is what is possible when one selects one word in a textbox, every similar word will be displayed in a MessageBox.Show.
The principle is to retrieve those similar words and the user can see.
Of course it's possible! The problem is how do YOU define "similar". Once you have that, YOU have to write the code to get the currently selected text in the textbox, then search the textbox contents for the "similar" works.
Nope. I've got my own pile of code to write and I'm not getting paid to write yours too.
You first need to define exactly what you mean by "similar" and then probably do some research on "Regular Expressions".