VB.net DownloadDataAsync 到 MemoryStream 不工作

Question

我有以下代码可以将互联网上的图片直接加载到我的图片框中（从内存中）：

PictureBox1.Image = New Bitmap(New IO.MemoryStream(New Net.WebClient().DownloadData("LINK")))

这里的问题是我的应用程序在 WebClient 下载时冻结，所以我想我会使用 DownloadDataAsync

但是，使用此代码根本不起作用：

PictureBox1.Image = New Bitmap(New IO.MemoryStream(New Net.WebClient().DownloadDataAsync(New Uri("LINK"))))

它returns错误"Expression does not produce a value"

Answer 1

如错误消息所述，您不能简单地将 DownloadDataAsync 作为 MemoryStream 参数传递，因为 DownloadDataAsync 是一个 Sub 而 DownloadData 是一个返回 Bytes().

为了使用 DownloadDataSync，查看下面的示例代码：

Dim wc As New Net.WebClient()
AddHandler wc.DownloadDataCompleted, AddressOf DownloadDataCompleted
AddHandler wc.DownloadProgressChanged, AddressOf DownloadProgressChanged ' in case you want to monitor download progress

wc.DownloadDataAsync(New uri("link"))

以下是事件处理程序：

Sub DownloadDataCompleted(sender As Object, e As DownloadDataCompletedEventArgs)
    '  If the request was not canceled and did not throw
    '  an exception, display the resource.
    If e.Cancelled = False AndAlso e.Error Is Nothing Then

        PictureBox1.Image =  New Bitmap(New IO.MemoryStream(e.Result))
    End If
End Sub

Sub DownloadProgressChanged(sender As Object, e As DownloadProgressChangedEventArgs)
    ' show progress using : 
    ' Percent = e.ProgressPercentage
    ' Text = $"{e.BytesReceived} of {e.TotalBytesToReceive}"
End Sub

VB.net DownloadDataAsync 到 MemoryStream 不工作

VB.net DownloadDataAsync to MemoryStream not working

vb.net

webclient

memorystream

bitmap