TIL: How to write tests for HttpClient using Moq

I originally published this post on my blog. It's part of my personal C# Advent of Code.

These days I needed to unit test a service that used the built-in HttpClient. It wasn't as easy as creating a fake for HttpClient. This is how to write tests for HttpClient with Moq and a set of extension methods to make it easier.

To write tests for a service that requires a HttpClient, create a fake for HttpMessageHandler and set up the protected SendAsync() method to return a HttpResponseMessage. Then, create a new HttpClient passing the fake instance of HttpMessageHandler created before.

How to Create a Testable HttpClient

For example, let's write a test for a AnyService class that receives a HttpClient, using MSTest and Moq,

using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;
using Moq.Protected;
using Newtonsoft.Json;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace MyProject.Services.Tests;

[TestClass]
public class AnyServiceTests
{
    [TestMethod]
    public async Task DoSomethingAsync_ByDefault_ReturnsSomethingElse()
    {
        var fakeHttpMessageHandler = new Mock<HttpMessageHandler>();
        fakeHttpMessageHandler
                .Protected()
                // 👆👆👆
                .Setup<Task<HttpResponseMessage>>(
                    "SendAsync",
                    ItExpr.IsAny<HttpRequestMessage>(),
                    ItExpr.IsAny<CancellationToken>()
                )
                .ReturnsAsync(new HttpResponseMessage
                {
                    StatusCode = HttpStatusCode.OK,
                    Content = new StringContent(JsonConvert.SerializeObject(new AnyResponseViewModel()))
                    // We add the expected response here:                   👆👆👆
                });
        using var httpClient = new HttpClient(fakeHttpMessageHandler.Object);
        //                                    👆👆👆
        var service = new AnyService(client);

        var someResult = await service.DoSomethingAsync();

        // Assert something here...
        Assert.IsNotNull(someResult);
    }
}

Notice how we used the Protected() and Setup() methods from Moq to create a fake for HtttpMessageHandler. Then, inside the ReturnsAsync() method, we created a response message with a response object. And, finally, we used the fake handler to create a new HttpClient to pass it to our AnyService instance.

That's how we created a fake HttpClient. But, as soon as we start to write more tests, all of them get bloated with lots of duplicated code. Especially, if we create new tests by copy-pasting an existing one.

We should reduce the noise in our tests using factory methods or builders to make our tests more readable. Let's do that!

Some extensions methods to set up the faked HttpClient

It would be great if we could reduce the Arrange phase of our sample test to one or two lines. Something like this,

[TestMethod]
public async Task DoSomethingAsync_ByDefault_ReturnsSomethingElse()
{
    using var client = new Mock<HttpMessageHandler>()
                  .WithSuccessfulResponse(new AnyResponseViewModel())
                  //                      👆👆👆
                  // Alternatively,
                  // .WithUnauthorizedResponse()
                  // or
                  // .WithException<HttpRequestException>()
                  .ToHttpClient();
                  //  👆👆👆
    var service = new AnyService(client);

    var someResult = await service.DoSomethingAsync();

    // Assert something here...
    Assert.IsNotNull(someResult);
}

It's not that difficult to write some extension methods on top of the Mock<HttpMessageHandler> to simplify the creation of testable HttpClient instances.

In fact, here they are,

using Moq;
using Moq.Language.Flow;
using Moq.Protected;
using Newtonsoft.Json;
using System;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace HttpMessageHandlerTests.Extensions;

public static class MockHttpMessageHandlerExtensions
{
    public static Mock<HttpMessageHandler> WithSuccessfulResponse<T>(
        this Mock<HttpMessageHandler> fakeHttpMessageHandler,
        T responseContent)
    {
        fakeHttpMessageHandler
            .GetProtectedSetup()
            .ReturnsAsync(new HttpResponseMessage
            {
                StatusCode = HttpStatusCode.OK,
                Content = new StringContent(JsonConvert.SerializeObject(responseContent))
            });

        return fakeHttpMessageHandler;
    }

    public static Mock<HttpMessageHandler> WithUnauthorizedResponse(
        this Mock<HttpMessageHandler> fakeHttpMessageHandler)
    {
        fakeHttpMessageHandler
            .GetProtectedSetup()
            .ReturnsAsync(new HttpResponseMessage
            {
                StatusCode = HttpStatusCode.Unauthorized,
                RequestMessage = new HttpRequestMessage()
            });

        return fakeHttpMessageHandler;
    }

    public static Mock<HttpMessageHandler> WithDelegate(
        this Mock<HttpMessageHandler> fakeHttpMessageHandler,
        Func<HttpRequestMessage, CancellationToken, HttpResponseMessage> func)
    {
        fakeHttpMessageHandler
            .GetProtectedSetup()
            .ReturnsAsync(func);

        return fakeHttpMessageHandler;
    }

    public static Mock<HttpMessageHandler> WithException<TException>(
        this Mock<HttpMessageHandler> fakeHttpMessageHandler)
        where TException : Exception, new()
    {
        fakeHttpMessageHandler
            .GetProtectedSetup()
            .Throws<TException>();

        return fakeHttpMessageHandler;
    }

    public static HttpClient ToHttpClient(this Mock<HttpMessageHandler> fakeHttpMessageHandler)
    {
        return new HttpClient(fakeHttpMessageHandler.Object);
    }

    private static ISetup<HttpMessageHandler, Task<HttpResponseMessage>> GetProtectedSetup(
        this Mock<HttpMessageHandler> fakeHttpMessageHandler)
    {
        return fakeHttpMessageHandler
            .Protected()
            .Setup<Task<HttpResponseMessage>>(
                "SendAsync",
                ItExpr.IsAny<HttpRequestMessage>(),
                ItExpr.IsAny<CancellationToken>());
    }
}

We can add other methods like WithNotFoundResponse(), WithInternalServerResponse() or WithTooManyRequestsResponse() to cover other response codes. Even, we can setup the fake HttpMessageHandler passing an Uri with a method ForUri(), for example.

Voilà! That's how to write tests with HttpClient and Moq. With some extension methods, we could have a small DSL to write more readable tests. For a more fully-featured alternative to write tests for HttpClient, check mockhttp, "a testing layer for Microsoft's HttpClient library."

If you want to read more about unit testing, check my Unit Testing 101 series on my blog where we cover from what a unit test is, to fakes and mocks, to best practices. And, stay tuned to my C# Advent of Code.

Hey! I'm Cesar, a software engineer and lifelong learner. If you want to upgrade your unit testing skills, check my course: Mastering C# Unit Testing with Real-world Examples on Udemy. Practice with hands-on exercises and learn best practices by refactoring real-world unit tests.

Happy testing!

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TIL: How to write tests for HttpClient using Moq

Cesar Aguirre

How to Create a Testable HttpClient

Some extensions methods to set up the faked HttpClient

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