HOWTO: Cast an Object to a Different Interface

ID: Q168830

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 6.0
Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0

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

Create a new project with the following objects:


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

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

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

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

Add a CommandButton to the lower edge of the form.

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

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

Keywords : kbGrpVBDB
Version :
Platform : WINDOWS
Issue type :