Get file size before download it in vb

问题:

I have been working on a web browser in visual basic..Now,what I want to do is to the get file size before download it and when I click download I want to to get the number of the alrady downloaded Mbs(watch the picture)

Thank's for help!

回答1:

I've done some research, and this would probably be the most simple and "cleanest" way of getting a download's size (in bytes):

Public Function GetDownloadSize(ByVal URL As String) As Long
    Dim r As Net.WebRequest = Net.WebRequest.Create(URL)
    r.Method = Net.WebRequestMethods.Http.Head
    Using rsp = r.GetResponse()
        Return rsp.ContentLength
    End Using
End Function

Credit to Reed Kimble, who told me to dispose the WebResponse in my initial MSDN question.

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The above code will read the response headers of the file, rather than reading the body of it. This means that the file does not require to get downloaded, just to check it's size.

This is the reason to why some codes requires the file to actually get downloaded at first; they're reading the file's body rather than it's headers.

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Hope this helps!

回答2:

Use the WebClient ResponseHeaders:

Public Shared Function GetFileSize(url As String) As Long
    Using obj As New WebClient()
        Using s As Stream = obj.OpenRead(url)
            Return Long.Parse(obj.ResponseHeaders("Content-Length").ToString())
        End Using
    End Using
End Function

Request file size before download it

回答3:

The WebClient's DownloadProgressChanged event's args contains the property TotalBytesToRecieve. That tells you how many bytes the file you're downloading is.

Not the prettiest way, but if you want to get the size of the file before downloading you can start downloading the file then immediately cancel it:

Dim DownloadSize As Long
Private Sub CheckDownloadSize(ByVal URL As String)
    WebClient.DownloadFile(URL, IO.Path.Combine(My.Computer.FileSystem.SpecialDirectories.Temp, "tempdownload.tmp"))
End Sub

Private WithEvents WebClient As New WebClient
Private Sub WebClient_DownloadProgressChanged(ByVal sender As Object, ByVal e As System.Net.DownloadProgressChangedEventArgs) Handles WebClient.DownloadProgressChanged
    DownloadSize = e.TotalBytesToReceive
    WebClient.CancelAsync()
End Sub

Otherwise, just remove the .CancelAsync() line.