I'm trying to replace the specific text (space or special text) to tab.
First, the text is selected by NSelection. 
But it does not have method for InsertTab.
How can I do this?

Hi Junghyron,

You can use the following code insert a tab at the currently selected text selection:

richTextView.View.Selection.InsertText("\t");

If the selection object currently selects a range then the text will be replaced by a tab character. 

We hope this helps - let us know if you meet any problems.

Best Regards,
Nevron Support Team