How to Compare User-Defined Type Variables in Visual Basic

The relational operators (=, <>, and so on) do not support the comparison of user-defined type variables. However, you can compare user-defined type variables by converting the variables to strings, and then comparing the strings. The Windows version 3.1 API hmemcpy can be used to convert a user-defined type variable to a string.

The hmemcpy API was introduced in Microsoft Windows version 3.1, so this technique requires Windows version 3.1 or later.

If you attempt to compare user-defined type variables using the relational operators, the error "Type mismatch" is displayed.

The following steps demonstrate how to compare user-defined type variables by first converting the variables to strings and then comparing the strings by using the relational operators.

Step-by-Step Example

1. Start Visual Basic or from the File menu, choose New Project (ALT, F, N) if Visual Basic is already running. Form1 is created by default.
2. Enter the following code into the global module:

      Type myType
         f1 As String * 2
         f2 As Single
      End Type
   
      ' Enter the following Declare statement entirely as one, single line:
      Declare Sub hmemcpy Lib "kernel" (hpvDest As Any, hpvSource As Any,
         ByVal cbCopy As Long)
   						

3. Enter the following code into the general Declarations section of Form1:

      ' type2str converts a user-defined type variable to a string.
      Function type2str (t As myType) As String
         Dim s As String
         s = Space$(Len(t))
         Call hmemcpy(ByVal s, t, Len(t))
         type2str = s
      End Function
   						

4. Enter the following code into the Form1 Click event procedure:

      Sub Form_Click ()
         Dim x As myType
         Dim y As myType
   
         x.f1 = "ab"
         x.f2 = 2
         y = x
   
         If type2str(x) = type2str(y) Then
            Print "x = y"
         Else
            Print "x <> y"
         End If
   
         y.f1 = "ba"
         If type2str(x) > type2str(y) Then
            Print "x > y"
         Else
            Print "x <= y"
         End If
      End Sub
   						

5. Press the F5 key to run the program.

The program prints "x = y" and "x <= y" on Form1.