使用 DownloadDataCompleted 方法时需要将附加参数传递给事件处理程序

Question

我需要使用WebClient.DownloadDataAsync Method实现异步下载

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附加参数以及下载的结果，我用它创建 URI 的值。例如：

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

我需要在 DownloadCompleted 子回调中获得 myUniqueUserIdentifier。

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 对象并覆盖 DownloadDataCompleted 对象。但在执行此操作之前，我想检查是否有更简单的解决方案？

谢谢。

Answer 1

您可以使用 WebClient.DownloadDataAsync overload which takes an additional object, and then recieve it again via the AsyncCompletedEventArgs.UserState 属性.

这是一个例子：

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

您可以使用函数传递参数而不是AddressOf

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

使用 DownloadDataCompleted 方法时需要将附加参数传递给事件处理程序

Need to pass additional parameter to event handler while using DownloadDataCompleted method

vb.net

webclient