HttpWebRequest WebExcepton: The remote server returned an error: (407) Proxy Authentication Required.

1. Supply the credentials of the Currently Logged on User to the Proxy object similar to this:

// Begin code change by jeff

                    //  Obtain the 'Proxy' of the  Default browser.
                    IWebProxy theProxy = aReq.Proxy;
                    // Print the Proxy Url to the console.
                    if (theProxy != null)
                    {
// Use the default credentials of the logged on user.
                        theProxy.Credentials = CredentialCache.DefaultCredentials;
                    }
                    // End code change by jeff

                    HttpWebResponse aResp = aReq.GetResponse() as HttpWebResponse;

　　2. Add this property to the <<application>>.exe.config or machine.config file:

<system.net>
    <defaultProxy useDefaultCredentials="true"> </defaultProxy>
</system.net>

　　link:http://blogs.msdn.com/b/jpsanders/archive/2009/03/24/httpwebrequest-webexcepton-the-remote-server-returned-an-error-407-proxy-authentication-required.aspx