Functions in VB6 are often used to perform data manipulation
on numbers or strings and return the result. Suppose there is the possibility
of an error in the function—not a run-time error but a program-definer error
such as invalid data. If you want to return a value from the function that
signals an error to the calling code, you simply use the CVErr
and IsError functions.

The CVErr function converts a type
Variant into the subtype Error. You can detect this with the IsError function. Here’s the strategy:

  1. Declare
    your function with a Variant return type, which will work for both string
    and number data.
  2. If
    the function executes properly, assign the return value as usual:
MyFunction = value
  1. If
    there’s a problem and you need to return an error value, use CVErr to return an error with some numerical value:
MyFunction = CVErr(SomeNumericalValue)
  1. In
    the calling program, use IsError like this:
ReturnValue = MyFunction()

If IsError(ReturnValue) Then
    ' Deal with error here.

Else
    ' Deal with non-error return value here.

End If

The ability to signal an error with a function’s return
value is very useful in some situations.

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