I think you mean that if the user types the name of a function in the textbox and presses a button, that the text he typed should be treated as code and executed, yes?
If so, then it is possible, but I would very strongly recommend against it. What if what they type is
File.Delete(@"%USERPROFILE%\*.*");
Would it be a good idea to execute that? And there are much, much worse things they could do...
Please, think again about what you want to do - there are some programs you shouldn't let your mates get anywhere near...