.NET MSIE OnBeforeNavigate2 fix

The important thing to note is that the IDL defines DWebBrowserEvents2 as the default event source, not DWebBrowserEvents. Because of that, the interop wrapper generator (tlbimp.exe) will provide us with marshaling code reflecting just that, namely AxInterop.SHDocVw.dll (ActiveX layer) and Interop.SHDocVw.dll (shdocvw.dll wrapper). As a result, if you type axWebBrowser1. (notice the dot), then intellisense will show you methods from this interface, not from DWebBrowserEvents. Casting is of no help here : the compiler would be ok, but it would fail at run-time. Looks like we are a bit stuck here.

To go on, we are actually going to ask the interop marshaler to produce at run-time a wrapper for the DWebBrowserEvents interface. Let's show some code now:

/// <summary>

/// Summary description for Form1.

/// </summary>

public class Form1 : System.Windows.Forms.Form
{
    private AxSHDocVw.AxWebBrowser axWebBrowser1;
    private SHDocVw.WebBrowserClass ie_events;
    private System.Windows.Forms.TextBox textBox1;

    public Form1()
    {
        //

        // Required for Windows Form Designer support

        //

        InitializeComponent();

        // -- begin code snippet --


        ie_events = (SHDocVw.WebBrowserClass) 
                    Marshal.CreateWrapperOfType(
                        axWebBrowser1.GetOcx(),
                        typeof(SHDocVw.WebBrowserClass)
                    );

        // -- end code snippet --


        ...
    }
}

The CreateWrapperOfType call performs the magic of creating an RCW (layer to execute COM interfaces and methods) for us. Instead of passing the SHDocVw.DWebBrowserEvents interface type we want, we pass the SHDocVw.WebBrowserClass instead. Why ? That's a trick again, the marshaler expects a coclass type to build the RCW, instead of a simple interface. WebBrowserClass is the .NET name of coclass WebBrowser declared in the IDL.

The resulting RCW is stored in a member of our Form. Now we have the right interface to play with. By virtue of the IDL COM declaration, if we use intellisense on ie_events, we are going to see both interface's methods and events. And there we have BeforeNavigate.

We are done, let's show how we use this event to get the actual notification. In .NET, we just create a delegate, and attach an event handler to it:

public Form1()
{
    //

    // Required for Windows Form Designer support

    //

    InitializeComponent();

    // -- begin code snippet --


    ie_events = (SHDocVw.WebBrowserClass) Marshal.CreateWrapperOfType(
        axWebBrowser1.GetOcx(),
        typeof(SHDocVw.WebBrowserClass)
    );

    SHDocVw.DWebBrowserEvents_BeforeNavigateEventHandler BeforeNavigateE = 
        new SHDocVw.DWebBrowserEvents_BeforeNavigateEventHandler( 
            OnBeforeNavigate 
        );

    ie_events.BeforeNavigate += BeforeNavigateE;

    // -- end code snippet --


    ...
}

public void OnBeforeNavigate(string url, 
                             int flags, 
                             string targetFrame, 
                             ref object postData, 
                             string headers, 
                             ref bool Cancel)
{
    int c = 0; // PUT A BREAKPOINT HERE

}

A demo app

Just to see something happen on screen, we immediately ask the web browser to show CodeProject (face of relief...):

textBox1.Text = "http://www.codeproject.com";
OnNewUrl(null,null);

// KeyUp handler (used to trap VK_RETURN from the text box)

private void OnNewUrl(object sender, KeyEventArgs e)
{
    object o = null;

    if (e==null || e.KeyCode==Keys.Enter)
        axWebBrowser1.Navigate(textBox1.Text, ref o, ref o, 
            ref o, ref o);
}

Eh voilà.

Stephane Rodriguez - Oct 28 2002.

History

• October 28, 2002 - Initial Posting