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 方法来实现异步下载

我已经定义了事件处理程序如下：

 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以及我创建 URI 的下载结果值。例如：

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

我需要myUniqueUserIdentifier在DownloadCompletedsub 中进行回调。

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。但在我要执行此操作之前，我想检查一下是否有更简单的解决方案？

谢谢你。

Answer 1

您可以使用WebClient.DownloadDataAsync带附加对象的重载，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