You could capture the KeyPress Event of the TextBox and allow the user to only enter what you want. You check the value of the KeyPressEventArgs. I usually use it so I don't have to code the ooops you didn't enter the proper value sub/functions. There is also a nice RegEx pattern you can use to match a good value when the user is finished. Let me know if you are interested.
In the key press event of the text box try writing the below code....it will allow user to enter only number and when the user will enter alphabet or any other characters it wont allow user to enter it....
'accepts only numbers
If (Microsoft.VisualBasic.Asc(e.KeyChar) < 48) Or (Microsoft.VisualBasic.Asc(e.KeyChar) > 57) Then
e.Handled = True
If (Microsoft.VisualBasic.Asc(e.KeyChar) = 8) Then
e.Handled = False