为了获取微信的 AccessToken  代码如下：

 public static string WebRequestPostOrGet(string sUrl, string sParam)         {            

byte[] bt = System.Text.Encoding.UTF8.GetBytes(sParam);

 Uri uriurl = new Uri(sUrl);            
HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(uriurl);           

req.Method = "Post";            
req.Timeout = 120 * 1000;            
req.ContentType = "application/x-www-form-urlencoded;";             req.ContentLength = bt.Length;

try            
{                
using (Stream reqStream = req.GetRequestStream())   //代码执行到这报“基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系”                
{                    
reqStream.Write(bt, 0, bt.Length);                    
reqStream.Flush();                
}            
}            
catch (Exception e)            
{            

}
 try            
{                
using (WebResponse res = req.GetResponse())                
{                    
//在这里对接收到的页面内容进行处理

Stream resStream = res.GetResponseStream();

StreamReader resStreamReader = new StreamReader(resStream, System.Text.Encoding.UTF8);

string resLine;

System.Text.StringBuilder resStringBuilder = new System.Text.StringBuilder();

 while ((resLine = resStreamReader.ReadLine()) != null)                     {                        
resStringBuilder.Append(resLine + System.Environment.NewLine);                   
}

resStream.Close();                    
resStreamReader.Close();
return resStringBuilder.ToString();                
}            
}            
catch (Exception ex)            
{               
  return ex.Message;//url错误时候回报错            
}        
}   

结果出错 提示“基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系”。
请问如何解决此问题，谢谢！