Loading HTML Content Into A WebBrowser Control From Memory?

WebBrowser Example.zip IntroductionBecause the WebBrowser control that we use in .NET is a COM control, not all of its uses are straightforward and some of them (even those which seem like they should be easy) require that we dip into our Interop toolbox in order to properly implement them.

A perfect example of this is loading HTML content into the WebBrowser from memory, rather than a file or a URL. Anyone who's ever used the WebBrowser control before is familiar with the Navigate2 method, which tells the control to load content from a URL (or path to a file). Loading HTML content from memory, however, is a rather elusive practice because of the many steps involved in making it work.

MSHTML.HTMLDocumentYou might notice that the WebBrowser control exposes a "document" property. The object returned by this property can be coerced to the type of "mshtml.HTMLDocument" (you must add a reference to MSHTML to your project in order to make this work) as follows:

Code:Dim clsDocument As mshtml.HTMLDocument = CType(WebBrowser.Document, mshtml.HTMLDocument)

(NOTE: You will have to add a reference to the COM library MSHTML to your project to make this compile)

Once we create an instance of HTMLDocument, a whole new world opens up to us, providing all sorts of DOM access to the content of any given Web page.

If we were to create our own HTMLDocument object from memory, we could use the "write" method to write HTML content to the document from a string variable, like this:

Code:'initialize the document object within the HTMLDocument class... clsDocument.close() clsDocument.open("about:blank")

'write the HTML to the document using the MSHTML "write" method... Dim clsHTML() As Object = {sHTML} m_clsDocument.write(clsHTML) clsHTML= Nothing

WebBrowser Control ImplementationUsing the HTMLDocument returned by the "document" property of the WebBrowser control, however, is not as straight-forward. Because of the way that this object is created and initialized in memory (by the COM WebBrowser control), the "write" method fails when called as above. In order to write content to the HTMLDocument exposed by the WebBrowser control, we must first marshal the string value to a memory space that is compatible with COM. Once the string is properly marshalled, the COM interface IPersistStreamInit (implemented by the HTMLDocument class) must be used to pass the value into the object.

Interop DeclarationsIn order to pull all of this off, we must declare several Interop pieces, including an enumeration, a function, and two interfaces. The declarations for these pieces are as follows:

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