Try-Catch exception handler VB.NET - vb.net

Question

I'm working through a Hair Salon application in VB.NET. Basically the user is able to select the stylist, service(s), age category and enter the number of visits. I then have a Calculate button in order to calculate the total price. I'm required to use Try-Catch for exception handling but I'm not too familiar with it. I'm assuming it's used for error handling, in which case how would I check to see if client visits is greater than 0 and to check to see if the value entered is an integer? Here is what I've tried with the client visits:
Try
(intClientVists > 0)
Exit Try
Catch ex As Exception When
(intClientVists < 0)
Exit Try
End Try
Just hoping someone can steer me in the right direction because I'm not really sure what I'm doing for this particular aspect of the application. Thanks.

Answer 1

Yes, Try/Catch blocks are used for error handling. However, they're not really for this kind of error. Try/Catch blocks are more for errors made by the programmer or the computer (could not allocate memory, could not connect to database, could not open a file, tried to divide by zero, could not cast a value to the specified type) than for errors made by the user (entered a wrong number).
Instead, you want a simple If/Else block:
If intClientVists > 0 Then
'Do something
Else
'Do something else
End If
If you really want to use exception handling for this (again: not normally the best choice), here is how it might look:
Try
If intClientVists < 0 Then Throw New InvalidOperationException()
'Data is OK. Continue processing here
Catch Ex As InvalidOperationException When intClientVisits = 0
'Show one error message
Catch Ex As InvalidOperationException When intClientVisits < 0
'Show different error message
End Try

Answer 2

This MSDN article gives you a list of .NET exceptions that you can use in a Try-Catch. These are actual errors as opposed to input validation checking, like Joel explained.
Basically, you put some logic in the Try block, and if you want to do something in the case of a specific exception, then you catch that exception type and put your logic in that Catch block. Then as a "catch-all" just simply catch "Exception" to do something no matter what type the exception is.
In your particular case, it sounds like the most likely use of a Try-Catch would be if the user inputs the number of visits into a regular text box in which they could enter letters. Here is an example of what you could do for that:
Try
If CInt(txtNumVisits.Text) > 0 Then
'logic here
End If
Catch ex As Exception
'If user entered something other than an integer in that box then an InvalidCastException will be caught
'enter logic here for if that's the case. For example:
MessageBox.Show(Me, "Please enter a number >= 0", "Invalid Input")
txtNumVisits.Focus()
End Try