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        WebClient[] wc = new WebClient[10];
            ...
            ...
            wc[downloads] = new WebClient();
            wc[downloads].Proxy = new WebProxy();
            wc[downloads].DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileCompleted);
            wc[downloads].DownloadProgressChanged += new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);
            wc[downloads].DownloadFileAsync(new Uri(gefilterteURL), file_name);					

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ServicePointManager.DefaultConnectionLimit = 10;					

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<configuration>
 <system.net>
  <connectionManagement>
   <add address="*" maxconnection="10" />
  </connectionManagement>
 </system.net>
</configuration>