.NET: HttpClient mocking it with my Interface IHttpClient, but there is an internal object that is NULL and it is sealed

Just inject the concrete HttpClient and mock the underlying HttpMessageHandler...

http://geekswithblogs.net/abhi/archive/2013/11/20/unit-tests-for-httpclient-using-httpmessagehandler.aspx

You can unit test a class that uses HttpClient by giving that HttpClient a mock HttpMessageHandler. This way, you can capture the request and prevent it from actually going over the wire.

Here is an example using Moq. HttpClient depends on HttpMessageHandler’s SendAsync() method, so give SendAsync() a stub implementation and use Moq’s Callback() to capture arguments.

var handler = new Mock<HttpMessageHandler>();

handler.Protected()

    .Setup<Task<HttpResponseMessage>>("SendAsync", ItExpr.IsAny<HttpRequestMessage>(), ItExpr.IsAny<CancellationToken>())

    .Returns(Task<HttpResponseMessage>.Factory.StartNew(() =>

    {

        return new HttpResponseMessage(HttpStatusCode.OK);

    }))

    .Callback<HttpRequestMessage, CancellationToken>((r, c) =>

    {

        Assert.AreEqual(HttpMethod.Get, r.Method);

    });

 

using (var client = new HttpClient(handler.Object))

{

    var request = new HttpRequestMessage(HttpMethod.Get, "http://www.google.com");

    var response = client.SendAsync(request).Result;

    Console.WriteLine(response.StatusCode);

}

HttpRequestHeaders has only internal constructor. But you can do it by creating HttpRequestMesssage object:

var message = new HttpRequestMessage();
.Setup(x => x.DefaultRequestHeaders).Returns(message.Headers);