Mocking HttpClient in unit tests

HttpClient's extensibility lies in the HttpMessageHandler passed to the constructor. Its intent is to allow platform specific implementations, but you can also mock it. There's no need to create a decorator wrapper for HttpClient.

If you'd prefer a DSL to using Moq, I have a library up on GitHub/Nuget that makes things a little easier: https://github.com/richardszalay/mockhttp

var mockHttp = new MockHttpMessageHandler();

// Setup a respond for the user api (including a wildcard in the URL)
mockHttp.When("http://localost/api/user/*")
        .Respond("application/json", "{'name' : 'Test McGee'}"); // Respond with JSON

// Inject the handler or client into your application code
var client = new HttpClient(mockHttp);

var response = await client.GetAsync("http://localhost/api/user/1234");
// or without async: var response = client.GetAsync("http://localhost/api/user/1234").Result;

var json = await response.Content.ReadAsStringAsync();

// No network connection required
Console.Write(json); // {'name' : 'Test McGee'}

I agree with some of the other answers that the best approach is to mock the HttpMessageHandler inside HttpClient, rather than wrap HttpClient. This answer is unique in that it still injects HttpClient, allowing it to be a singleton or managed with dependency injection.

HttpClient is intended to be instantiated once and re-used throughout the life of an application.

(Source).

Mocking HttpMessageHandler can be a little tricky because SendAsync is protected. Here's a complete example, using xunit and Moq.

using System;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using Moq;
using Moq.Protected;
using Xunit;
// Use nuget to install xunit and Moq

namespace MockHttpClient {
    class Program {
        static void Main(string[] args) {
            var analyzer = new SiteAnalyzer(Client);
            var size = analyzer.GetContentSize("http://microsoft.com").Result;
            Console.WriteLine($"Size: {size}");
        }

        private static readonly HttpClient Client = new HttpClient(); // Singleton
    }

    public class SiteAnalyzer {
        public SiteAnalyzer(HttpClient httpClient) {
            _httpClient = httpClient;
        }

        public async Task<int> GetContentSize(string uri)
        {
            var response = await _httpClient.GetAsync( uri );
            var content = await response.Content.ReadAsStringAsync();
            return content.Length;
        }

        private readonly HttpClient _httpClient;
    }

    public class SiteAnalyzerTests {
        [Fact]
        public async void GetContentSizeReturnsCorrectLength() {
            // Arrange
            const string testContent = "test content";
            var mockMessageHandler = new Mock<HttpMessageHandler>();
            mockMessageHandler.Protected()
                .Setup<Task<HttpResponseMessage>>("SendAsync", ItExpr.IsAny<HttpRequestMessage>(), ItExpr.IsAny<CancellationToken>())
                .ReturnsAsync(new HttpResponseMessage {
                    StatusCode = HttpStatusCode.OK,
                    Content = new StringContent(testContent)
                });
            var underTest = new SiteAnalyzer(new HttpClient(mockMessageHandler.Object));

            // Act
            var result = await underTest.GetContentSize("http://anyurl");

            // Assert
            Assert.Equal(testContent.Length, result);
        }
    }
}

Mocking HttpClient in unit tests

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