Question

我想在网站上下载多个图片。查询链接的格式为：

abc.com/GetImage.aspx?MSSV=0101012018

当我在C＃中的 WebClient 类中使用DownloadFileAsync()方法时，如果我只获得1张图片，它仍然有用。但是当我试图获得多个图像时，我把它放在循环中。如果它到达循环，则返回MessageBox包含字符串：＆＃34;下载完成＆＃34;或＆＃34;下载失败＆＃34;。

但是当我运行它时，当它超出循环时，它会停留在wc_DownloadProgressChanged()方法中，并且永远不会转到wc_DownloadFileComplete()来打印消息。

private void wc_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
    progressBar1.Value = e.ProgressPercentage;
}
private void wc_DownloadFileComplete(object sender, AsyncCompletedEventArgs e)
{
    if (e.Error == null)
    {
        MessageBox.Show("Download sucessfully!");
    }
    else
    {
        MessageBox.Show(e.Error.Message);
    }
    ((WebClient)sender).Dispose();
}

private void button1_Click(object sender, EventArgs e)
{
    int mssv = Convert.ToInt32(textBox2.Text);
    int amount = Convert.ToInt32(textBox3.Text);
    for(int i = 0; i < amount; i++)
    {
        progressBar1.Value = 0;
        WebClient wc = new WebClient();                
        wc.DownloadProgressChanged += new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);                    
        wc.DownloadFileAsync(new Uri("abc.com/GetImage.aspx?MSSV="+mssv), mssv.ToString()+".jpg");
        mssv++;
        if (i == amount)
        {
            wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileComplete);
        }
    }

}

Answer 1

这个代码块永远不会被执行，因为你的for循环条件会检查i < amount。因此i永远不会在循环内等于amount。它总是小于。

if (i == amount)
{
    wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileComplete);
}

你可以通过多种方式纠正这个......

将for循环更改为
```
for(int i = 1; i <= amount; i++)
```
将if语句更改为
```
if (i == amount-1 )
```

将DownloadFileCompleted事件处理程序移出循环，并重用WebClient（未经测试）。

WebClient wc = null;
for(int i = 0; i < amount; i++)
{
    progressBar1.Value = 0;
    wc = new WebClient();
    wc.DownloadProgressChanged += new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);                        
    wc.DownloadFileAsync(new Uri("abc.com/GetImage.aspx?MSSV="+mssv), mssv.ToString()+".jpg");
    mssv++;
}
if( wc != null )
    wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileComplete);

我的个人推荐......现在，随着不同的下载报告进度，您的进度条将会遍布整个地方。（也未经测试，我可能犯了几个拼写错误）。

private void wc_DownloadFileComplete(object sender, AsyncCompletedEventArgs e)
{
    progressBar1.PerformStep();
    if( e.Error != null )
    {
        string msg = String.Format("Error downloading MSSV {0}\r\n\r\n{1}",progressBar1.Value,e.Error.Message);
        MessageBox.Show(msg);
    }
    if( progressBar1.Maximum == progressBar1.Value )
    {
        MessageBox.Show("All downloads completed.");
        WebClient wc = sender as WebClient;
        if( wc != null )
            wc.Dispose();
    }
}

private void button1_Click(object sender, EventArgs e)
{
    int mssv = Convert.ToInt32(textBox2.Text);
    int amount = Convert.ToInt32(textBox3.Text);
    progressBar1.Value = 0;
    progressBar1.Maximum = amount;
    WebClient wc = new WebClient();
    wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileComplete);
    for(int i = 0; i < amount; i++, mssv++)
    {
        Uri url = new Uri(String.Format("abc.com/GetImage.aspx?MSSV={0}",mssv));
        string path = String.Format("{0}.jpg",mssv);
        wc.DownloadFileAsync(url,path);
    }
}

此外，我质疑在调用DownloadFileAsync之后添加事件处理程序的逻辑。它可能永远不会发生，但从理论上讲，对DownloadFileAsync的调用可以在添加事件处理程序之前完成。最好在调用DownloadFileAsync之前添加事件处理程序。

DownloadFileAsync陷入循环C＃

1 个答案: