ICSharpCode.TextEditor.TextArea.SelectionManager provides following members:

ClearSelection()	Removes the current selection
IsSelected(int offset)	Returns true, if the offset is in the selected range.

HasSomethingSelected	Returns true, if there is something selected in the TextArea, false if not.
SelectedText	Returns the currently selected text.
SelectionCollection	A collection of ISelection objects, which each represents a selection. Currently maximal 1 selection is possible.
SelectionIsReadonly	Returns true, if the current selection is readonly, false if not.