Question

我的自定义Class事件不起作用。这是我的代码：

public delegate void ChangedEventHandler(object sender, EventArgs e, int p);

当我不使用delegate { }时，我收到错误：

对象引用未设置为Object的实例。

public event EventHandler DownloadCompleted = delegate { };
public event ChangedEventHandler DownloadProgressChanged = delegate { };

public void DownloadFile(String url, String localFilename)
{

    try
    {
        Application.DoEvents();
        WebRequest req = WebRequest.Create(url);
        WebResponse response = req.GetResponse();
        Stream stream = response.GetResponseStream();
        byte[] downloadedData = new byte[0];

        int downloaded = 0;

        byte[] buffer = new byte[1024];
        int totalData = (int)response.ContentLength;
        int a = 0;

        FileStream file = File.Create(localFilename);

        while (true)
        {

            if (paused == false)
            {
                Application.DoEvents();
                int bytesRead = stream.Read(buffer, 0, buffer.Length);

                if (bytesRead == 0)
                {
                    file.Flush();
                    file.Close();
                    DownloadCompleted(this, new EventArgs());
                    break;
                }
                file.Write(buffer, 0, bytesRead);

                downloaded += bytesRead;
                DownloadProgressChanged(this, EventArgs.Empty, downloaded);
            }
        }
    }

    catch (Exception ex)
    {
        MessageBox.Show(ex.Message.ToString());
    }

}

在其他类中使用：

Downloader dw = new Downloader();
dw.DownloadFile(txt_url.Text, txt_path.Text);
dw.DownloadCompleted += dw_DownloadCompleted;
dw.Changed +=dw_Changed;

void wb_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
{
    MessageBox.Show("Completed!");
}

void wb_DownloadProgressChanged(object sender,DownloadProgressChangedEventArgs e)
{
    //Setting ProgressBar Value
    progressBar1.Maximum = 100;
    progressBar1.Value = e.ProgressPercentage;
}

但ProgressBar无效。

我的代码出了什么问题？

Answer 1

您必须在＆＃34;其他＆＃34;中分配事件处理程序。在调用前强 DownloadFile：

Downloader dw = new Downloader(); dw.DownloadCompleted += dw_DownloadCompleted; dw.Changed +=dw_Changed; dw.DownloadFile(txt_url.Text, txt_path.Text);

您也不需要分配= delegate { }等空事件处理程序。

否则在DownloadFile尝试呼叫DownloadCompleted和Changed事件处理程序时 - 它们是空引用，这就是您收到该错误的原因。分配空事件处理程序会阻止NullReferenceException，但由于您要为事件分配空事件处理程序 - 它们显然什么都不做。

活动不起作用

1 个答案: