No need to write this in C# from scratch. What's wrong with using the clipboard? This script ensures that it restores what was on the clipboard when it has finished.
Autohotkey makes this much simpler.
; Hotkey: Ctrl Shift t
^!t::
; Remember what was in the clipboard
clipboardPrev = %clipboard%
; Clear the clipboard
clipboard:=
Sleep,200
; Send a Ctrl C to copy the current selection
SendInput, {Ctrl down}c{Ctrl up}
Sleep,200
; Get the current selection from the clipboard
selectedText=%Clipboard%
if SelectedText =
{
; If the first attempt didn't get any test, try again
Sleep,200
; Send a Ctrl C to copy the current selection
SendInput, {Ctrl down}c{Ctrl up}
; Get the current selection from the clipboard
selectedText=%Clipboard%
}
; Restore the clipboard
clipboard=%clipboardPrev%
MsgBox, %selectedText%
return