Can't catch delete key in extended richtextbox

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VS2008

C#

.NET

Visual-Studio

I've been writing a small extension app (to run under Revit Architecture) using the .NET framework and VS2008 express. I've created a small editor that uses Delcan Brennan's rich textbox Extended class. However I have noticed that when I press the delete key the key event falls back through to the Revit program instead of deleting a character in the textbox.

I've tried setting the KeyPreview property of the form and also overriding the PreviewKeyDown and KeyDown events and IsInputKey of the richTextBoxExtended component and form but I still can't make it catch the delete key. My form is running non-modal.

I've Goggle searched this but all the posts are concerning applications and not extensions. I can't see that making a difference however.

Can someone point me to a tutorial or resource on this?