Silverlight 网络客户问题

我使用web客户端每5分钟下载一次数据，但有时会显示并发错误

                TimerCallback call = down;
                temp = new Timer(call);
                temp.Change(1000, System.Threading.Timeout.Infinite);

                 public void down(object obj)
    {
        if(webflag == true)
        webClient.DownloadStringAsync(new Uri(url));

        webflag = false;
    }

    private void FeedsDownloaded(object sender, DownloadStringCompletedEventArgs e)

    {

        if (e.Error == null)
        {
          // some processing
        }

            webflag = true;
            temp.Change(5000, System.Threading.Timeout.Infinite);

    }

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我是初学者，所以上面的代码看起来一定很混乱。感谢指导，谢谢

谢谢Drew Marsh，解决方案是使用webClient.isbusy标志

    if(webClient.isbusy == false)
    webClient.DownloadStringAsync(new Uri(url));

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你能发布你得到的异常的详细信息吗（请提供完整的堆栈跟踪）？