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vb6

InStr returns 1 when used with an empty string

I'm working on migrating some old VB6 code to C# at the moment, I've been testing my C# code against the VB code to check that the counterpart methods in each language return the same value.

I'm having problems with this if statement:

If InStr(1, "LPTVJY", strTempTaxCode) <> 0 Then
  strTempTaxCode = "0" & strTempTaxCode
End if

When strTtempTaxCode = "" The value 1 is returned from the InStr(1, "LPTVJY", strTempTaxCode) call. Why is this? As far as I am aware I should be returned 0 as none of the characters from "LPTVJY" are within strTempTaxCode

Solution

  • All strings of nonzero length, including single-character strings, contain the empty string.

    Debug.Print InStr("abc", "")     ' => 1
Debug.Print InStr("a", "")       ' => 1
Debug.Print "abc" & "" = "abc"   ' => True

    If you have trouble wrapping your head around the concept, consider that all numbers have a factor of 1:

    ' (not a real function)
Debug.Print Factors(2)           ' => 1, 2
Debug.Print 2 * 1 = 2            ' => True

    This is known as the Multiplicative Identity Property. You can think of strings as having a similar identity property involving the empty string.

    If you don't want InStr() to match the empty string, do a preliminary test before calling it:

    If Len(strTempTaxCode) > 0 Then
    If InStr(1, "LPTVJY", strTempTaxCode) <> 0 Then
      strTempTaxCode = "0" & strTempTaxCode
    End If
End If