Knowledge Base

BUG: OnBeforeUnload() Does Not Fire When Hosting WebBrowser Control

Article ID: 253201

Article Last Modified on 5/12/2003

APPLIES TO

Microsoft Internet Explorer 4.0 128-Bit Edition
Microsoft Internet Explorer 4.01 Service Pack 2
Microsoft Internet Explorer 4.01 Service Pack 1
Microsoft Internet Explorer 4.01 Service Pack 2
Microsoft Internet Explorer 5.0
Microsoft Internet Explorer 5.01
Microsoft Internet Explorer 5.5

This article was previously published under Q253201

SYMPTOMS

When hosting a WebBrowser control in either a Microsoft Visual C++ or Microsoft Visual Basic application, an HTML page's OnBeforeUnload event handler is never called when the application is terminated, even though the OnUnload event handler is called. The handler is called properly when viewing the page in a standalone instance of Internet Explorer.

CAUSE

In Internet Explorer, this event is fired from outside of the WebBrowser control by calling the WebBrowser's IOleCommandTarget's Exec() method with the OLECMDID_ONUNLOAD.

RESOLUTION

To resolve this problem, send the WebBrowser the OLECMDID_ONUNLOAD command in your host application. This can be accomplished by calling the WebBrowser control's ExecWB() method.

To fully emulate Internet Explorer's behavior, you must examine the return value of ExecWB's fourth argument, which tells you whether or not the shutdown attempt failed or succeeded. This is because script authors can use the OnBeforeUnload event to ask the user whether they want to leave the page or not. If the user answers no, the WebBrowser should remain activated.

Visual C++

HRESULT hr;
VARIANT vOut;
VariantInit(&vOut);
hr = pWebBrowser2->ExecWB(OLECMDID_ONUNLOAD, OLECMDEXECOPT_DODEFAULT, NULL, &vOut);
if (SUCCEEDED(hr)) {
    // Check VT_BOOL return value of vOut to make sure we're really shutting down - IE or the user
    // might decide not to honor this request.
    if (vOut.bVal == TRUE) {
        // Continue shutdown process
    } else {
        // recover
    }
}

Visual Basic

Public Sub Form_Unload(Cancel as Integer)
    WebBrowser1.ExecWB OLECMDID_ONUNLOAD, OLECMDEXECOPT_DODEFAULT, 0, ret
    If ret = False Then
        Cancel = 1
    End If
End Sub

STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

MORE INFORMATION

Steps to Reproduce this Behavior

Open Visual Basic and create a new Standard EXE.
Select Components from the Project menu and include Microsoft Internet Controls in your project.
Place a copy of the WebBrowser control on your application's form.

Save the following file onto your hard drive as Test.htm:

<html>

<HEAD>
<SCRIPT>
function unload() {
    alert("Inside unload");
}
function beforeunload() {
    alert("Inside beforeunload");
}
</SCRIPT>
</HEAD>

<body onunload="unload();" onbeforeunload="beforeunload();">
</body>

</html>

In your form's Load function, add code to navigate to this file:

    WebBrowser1.Navigate "C:\temp\test.htm"

Run this application from the Visual Basic Integrated Development Environment (IDE). As soon as the page fully loads, close the form. You will see an "Inside unload" message, but will never see an "Inside beforeunload" message.

REFERENCES

See the MSDN Library for more information on the OnBeforeUnload event.

Additional query words: onunload onbeforeunload script

Keywords: kbbug kbnofix kbwebbrowser KB253201

Microsoft KB Archive/253201

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