public class MessagesController : ApiController
{
    [Route("api/messages/user-{userId}/unread/count")]
    public async Task<HttpResponseMessage> GetUserUnreadMessageCount(int userId)
    {
        var unreadCount = 10;
        var response = Request.CreateResponse(HttpStatusCode.OK);
        response.Headers.Add("X-RESULT-COUNT", unreadCount.ToString());
        return response;
    }
}

public class WebApiTest
{
    [Fact]
    public async Task Get_User_Unread_Message_Count()
    {
        using (var client = new HttpClient())
        {
            client.BaseAddress = new System.Uri("www.cnblogs.com");
            var userId = 1;
            var response = await client.GetAsync($"/api/messages/user-{userId}/unread/count");
            if (response.IsSuccessStatusCode)
            {
                var unreadCount = response.Headers.GetValues("X-RESULT-COUNT").FirstOrDefault();
                Console.WriteLine(unreadCount);
                Assert.Equal(10, int.Parse(unreadCount));
            }
            else
            {
                Console.WriteLine(response.StatusCode);
                Console.WriteLine(await response.Content.ReadAsStringAsync());
            }
        }
    }
}