C# – Mocking HttpClient in unit tests

cmoqunit testing

I have some issues trying to wrap my code to be used in unit tests. The issues is this. I have the interface IHttpHandler:

public interface IHttpHandler
{
    HttpClient client { get; }
}

And the class using it, HttpHandler:

public class HttpHandler : IHttpHandler
{
    public HttpClient client
    {
        get
        {
            return new HttpClient();
        }
    }
}

And then the Connection class, which uses simpleIOC to inject the client implementation:

public class Connection
{
    private IHttpHandler _httpClient;

    public Connection(IHttpHandler httpClient)
    {
        _httpClient = httpClient;
    }
}

And then I have a unit test project which has this class:

private IHttpHandler _httpClient;

[TestMethod]
public void TestMockConnection()
{
    var client = new Connection(_httpClient);
     
    client.doSomething();  

    // Here I want to somehow create a mock instance of the http client
    // Instead of the real one. How Should I approach this?     

}

Now obviously I will have methods in the Connection class that will retrieve data (JSON) from a my back end. However, I want to write unit tests for this class, and obviously I don't want to write tests against the real back end, rather a mocked one. I Have tried to google a good answer to this without great success. I can and have used Moq to mock before, but never on something like HttpClient. How should I approach this problem?

Best Answer

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'}
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