How do I use Do Until with a previous Try Catch Exception to avoid running the code twice - vb.net

Question

i have this line of code to catch an exception if a letter is inputed, or if it is out of rang as a number, but I have added WHEN to avoid catching numberical data. Now how can I use an exception error to use it before my case statement in order to avoid running the code twice, cause once the case codes has been through it will run a clear txtbox which is already taken care by the try catch, don`t if thats clear for you but i understand it. here is the code in parts...
Try
'Integer Levels: intLvls is egual to the assigned text box, the first one from
'the top, this line of code allow the user input to be captured into a variable.
intLvls = txtBoxLvl.Text
Catch ex As Exception When IsNumeric(intLvls)
ErrTypeLetterFeild1()
Finally
analysingvalues1()
End Try
WHAT I WOULD LIKE TO DO: Use loop until refencing the exception error to avoid running this following part of the code:
Private Sub analysingvalues1()
Do Until IsNumeric (ex As Exception)<------how do i do this???
Loop
the case part of the code:
Select Case intLvls
'User is prompt with the following label: lblLvl "Level of salespersons 1 - 4"
'to make a choice from 1 to 4 as available values.
Case 1 To 4
'This line regulates the range of acceptable values, first textbox: must be egual
'or higher than 1 and lower or egual to 4. Upon such rules a validation becomes
'correct and is directed to the isValidCalculation sub.
isValidCalculation()
Case Is < 1
ErrType1NumberRangeFeild()
Case Is > 4
ErrType1NumberRangeFeild()
Case Else
If txtBoxLvl.Text = "" Then
ErrTypeClear1()
Else
If Not IsNumeric(txtBoxLvl.Text) Then
ErrType1NumberRangeFeild()
Else
ErrTypeLetterFeild1()
ErrTypeClear1()
End If
End If
End Select 'Ending choices.
End Sub
Tks for your help!

Answer 1

If you turn on Option Strict this:
intLvls = txtBoxLvl.Text
Will no longer compile. This should tell you that your doing something smelly.
Turn on Option Strict
The correct solution is not to blindly allow the runtime to cast string to int for you, and catch the exceptions.
When you are converting string user input to an integer, bad input is not an exceptional condition, it is something you should expect and code defensively for.
I would rewrite it to something like this:
'Integer Levels: intLvls is egual to the assigned text box, the first one from
'the top, this line of code allow the user input to be captured into a variable.
if integer.TryParse( txtBoxLvl.Text, intLvls )
analysingvalues1()
else
ErrTypeLetterFeild1()
Edit - As pointed out by Chris below, I meant Option Strict. I recommend using but Explicit and Strict, and Infer if available.