在使用downloaddataccompleted方法时需要传递额外的参数给事件处理程序

Question

我需要通过使用WebClient实现异步下载。downloaddatasync Method

我定义的事件处理程序如下:

 Private Sub DownloadCompleted(sender As Object, e As DownloadDataCompletedEventArgs)
        Dim bytes() As Byte = e.Result
        'do somthing with result
    End Sub

但是我需要向这个事件处理程序传递一个额外的参数，连同下载的结果，我创建URI的值。例如:

Dim pictureURI = "http://images.server.sample.com/" + myUniqueUserIdentifier

我需要在DownloadCompleted子中获得myUniqueUserIdentifier回调。

我正在使用以下代码注册我的事件处理程序:

AddHandler webClient.DownloadDataCompleted, AddressOf DownloadCompleted

我只有一个猜测，扩展WebClient对象和覆盖DownloadDataCompleted对象。但在我执行这个之前我想检查一下有没有更简单的解?

谢谢。

Answer 1

你可以使用WebClient.DownloadDataAsync重载，它接受一个额外的对象，然后通过AsyncCompletedEventArgs.UserState属性再次接收它。

下面是一个例子:

Sub Main
    Dim url = "http://google.com"
    Dim client = new WebClient()
    AddHandler client.DownloadDataCompleted, AddressOf DownloadDataCompleted
    client.DownloadDataAsync(new Uri(url), url) ' <- pass url as additional information...'
End Sub
Sub DownloadDataCompleted(sender as object, e as DownloadDataCompletedEventArgs)
    Dim raw as byte() = e.Result
    ' ... and recieve it in the event handler via the UserState property'
    Console.WriteLine(raw.Length & " bytes received from " & e.UserState.ToString())
End Sub

Answer 2

您可以使用函数来传递参数，而不是AddressOf

AddHandler webClient.DownloadDataCompleted, Function(sender, e) DownloadCompleted(param)