Access Cookbook, 2nd Edition [Electronic resources] نسخه متنی

Recipe 17.2 Call a .NET Component Containing a Parameterized Constructor

17.2.1 Problem

Attempting to call a .NET class containing a parameterized constructor generates
the compile error "Invalid use of New
keyword". Is there some sort of workaround so that I
can call a .NET class containing a parameterized constructor?

17.2.2 Solution

To see the problem, you will need to follow these steps:

1. Start a Visual Studio .NET command prompt and run the
   RegAsm
   utility program on the Geometry.dll file found in the Geometry\bin
   folder of this chapter's sample files using the
   following syntax (see topic 17.1 for more details on the Visual
   Studio .NET command prompt and running the RegAsm utility):

   regasm Geometry.dll /tlb: Geometry.tlb /codebase

   RegAsm will display a warning about this being an unsigned assembly
   but you can safely ignore the warning.

2. Load the

   17-02.MDB database and open the
   frmCircleDirect form in Design view.

3. From the VBA IDE, select Tools References. At the
   References dialog, select the Geometry component.

4. Close and save the form.

5. Open frmCircleDirect in form view. Enter a numeric
   value into the Radius textbox and click the Calculate button. Access
   should respond with the error shown in Figure 17-3.

Figure 17-3. This compile error is triggered when you attempt to instantiate a .NET class containing a parameterized constructor

6. Click OK. Select Run Reset and close the form.

The Circle class is shown here:

    Public Class Circle
' NOTE: This class contains a 
' parameterized constructor which
' prevents it from being called
' by a COM program. 
Private RadiusVal As Double
Public Sub New(ByVal Radius As Double)
' This constructor takes a parameter
RadiusVal = Radius
End Sub
Public Property Radius( ) As Double
Get
Return RadiusVal
End Get
Set(ByVal Value As Double)
RadiusVal = Value
End Set
End Property
Public Function Area( ) As Double
Return Radius ^ 2 * System.Math.PI
End Function
Public Function Circumference( ) As Double
Return 2 * Radius * System.Math.PI
End Function
End Class

This class is inaccessible from Access because its constructor (the
New subroutine) contains a parameter. The trick to being able to call
the inaccessible class from Access is to
create a helper class that you can use to call the unavailable class.
To create a helper class that you can use to call the Circle class,
follow these steps:

1. Exit Access completely. This is necessary because otherwise Access
   may place a lock on the existing Geometry.tlb type library.

2. Start Visual Studio .NET and load the Geometry project.

3. Open Geometry.vb and add the following class after the Circle
   class's End Class statement:

   Public Class CircleCOM
   Inherits Circle
   Sub New( )
   ' Call base class' constructor
   ' with dummy radius value.
   MyBase.New(1)
   End Sub
   End Class

   Notice that the new class, CircleCOM, inherits
   from the original inaccessible Circle class.

4. Compile the project by selecting Build Build Solution.

5. Start a Visual Studio .NET command prompt and run the RegAsm utility
   program on the updated Geometry.dll file found in the Visual Studio
   Projects\Geometry\bin folder of this chapter's
   sample files using the following syntax (see topic 17.1 for more
   details on running the RegAsm utility):

   regasm Geometry.dll /tlb: Geometry.tlb /codebase

   RegAsm will display a warning about this being an unsigned assembly
   but you can safely ignore the warning.

6. Restart Access and load the

   17-02.MDB database.
   Open the frmCircleUsingHelper form in design view.

7. From the VBA IDE, select Tools References. Verify that the
   Geometry component is selected.

8. Note the source code behind the Calculate command button:

   Private Sub cmdCalculate_Click( )
   Dim cirCOM As Geometry.CircleCOM
   Set cirCOM = New Geometry.CircleCOM
   cirCOM.Radius = txtRadius
   lblMsg.Caption = "Area of circle: " & cirCOM.Area( )
   End Sub

9. Close and save the form.

10. Open frmCircleUsingHelper in form view. Enter a
    numeric value into the Radius textbox and click the Calculate button.
    Access should display the result as shown in Figure 17-4.

Figure 17-4. frmCircleUsingHelper instantiates a helper class, CircleCOM, which calls the inaccessible class, Circle

17.2.3 Discussion

The helper class could have been constructed in a number of ways.
Although we chose to use a derived class, the helper class could also
have been independent of the original class. The helper class could
live within the same component or in a separate component. In this
example, we chose to make the helper class a derived class that lives
in the same component as the inaccessible class.

In this example, you were able to use the CircleCOM class to call the
Circle class. In instantiating the Circle class,
CircleCOM passed a dummy radius value to the
constructor. Because Circle also included a Radius property, you were
able to specify the radius value prior to calling the Area method.
There may be some classes where properties that duplicate the
constructor parameters are not available. In these cases, it may be
difficult if not impossible to create a helper class that is able to
instantiate the inaccessible class for you.

Many of the built-in classes of the .NET Framework contain
parameterized constructors. This means that you will need to create a
lot of helper classes in order to work with these classes.