ACC97: Cannot Call Class Module Methods from MS Access Objects

• 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.

This article assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to the "Building Applications with Microsoft Access 97" manual.

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.

1. Open the sample database Northwind.mdb.

2. On the Insert menu, click Class module.

3. Type the following line in the Declarations section if it is not already there:

   Option Explicit

4. Type the following procedure:

   Function MultiplyByTen(clsVar As Variant) As Variant

             MultiplyByTen = clsVar * 10
          End Function
   
   

5. Close and save the class module as MultiplyClass.

6. Create a standard module and type the following line in the Declarations section if it is not already there:

   Option Explicit

7. Type the following procedure:

   Function CallMultiplyByTen(stdVar As Variant) As Variant

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

8. 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.

9. Close and save the module as Module1.

10. 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])
    
    

11. 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:

1. Open the sample database Northwind.mdb.

2. On the Insert menu, click Class module.

3. Type the following line in the Declarations section if it is not already there:

         Option Explicit
   

4. Type the following procedure:

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

5. Close and save the class module as MultiplyClass.

1. Create a new query based on the Orders table:

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

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

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

         Undefined function 'MultiplyByTen' in expression.
   

1. 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])
   

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

1. 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])
   

2. 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."

1. Create a new macro:

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

2. 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: