How to Prevent an Empty TextBox in Visual Basic.Net

How to Prevent an Empty TextBox in Visual Basic.Net, using For Loop and If Statement
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[@maxtertj8775]

Do you know the opposite of Textbox1.text = "" ?
Or how to do this Textbox1.text = not empty ?
Because I want the user to enter characters on the textbox.
Example:
If the user checked this radiobutton(Yes/No), the textbox beside it must be filled.
Just like the question: Are you confident? If yes, Why?

[@DJOamen]

Textbox1.text = Not vbEmpty
Textbox1.text = vbEmpty

[@maxtertj8775]

@@DJOamen Thank you, I will try this.