ACC2000: Cannot Call Class Module Methods from MS Access Objects

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

In order to call a class module procedure, the calling object must initialize an instance of the class. 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.

The following are two possible workarounds:

Method 1

If you plan to call the procedure from a query, form, report, or macro, then store it in a standard module. Having it in a standard module avoids the 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 creates more overhead than expected. For instance, calling a wrapper procedure from a query creates additional overhead for each record contained in the query. To increase query efficiency and to lessen resource usage, move the code in the class module to a standard module; this will eliminate additional overhead.

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

1. Open the sampe 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 Immediate window, and then press ENTER.

    ?CallMultiplyByTen(5)
   					

   Note that the procedure returns the number 50 to the Immediate 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.

Steps to Reproduce Behavior

CAUTION: If you follow the steps in this example, you modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and follow these steps on a copy of the database.

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.

Call the Class Module Method from a Query

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.

Call the Class Module Method from a Form

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

      Form: ClassTestForm
      -------------------------
      Caption: TestForm
      ControlSource: Test 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.

Call the Class Module Method from a Report

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: You see Text1 contains "#Error."

Call the Class Module Method from a Macro

1. Create a new macro:

      Macro Name            Action
      --------------------------------
      ClassTestMacro        MsgBox
   
      ClassTestMacro Action Arguments
      --------------------------------
      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.

For more information about class modules, in the Visual Basic Editor, click Microsoft Visual Basic Help on the Help menu, type class modules in the Office Assistant or the Answer Wizard, and then click Search to view the topic.

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