C# WebClient 使用异步并返回数据

Question

好的，我在使用 DownloadDataAsync 并将字节返回给我时遇到了问题。这是我正在使用的代码:

    private void button1_Click(object sender, EventArgs e)
    {
        byte[] bytes;
        using (WebClient client = new WebClient())
        {
            client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressChanged);
            bytes = client.DownloadDataAsync(new Uri("http://example.net/file.exe"));
        }
    }
    void DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        double bytesIn = double.Parse(e.BytesReceived.ToString());
        double totalBytes = double.Parse(e.TotalBytesToReceive.ToString());
        double percentage = bytesIn / totalBytes * 100;
        label1.Text = Math.Round(bytesIn / 1000) + " / " + Math.Round(totalBytes / 1000);

        progressBar1.Value = int.Parse(Math.Truncate(percentage).ToString());
        if (progressBar1.Value == 100)
        {
            MessageBox.Show("Download Completed");
            button2.Enabled = true;
        }
    }

我得到的错误是“无法将类型‘void’隐式转换为‘byte[]’”

有没有办法让这成为可能，并在下载完成后给我字节数？删除“bytes =”时效果很好。

Answer 1

由于 DownloadDataAsync 方法是异步的，因此它不会立即返回结果。您需要处理 DownloadDataCompleted 事件:

client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(DownloadCompleted);
...


private static void DownloadCompleted(Object sender, DownloadDataCompletedEventArgs e)
{
    byte[] bytes = e.Result;
    // do something with the bytes
}