpMessageHandler))
{
    var response = await httpClient.GetAsync("http://zheludov.com");
}

Sample Class and Unit Test

ApiClient.cs

public class ApiClient : IDisposable
{
    private readonly HttpMessageHandler _httpMessageHandler;
    public ApiClient(HttpMessageHandler httpMessageHandler = null)
    {
        _httpMessageHandler = httpMessageHandler;
    }

    public async Task<string> GetResponseAsync()
    {
        using (var httpClient = new HttpClient(_httpMessageHandler ?? new HttpClientHandler()))
        {
            var response = await httpClient.GetAsync("http://zheludov.com");
            if(response.IsSuccessStatusCode)
            {
                return await response.Content.ReadAsStringAsync();
            }
            return null;
        }
    }

    public void Dispose()
    {
        _httpMessageHandler?.Dispose();
    }
}

ApiClientTest.cs

[TestClass]
public class ApiClientTest
{
    [TestMethod]
    public async Task ApiClient_GetResponseAsync_VerifyHttpRequestIsMade()
    {
        var httpHandler = new FakeHttpMessageHandler<string>("Response");
        using (var client = new ApiClient(httpHandler))
        {
            var response = await client.GetResponseAsync();
            response.Should().NotBeNullOrWhiteSpace();
        }    
    }
}