I'm basically looking for a replacement for RichTextBox or TextBox components that is fully written in c# without using pinvokes in order to keep cross platform compatibility.
RichTextBox contains for example functionality to highlight URLs in the text and provide a handler for event when user clicks on the link. But there is no way to specify what regex is used for that highlighting so there is no way to add support for URI schemes other than http/https.
why would anyone provide a html component thats only compatible with c# developers.
This might be a little overkill for what you're looking for, but have you considered SyntaxEditor from Actipro? (http://www.actiprosoftware.com/Products/DotNet/WindowsForms/SyntaxEditor/Default.aspx)
Yes, it is a commercial (i.e. not free) product, but it is capable of doing what you wanted, and more. Its 100% managed code, and supports some language plugins, e.g. a web-language add-in.
Might be worth a look.
Mono's RichTextBox is written in C# and is open source:
It will take some work to separate it from Mono and get it running the way you want it, but it's better than starting from scratch.
The text editors with SharpDevelop and MonoDevelop may be useful.