发送Http请求时报错，请各位大仙们多帮忙，谢谢！

{"请求被中止: 请求已被取消。"}
    [System.Net.WebException]: {"请求被中止: 请求已被取消。"}
    Data: {System.Collections.ListDictionaryInternal}
    HelpLink: null
    InnerException: {"在写入所有字节之前不能关闭流。"}
    Message: "请求被中止: 请求已被取消。"
    Source: "System"
    StackTrace: "   在 System.Net.ConnectStream.CloseInternal(Boolean internalCall, Boolean aborting)\r\n   在 System.Net.ConnectStream.System.Net.ICloseEx.CloseEx(CloseExState closeState)\r\n   在 System.Net.ConnectStream.Dispose(Boolean disposing)\r\n   在 System.IO.Stream.Close()\r\n   在 System.IO.StreamWriter.Dispose(Boolean disposing)\r\n   在 System.IO.StreamWriter.Close()\r\n   在 HttpRequestAndReponse.SendRequest(String indata) 位置 c:\\Inetpub\\wwwroot\\QPayment\\App_Code\\Common\\HttpRequestAndReponse.cs:行号 45"
    TargetSite: {Void CloseInternal(Boolean, Boolean)}代码：
protected static HttpWebRequest SendRequest(string indata)
    {
        HttpWebRequest req;
        try
        {            
            //发起HTTP请求
            req = (HttpWebRequest)WebRequest.Create(Constant.HTTP_URL);
            req.Method = Constant.HTTP_METHOD;
            req.ContentType = Constant.HTTP_CONTENTTYPE;
            req.ContentLength = indata.Length;
            req.UserAgent = Constant.HTTP_USERAGENT;
            req.KeepAlive = true;
            req.Headers.Add("Cmbc-E2bTxcode", "yq0100");
            req.Headers.Add("Cmbc-E2bEcode", "rtdl");
            Stream requestStream = req.GetRequestStream();
            StreamWriter streamWriter = new StreamWriter(requestStream, Encoding.GetEncoding("GB2312"));
            streamWriter.Write(indata);
            req.Timeout = 30000;
            streamWriter.Close();
            requestStream.Close();
            return req;
        }
        catch(Exception err)
        {
            throw new System.Exception("0");// 0:发送请求失败
        }
    }