Question

变量'totalBytes'始终为-1，因此我无法正确计算/更新进度条，为什么会发生这种情况？

private void button1_Click(object sender, EventArgs e)
{
    WebClient client = new WebClient();
    client.DownloadFileCompleted += new AsyncCompletedEventHandler(Completed);
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged);
    client.DownloadFileAsync(new Uri("http://example.com/test.mp3"), @"E:\Test.mp3");
}

private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
    double bytesIn = double.Parse(e.BytesReceived.ToString());
    label1.Text = Convert.ToString(bytesIn);

    double totalBytes = double.Parse(e.TotalBytesToReceive.ToString()); //stays at -1
    label2.Text = Convert.ToString(totalBytes);

    double percentage = bytesIn / totalBytes * 100;
    label3.Text = Convert.ToString(percentage);

    progressBar1.Value = int.Parse(Math.Truncate(percentage).ToString());
}

Answer 1

WebClient在内部使用WebRequest，问题可能是您从中下载文件的服务器未发送Content-Length HTTP标头，在这种情况下您应该使用Indeterminate ProgressBar样式（例如Marquee）。

Answer 2

您可以在响应标题中手动检查Content-Length

client.ResponseHeaders["Content-Length"]

使用WebClient（）计算要下载的总字节数

2 个答案: