How To Cast an Object to a Different Interface

MORE INFORMATION

The sample code below illustrates a common problem when utilizing objects that have multiple interfaces:

   Dim Shape As IShape, C As ICircle, S As ISquare
   Set C = New ICircle
   Set Shape = C
   Shape.SetXY 100, 100
   C.Radius = 50
   Shape.Draw Me
   Set C = Nothing
   Set S = New ISquare
   Set Shape = S            ' Critical line
   Shape.SetXY 300, 200
   S.Side = 50
   Shape.Draw Me
   Set Shape = Nothing
   Set S = Nothing
				

If you forget to set the Shape object to S on the "Critical line," you will still be manipulating the Circle object in subsequent code. In addition, it's difficult to tell you are using the Shape object to get at additional interfaces or what object it's currently set to.

To clear up this problem, create global functions in your application to cast the variables to their alternate interfaces. The example above requires only one function to cast to the IShape interface:

   Function IShape(oShape As IShape) As IShape
     Set IShape = oShape
   End Function
				

You can then reference this function to perform the casting:

   IShape(C).SetXY 100, 100
   C.Radius = 50
   IShape(C).Draw Me
				

Or more efficiently:

   With IShape(C)
     .SetXY 100, 100
     C.Radius = 50
     .Draw Me
   End With
				

You no longer have to define, set, or otherwise keep track of additional variables for casting purposes. Giving the function the same name as the interface helps with program clarity.

Step-by Step Example

1. Create a new project with the following objects:

      Type            Name
      ---------------------------
      Class Module    IShape
      Class Module    ICircle
      Class Module    ISquare
      Form            Form1
   						

2. Type the following code in the IShape Class Module:

         Option Explicit
   
         Public Sub SetXY(ByVal NewX As Long, ByVal NewY As Long)
         End Sub
   
         Public Sub Draw(oDrawSpace As Object)
         End Sub
   						

3. Type the following code into the ICircle Class Module:

         Option Explicit
         Implements IShape
   
         Dim X As Long, Y As Long, iRadius As Long
   
         Private Sub IShape_Draw(oDrawSpace As Object)
           oDrawSpace.Circle (X, Y), iRadius
         End Sub
   
         Private Sub IShape_SetXY(ByVal NewX As Long, ByVal NewY As Long)
           X = NewX
           Y = NewY
         End Sub
   
         Public Property Let Radius(ByVal NewVal As Long)
           iRadius = NewVal
         End Property
   						

4. Type the following code in the ISquare Class Module:

         Option Explicit
         Implements IShape
   
         Dim X As Long, Y As Long, Side As Long
   
         Private Sub IShape_Draw(oDrawSpace As Object)
           oDrawSpace.Line (X, Y)-(X + Side, Y + Side), , B
         End Sub
   
         Private Sub IShape_SetXY(ByVal NewX As Long, ByVal NewY As Long)
           X = NewX
           Y = NewY
         End Sub
   
         Public Property Let SideLength(ByVal NewVal As Long)
           Side = NewVal
         End Property
   						

5. Add a CommandButton to the lower edge of the form.
6. Type the following code in the form module:

         Option Explicit
   
         Private Sub Command1_Click()
   
         Dim C As ICircle, S As ISquare
           Set C = New ICircle
           Set S = New ISquare
         ' Manipulate circle shape (using With)
           C.Radius = 250
           With IShape(C)
             .SetXY 400, 400
             .Draw Me
           End With
         ' Manipulate square shape (without using With)
           IShape(S).SetXY 700, 300
           S.SideLength = 300
           IShape(S).Draw Me
         End Sub
   
         Function IShape(oShape As IShape) As IShape
           Set IShape = oShape
         End Function
   						

7. Run the project and click the CommandButton. You will see a circle and a square drawn on the form background.