c# - UploadStringDataAsync 什么都不发送

Question

我正在尝试将 json 发送到 php 页面，但结果始终为空。

try
{
   using (WebClient webclient = new WebClient())
   {
      webclient.Headers.Add("user-agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)");
      webclient.Headers.Add("Content-Type", "application/json");
      webclient.Encoding = System.Text.Encoding.UTF8;

      string result = await webclient.UploadStringTaskAsync(url,"POST",json);

      if (debug)
         Console.WriteLine(" => " + result); // array(0) { }

   }
}

json字符串是用

string json = Newtonsoft.Json.JsonConvert.SerializeObject(searchModel);

json输出是

{"term":"test","local":null,"localLabel":null,"result":null,"resultLabel":null,"genre":null,"costumer":null}

我的网址目标：

<?php echo var_dump($_POST); ?>

//doing  $test="test" and  adding $test inside var_dump shows the right value.. 

我在这里缺少什么？