ACC97: Cannot Call Class Module Methods from MS Access Objects

ID: Q170532

The information in this article applies to:

Microsoft Access 97

SYMPTOMS

Advanced: Requires expert coding, interoperability, and multiuser skills.

When you call a class module method directly from a query, form, report, or macro, you receive an error message.

CAUSE

In order to call a class module procedure, the calling object must initialize an instance of the class. Microsoft Access objects, such as queries, forms, reports, and macros, cannot automatically initialize new instances of a user-defined class. Only a Visual Basic for Applications procedure can initialize a new instance of a user-defined class.

RESOLUTION

There are two possible workarounds.

Method 1

Store the procedure in a standard module if you plan to call it from a query, form, report, or macro. If the procedure is in a standard module, you do not need to create a new instance of a user-defined class every time you call it. This is the recommended method.

Method 2

Create a procedure in a standard module that initializes an instance of the class. The procedure in the standard module then calls the procedure stored in the class module and passes it any necessary arguments. This is typically known as a "wrapper" procedure.

Using a wrapper procedure in this manner is not recommended because additional overhead is created when the object is initialized. In some instances, this can cause more overhead to be created than expected. For instance, calling a wrapper procedure from a query causes additional overhead to be created for each record that the query contains. To make the query more efficient and use less resources, move the code in the class module to a standard module so that the additional overhead can be eliminated.

The following example demonstrates how to create a class module method named MultiplyByTen and a wrapper procedure named CallMultiplyByTen, that makes the class method available to other Microsoft Access objects. It then demonstrates how to call the wrapper procedure from a query.

Open the sample database Northwind.mdb.

On the Insert menu, click Class module.

Type the following line in the Declarations section if it is not already there:
```
        Option Explicit 
```

Type the following procedure:


        Function MultiplyByTen(clsVar As Variant) As Variant
           MultiplyByTen = clsVar * 10
        End Function

Close and save the class module as MultiplyClass.

Create a standard module and type the following line in the Declarations section if it is not already there:
```
        Option Explicit 
```

Type the following procedure:


        Function CallMultiplyByTen(stdVar As Variant) As Variant
           Dim clsMultiply As New MultiplyClass
           CallMultiplyByTen = clsMultiply.MultiplyByTen(stdVar)
        End Function

To test this function, type the following line in the Debug window, and then press ENTER.

?CallMultiplyByTen(5)

Note that the procedure returns the number 50 to the Debug window.

Close and save the module as Module1.

Create a new query based on the Orders table with the following fields:


        Query: Query1
        -----------------------------------------
        Type: Select Query

        Field: OrderID
           Table: Orders
        Field: Freight
           Table: Orders
        Field: EXPR1: CallMultiplyByTen([Freight])

Run the query. Note that the class module method returns a value for each record.

MORE INFORMATION

Steps to Reproduce Behavior

Create a class module method:

Open the sample database Northwind.mdb.

On the Insert menu, click Class module.

Type the following line in the Declarations section if it is not already there:
```
      Option Explicit 
```

Type the following procedure:


      Function MultiplyByTen(clsVar As Variant) As Variant
         MultiplyByTen = clsVar * 10
      End Function

Close and save the class module as MultiplyClass.

Call the class module method from a query:

Create a new query based on the Orders table:


      Query: ClassTestQuery
      -------------------------------------
      Type: Select Query

      Field: Freight
         Table: Orders
      Field: Expr1:MultiplyByTen([Freight])

Run the query. Note that you receive the following error message:

Undefined function 'MultiplyByTen' in expression.

Call the class module method from a form:

Create a new form based on the Orders table:


      Form: ClassTestForm
      -------------------------------------
      ControlSource: Orders Table

      Text box:
      Name: Freight
      Caption: Freight
      ControlSource: Freight
      Text box:
      Name: Text1
      Caption: Text1
      ControlSource: =MultiplyByTen([Freight])

Switch the form to Form view. Note that the error "#Name?" appears in Text1.

Call the class module method from a report:

Create a new report based on the Orders table:


      Report: ClassTestReport
      ---------------------------------------
      ControlSource: Orders Table

      Text box:
      Name: Freight
      Caption: Freight
      ControlSource: Freight
      Text box:
      Name: Text1
      Caption: Text1
      ControlSource: =MultiplyByTen([Freight])

Preview the report. An "Enter Parameter Value" dialog box appears and prompts you to enter the value of MultiplyByTen. Click OK.

Note that Text1 contains "#Error."

Call the class module method from a macro:

Create a new macro:


      Macro Name            Action
      ----------------------------
      ClassTestMacro        MsgBox

      ClassTestMacro Action
      -----------------------------
      MsgBox
         Message: =MultiplyByTen(5)
         Beep: Yes
         Type: None

Save the macro and run it. Note that you receive the following error message:

The expression you entered has a function name that Microsoft Access can't find.

You may also receive the following error message when you call a class module method directly from a macro:

Microsoft Access can't find the name <class name> you entered in the expression.

REFERENCES

For more information about class modules, search the Help Index for "class modules, overview."

For more information about programming with class modules, please see the following article in the Microsoft Knowledge Base:

Q160007 ACC97: Introduction to Stand-Alone Class Module Programming


Keywords          : kbcode kbprg 
Version           : 97
Platform          : WINDOWS 
Issue type        : kbprb

Last Reviewed: April 22, 1999