【一】步骤：
1）将对象转化为Json字符串。
2）将Json字符串编码为byte数组。
3）设置传输对象（WebRequest或者HttpClient）的ContentType是"application/json"。
4）设置传输对象的ContentLength=Byte数组的长度。
5）开始传输
6）获取JSON结果：
【二】示例代码：
【对于WebRequest而言】
static void SendByWebRequesttoApi() ???????{ ???????????WebRequest req = WebRequest.Create("http://localhost:15203/api/ApiDefault"); ???????????var stu = new Student { ID = 1, Name = "董玮" }; ???????????string jsonString = JsonConvert.SerializeObject(stu); ???????????byte[] objectContent = Encoding.UTF8.GetBytes(jsonString); ???????????req.ContentLength = objectContent.Length; ???????????req.ContentType = "application/json"; ???????????req.Method = "POST"; ???????????using (var stream = req.GetRequestStream()) ???????????{ ???????????????stream.Write(objectContent, 0, objectContent.Length); ???????????????stream.Close(); ???????????} ???????????var resp = req.GetResponse(); ???????????using (StreamReader sr = new StreamReader(resp.GetResponseStream())) ???????????{ ???????????????string s = sr.ReadToEnd(); ???????????????System.Console.WriteLine(s); ???????????} ???????}【对于HttpClient而言】
static void SendByHttpClienttoApi() ???????{ ???????????var stu = new { ID = 1, Name = "董玮" }; ???????????using (var client = new HttpClient()) ???????????{ ???????????????string jsonString = JsonConvert.SerializeObject(stu); ???????????????byte[] bytes = Encoding.UTF8.GetBytes(jsonString); ???????????????using (StreamContent sc = new StreamContent(new MemoryStream(bytes))) ???????????????{ ???????????????????sc.Headers.ContentLength = bytes.Length; ???????????????????sc.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/json"); ???????????????????var result = client.PostAsync("http://localhost:15203/api/ApiDefault", sc).Result; ???????????????????var objectResult = JsonConvert.DeserializeObject<Student>(result.Content.ReadAsStringAsync().Result); ???????????????} ???????????} ???????}另外注意，以上是针对WebApi（WebApi默认是JSON格式数据传输）。如果是MVC的模式，那么默认是表单形式传输。因此：
static void SendByWebRequesttoMVC() ???????{ ???????????WebRequest req = WebRequest.Create("http://localhost:15203/Default/DoGetStudent"); ???????????var htmlFormPost = "ID=1&Name=董玮"; ???????????byte[] objectContent = Encoding.UTF8.GetBytes(htmlFormPost); ???????????req.ContentLength = objectContent.Length; ???????????req.ContentType = "application/x-www-form-urlencoded"; ??//必须写！ ???????????req.Method = "POST"; ???????????using (var stream = req.GetRequestStream()) ???????????{ ???????????????stream.Write(objectContent, 0, objectContent.Length); ???????????????stream.Close(); ???????????} ???????????var resp = req.GetResponse(); ???????????using (StreamReader sr = new StreamReader(resp.GetResponseStream())) ???????????{ ???????????????string s = sr.ReadToEnd(); ???????????????System.Console.WriteLine(s); ???????????} ???????}在HttpClient中，把StreamContent改为FormUrlEncodedContent，传入一个Dictionary<string,string>对象即可：
static void SendByHttpClienttoMVC() ???????{ ???????????using (var client = new HttpClient()) ???????????{ ???????????????FormUrlEncodedContent fc = new FormUrlEncodedContent(new Dictionary<string, string>() { { "ID", "1" }, { "Name", "董玮" } }); ???????????????var result = client.PostAsync("http://localhost:15203/Default/DoGetStudent", fc).Result; ???????????????System.Console.WriteLine(result.Content.ReadAsStringAsync().Result); ???????????} ???????}使用HttpClient和WebRequest时POST一个对象的写法
原文地址：https://www.cnblogs.com/coce/p/9721034.html