DOCUMENT:Q168830  11-JAN-2001  [vbwin]
TITLE   :HOWTO: Cast an Object to a Different Interface
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:
OPER/SYS:
KEYWORDS:kbGrpDSVBDB

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The information in this article applies to:

 - Microsoft Visual Basic Learning Edition for Windows, version 6.0 
 - Microsoft Visual Basic Professional Edition for Windows, version 6.0 
 - Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
 - Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 
 - Microsoft Visual Basic Learning Edition for Windows, version 5.0 
 - Microsoft Visual Basic Professional Edition for Windows, version 5.0 
 - Microsoft Visual Basic Enterprise Edition for Windows, version 5.0 
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SUMMARY
=======

When using an object that has multiple interfaces, you may be faced with
assigning it to multiple variables of different object types to get at relevant
interfaces. This is made more difficult by having to keep track of the different
variables and clean them up properly. This article gives code for a simple
function to allow casting an object to a different interface without the need to
create additional object variables or keep track of them.

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.

REFERENCES
==========

Microsoft Visual Basic Help topic "Implements Statement"

Additional query words: kbVBp500 kbVBp600 kbdse kbDSupport kbVBp

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Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbZNotKeyword3
Issue type        : kbhowto

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