使用WebRequest方法在C#中发送JSON POST -错误？

WebRequest request = WebRequest.Create(url);
request.Method = "POST";

request.Headers.Add("Content-Type", "application/json");

string json = JsonConvert.SerializeObject(data);
byte[] postData = Encoding.UTF8.GetBytes(json);
request.ContentLength = postData.Length;
using (Stream requestStream = request.GetRequestStream())
{
    requestStream.Write(postData, 0, postData.Length);
}

using (WebResponse response = request.GetResponse())
{
    using (Stream responseStream = response.GetResponseStream())
    {
        using (StreamReader reader = new StreamReader(responseStream))
        {
            string responseJson = reader.ReadToEnd();
            // 处理响应数据
        }
    }
}