Using MSHTML Advanced Hosting Interfaces

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Overview

This article shows you how to use the MSHTML Advanced Hosting Interfaces, specifically IDocHostUIHandler, from .NET. These interfaces allow you very fine control over the user interface presented by the Microsoft Web Browser Control, for example, you can show your own context menu. I'll show you an easy way to use this interface without having to re-write the interface definitions yourself. As a bonus, I'll show you how to receive events from elements in the document.

The Problem

It is easy enough to put a web browser control on a form and get a fully-functional browser right away. But in your application, you might want finer control over how the control interacts with the user. For example, the standard IE context menu does not look particularly nice when you application shows an ultra-cool DHTML-based user interface. An interface called IDocHostUIHandler affords you this level of control.

Implementing IDocHostUIHandler

IDocHostUIHandler has to be implemented by your application. You can then use ICustomDoc::SetUIHandler to tell MSHTML that you have this interface.

IDocHostUIHandler is defined in MsHtmHst.idl in the Internet Development SDK, part of the Platform SDK. One approach I've seen is to do the import by hand - write the interface definition yourself in your source code.

The other approach, the one I'm going to use, is to create a type library with the interfaces we need declared in it, and then use the tlbimp tool to create an Interop assembly. No fiddling with marshalling yourself.

The first step would be to create an IDL file with the interfaces we need declared in it. You shall also need a UUID for the generated type library. There are only a few interfaces we need, and the entire IDL becomes:

MsHtmHstInterop.idl

[
    uuid(47F05070-FD66-45cc-AD99-74260F94A16B)
]
library MsHtmHstInterop
{
    import "MsHtmHst.idl";
    
    enum tagDOCHOSTUIDBLCLK;
    enum tagDOCHOSTUIFLAG;
    enum tagDOCHOSTUITYPE;
    
    interface ICustomDoc;
    interface IDocHostShowUI;
    interface IDocHostUIHandler;
    interface IDocHostUIHandler2;
    interface IHostDialogHelper;
};

I've included all the Advanced Hosting interfaces and enumerations here.

The next step is to generate the type library. Just compile the IDL file to get a TLB, using the Platform SDK tool MIDL.

midl MsHtmHstInterop.idl /tlb bin\MsHtmHstInterop.tlb

Now we need an interop assembly. We just have to use tlbimp on the newly created TLB file:

tlbimp bin\MsHtmHstInterop.tlb /out:bin\MsHtmHstInterop.dll

Now we can use the interfaces we need with just a using statement in our C# code:

HtmlUI.cs (partial)

using MsHtmHstInterop;

Wiring up

For the sample application, I have put just a single web browser control on a form. The form class implements IDocHostUIHandler.

To hook up the interface to MSHTML, we first need to get the ICustomDoc interface, and then call the SetUIHandler method with our interface as an argument.

HtmlUI.cs (partial - HtmlUIForm constructor)

public HtmlUIForm()
{
    InitializeComponent();
            
    this.WebBrowser.DocumentComplete += 
          new DWebBrowserEvents2_DocumentCompleteEventHandler(this.WebBrowser_DocumentComplete);
            
    object flags = 0;
    object targetFrame = String.Empty;
    object postData = String.Empty;
    object headers = String.Empty;
    this.WebBrowser.Navigate("about:blank", ref flags, ref targetFrame, 
                             ref postData, ref headers);
            
    ICustomDoc cDoc = (ICustomDoc)this.WebBrowser.Document;
    cDoc.SetUIHandler((IDocHostUIHandler)this);
            
    this.WebBrowser.Navigate(@"res://HtmlUI.exe/Sample1.htm", ref flags, ref targetFrame, 
                             ref postData, ref headers);
}

That's all there is to it. Of course, the form class implements the IDocHostUIHandler member functions.

HTML files in resources

You'll notice that I've used the res: protocol in my code. This is a neat way to package your HTML files and other support files - they're in the EXE itself. There are several advantages to this approach: your users cannot easily change the application's user interface, and you don't have to bother with packing more files into your install package.

You just have to make an RC file defining the resource:

HtmlUI.rc

Sample1.htm HTML "Sample1.htm"

This can then be compiled to get a RES file. You can add the RES file to your assembly with the /win32res C# compiler switch.

Handling Document Events

If your application has a DHTML-based user interface, you would definitely need to catch events from elements on the page in order to make it functional. If you run the sample app, you can see that clicking the button on the page shows you a message box. This message box has been invoked from the C# app, not from any scripts on the page. Here's the code:

HtmlUI.cs (partial)

private void WebBrowser_DocumentComplete(object sender, 
                                    AxSHDocVw.DWebBrowserEvents2_DocumentCompleteEvent e)
{
    // Get the document.
    IHTMLDocument2 doc = (IHTMLDocument2)this.WebBrowser.Document;
    
    // Get a reference to the button.
    HTMLButtonElement button = (HTMLButtonElement)doc.all.item("theButton", null);
    
    // Attach the event handler with the events interface.
    ((HTMLButtonElementEvents2_Event)button).onclick += 
                   new HTMLButtonElementEvents2_onclickEventHandler(this.Button_onclick);
}
        
private bool Button_onclick(IHTMLEventObj e)
{
    MessageBox.Show("Alert from the app: Received theButton.onclick!");
    return true;
}

Building the app

I have included a makefile which you can use to build from the command prompt. Just use nmake all to build the app. Note that the tlbimp mshtml.tlb step takes a while.