PRB: Cannot Cancel AsyncRead When Used With Local Files

ID: Q190885

The information in this article applies to:

Microsoft Internet Explorer (Programming) versions 4.0, 4.01, 5.0
Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 6.0

SYMPTOMS

When using AsyncRead from a ActiveX Document or an ActiveX Control project running under Internet Explorer 4.x to download a local file, you cancel the operation by calling CancelAsyncRead. CancelAsyncRead does not work for local files.

CAUSE

Reading a local file is a Synchronous operation. For this reason the call to AsyncRead is completed before any subsequent lines of code are executed.

MORE INFORMATION

Steps to Reproduce Behavior

Create a blank ActiveX Document project in Visual Basic 6.0.

Insert the following code in the project:


Private Sub UserDocument_AsyncReadComplete(AsyncProp as
   AsyncProperty)
    MsgBox "CancelAsyncRead didn't work. Downloaded " &amp; _
           AsyncProp.BytesRead &amp; " Bytes"
End Sub

Private Sub UserDocument_Show()
   'use a large file here
   AsyncRead "c:\windows\system\mshtml.dll", vbAsyncTypeByteArray
   MsgBox "Notice that CancelAsyncRead hasn't been called yet!"
   CancelAsyncRead
End Sub

Run your project and load the Visual Basic Document in Internet Explorer 4.x

You will see the MsgBox in AsyncReadComplete is shown before the MsgBox in UserDocument_Show(). This means that the AsyncReadComplete event is fired before CancelAsyncRead could be called.

Additional query words:


Keywords          : kbActiveDocs kbIE400 kbIE401 kbVBp600 kbIE500 
Version           : WINDOWS:4.0,4.01,6.0
Platform          : WINDOWS 
Issue type        : kbprb

Last Reviewed: April 30, 1999