HttpClient

 private async Task sendGETRequest()
        {
            //连环using
            using (var handler = new HttpClientHandler()
            {
                UseCookies = true,
                UseDefaultCredentials = true,
                CookieContainer = new CookieContainer()
            })
            using (var client = new HttpClient(handler))
            {
                client.BaseAddress = new Uri(S.domain);
                client.DefaultRequestHeaders.Accept.Clear();
                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                handler.CookieContainer.Add(client.BaseAddress, new Cookie("keyOne", "ttc")); //add cookie
                HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Get, "api/getSetCookieAndHeader"); 
                requestMessage.Headers.Add("key", "value"); //add header
                HttpResponseMessage response = await client.SendAsync(requestMessage);
                if (response.StatusCode == HttpStatusCode.OK)
                {
                    string responseAsString = await response.Content.ReadAsStringAsync();
                }               
            }
        }
        public async Task sendPOSTRequest()
        {
            using (var client = new HttpClient())
            {
                client.BaseAddress = new Uri(S.domain);
                var query = S.ParseQueryString("");
                query["returnUrl"] = "gowhere";
                string queryStr = query.toQueryString();
                HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Post, "Home/Login?" + queryStr);
                requestMessage.Headers.Add("key", "value"); //add header  
                //Content-Type 不可以加进去 header哦, 要json 或 FormUrlEncodedContent 就要像下面这样写    
                //requestMessage.Content = new StringContent("{\"name\":\"John Doe\",\"age\":33}", Encoding.UTF8, "application/json"); 
                requestMessage.Content = new FormUrlEncodedContent(new[]
                {
                    new KeyValuePair<string, string>("name", "nameValue")
                });
                HttpResponseMessage response = await client.SendAsync(requestMessage);
                string responseAsString = await response.Content.ReadAsStringAsync(); 
            }
        }