Often a Visual Basic program is required to do calculations, and these calculations cause errors if all of the variables involved in the calculation are not defined. In some cases the calculations are being performed on data being input by a user in an input text field.

Since the user will typically type in character at a time instead of pasting in a complete numerical representation, and can often type in incorrect characters there needs to be a way to clear the errors caused when the data in the entry field won't work with the code that does the calculations.

The following example assumes the use of the text field's change event to trigger the calculations. This means that every time the data in any input field involved in the calculation changes, the code for the calculation runs.

For this example, put three text boxes and a label on your form, and put the following code in each of the input field's change events.

Private Sub Text1_Change()
      TheAnswer = Text1 + Text2 + Text3
      lblOut.Caption = CStr(TheAnswer)
End Sub

Now try typing some values into the input fields. It works ok until you try something like typing in a decimal only value, starting with the decimal as the first character typed into one of the fields, or hit the wrong key and enter a text character instead of a numerical digit. How do we fix this?

Simple, just add an error trap and clear the error in the error handling routine. Rewrite the change event code blocks for each of the text fields to look like the following one, and give it a try.

Private Sub Text1_Change()
      On Error GoTo Clear
      TheAnswer = Text1 + Text2 + Text3
      lblOut.Caption = CStr(TheAnswer)
Exit Sub
Clear:
    On Error GoTo 0
    Resume Next
End Sub

Now if the attempt to calculate raises an error, it is cleared by the On Error GoTo 0 in the error handling routine, and the Resume Next causes execution of your program to continue with the code after the statement which caused the error. The assignment of the value of the calculation's result is the next statement in this case.

The preceding example code assumes the use of a Public variable TheAnswer explicitly declared in the declarations section of the form the text boxes are on. For purposes of this example, declare it to be be of type Double.