private void webClient_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
    this.Dispatcher.Invoke(new Action(()=>{
            progressBar.Value = e.ProgressPercentage;
            txtStatus.Text = e.ProgressPercentage.ToString() + " %";
            txtReceivedSize.Text = string.Format("正在下载,当前进度 {0}/{1} (字节)", e.BytesReceived, e.TotalBytesToReceive);
   }));//调用委托
}
这个方法里面要用委托调用,找找