If you want to display the contents of a file then you have to get the contents of that file as a String, which you can then assign to the Text property of the TextBox. Just don't do the last bit. This is lijke saying you want to make a pizza but you don't want it in a box. To get a pizza in a box you have to make the pizza first. If you don't want it in a box then just don't put it in a box.

You already know how to use an OpenFileDialog from your other thread. To read a text file just call IO.File.ReadAllText.