C#

Configuration _usrConfigLoc = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.PerUserRoamingAndLocal);

string _filePath = _userConfigLoc.FilePath;

public delegate byte[] getHTTPdelegate(Uri address); // this is a delegate defined as a class member, used to perform async page download
public void downloadPage(string URL)
        {
            // creating new webclient
            client = new WebClient();
            //assigning download method to a delegate
            getHTTPdelegate dl = client.DownloadData;

            // starting async download
            IAsyncResult ar = dl.BeginInvoke(new Uri(URL), null, null);
            

            while (!ar.IsCompleted)
            {
                
                Thread.Sleep(10);
            }
            // rawpage contains HTML in terms of byte[], the result of async download
            rawPage = dl.EndInvoke(ar); // this is also the line where exception occurs
        }

<?xml version="1.0" encoding="UTF-8"?>
<result xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" method="auth.createToken" status="ok">
	<error code="21" text="One of the parameter values is invalid" info="api_key"/> 
</result>

[XmlRoot("result")]
 public class ResultMessage
 {
 [XmlIgnore] 
 public const string StatusOk = "ok";
 [XmlIgnore]
 public const string StatusFail = "fail";

 [XmlAttribute("method")]
 public string _method;
 public string Method
 {
 get { return _method; }
 }

 [XmlAttribute("status")]
 public string _status;
 public string Status
 {
 get { return _status; }
 }

 [XmlElement("error")] 
 [XmlAttribute("status")] 
 public int _errorcode;
 public ErrorCode ErrorCode
 {
 get { return (ErrorCode)_errorcode; }
 }

 [XmlElement("error")]
 [XmlAttribute("text")]
 public string _errortext;
 public string ErrorText
 {
 get { return _errortext; }
 }

 [XmlElement("error")]
 [XmlAttribute("info")]
 public string _errorinfo;
 public string ErrorInfo
 {
 get { return _errorinfo; }
 }

 public ResultMessage()
 {
 _errorcode = 0;
 }

{\rtf1\ansi\ansicpg1252\uc1\deff0{\fonttbl
{\f0\fswiss\fcharset0\fprq2 Arial;}
{\f1\froman\fcharset2\fprq2 Symbol;}}
{\colortbl;\red0\green0\blue0;\red255\green255\blue255;\red0\green0\blue0;}
{\stylesheet{\s0\itap0\nowidctlpar\f0\fs24 [Normal];}{\*\cs10\additive Default Paragraph Font;}}
{\*\generator TX_RTF32 14.0.520.503;}
\deftab1134\paperw12240\paperh15840\margl1138\margt1138\margr1138\margb1138\widowctrl\formshade\sectd
\headery720\footery720\pgwsxn12240\pghsxn15840\marglsxn1138\margtsxn1138\margrsxn1138\margbsxn1138\pard\itap0\nowidctlpar\plain\f0\fs18\cf3 hello\par }

{\rtf1\ansi\ansicpg1252\uc1\deff0{\fonttbl
{\f0\fswiss\fcharset0\fprq2 Arial;}
{\f1\froman\fcharset2\fprq2 Symbol;}}
{\colortbl;\red0\green0\blue0;\red255\green255\blue255;\red0\green0\blue0;}
{\stylesheet{\s0\itap0\nowidctlpar\f0\fs24 [Normal];}{\*\cs10\additive Default Paragraph Font;}}
{\*\generator TX_RTF32 14.0.520.503;}
\deftab1134\paperw12240\paperh15840\margl1138\margt1138\margr1138\margb1138\widowctrl\formshade\sectd
\headery720\footery720\pgwsxn12240\pghsxn15840\marglsxn1138\margtsxn1138\margrsxn1138\margbsxn1138\pard\itap0\nowidctlpar\plain\f0\fs18\cf3 world\par }