using System;

using System.Net;

using System.Net.Http;

using System.Net.Http.Json;

using System.Threading;

using System.Threading.Tasks;

using Moq;

using Moq.Protected;

public class Program

{

    public static async Task Main()

    {

        //Arrange

        var data = new { A = 1, B = "C"};

​

        var handlerMock = new Mock<HttpClientHandler>();

        handlerMock.Protected()

            .Setup<Task<HttpResponseMessage>>(

            "SendAsync",

            ItExpr.IsAny<HttpRequestMessage>(),

            ItExpr.IsAny<CancellationToken>()

        )

            .ReturnsAsync((HttpRequestMessage req, CancellationToken ct) => {

                //Assert

                var content = (JsonContent)req.Content;

                (content.Value == data).Dump();

​

                //Returns

                return new HttpResponseMessage

                {

                    StatusCode = HttpStatusCode.OK,

                    Content = new StringContent("{'foo':'bar'}")

                };

            });

​

        //Act

        var httpClient = new HttpClient(handlerMock.Object);

        await httpClient.PostAsJsonAsync("http://google.com", data);

    }

}