Overview

Since HttpClient is not backed by an interface, the approach to get an HttpClient for testing is different:

1. Mock HttpMessageHandler:

   var handlerMock = new Mock<HttpMessageHandler>(MockBehavior.Strict);
   

2. Set up the mock:

   // Call Protected() since since HttpMessageHandler's constructor is protected:
   handlerMock.Protected()
           // Override HttpMessageHandler's protected internal abstract SendAsync method:
           .Setup<Task<HttpResponseMessage>>("SendAsync", ItExpr.IsAny<HttpRequestMessage>(), ItExpr.IsAny<CancellationToken>())
           .ReturnsAsync(new HttpResponseMessage()
           {
               StatusCode = HttpStatusCode.OK,
               Content = new StringContent(httpGetResponseStringContent)
           });
   

3. Pass the mocked HttpMessageHandler to the HttpClient constructor to get a test HttpClient:

   var httpClient = new HttpClient(handlerMock.Object);
   

4. Pass that HttpClient to the system under test that would normally receive an injected HttpClient instance:

   var sut = new SomeService(httpclient, ...);