Microsoft KB Archive/170532

= ACC97: Cannot Call Class Module Methods from Microsoft Access Objects =

Article ID: 170532

Article Last Modified on 1/20/2007

-

APPLIES TO


 * Microsoft Access 97 Standard Edition

-



This article was previously published under Q170532



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



SYMPTOMS
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.</li>  Create a standard module and type the following line in the Declarations section if it is not already there:

<pre class="fixed_text">       Option Explicit </li>  Type the following procedure:

Function CallMultiplyByTen(stdVar As Variant) As Variant Dim clsMultiply As New MultiplyClass CallMultiplyByTen = clsMultiply.MultiplyByTen(stdVar) End Function </li> 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.</li> Close and save the module as Module1.</li>  Create a new query based on the Orders table with the following fields:

<pre class="fixed_text">       Query: Query1 -       Type: Select Query

Field: OrderID Table: Orders Field: Freight Table: Orders Field: EXPR1: CallMultiplyByTen([Freight]) </li> Run the query. Note that the class module method returns a value for each record.</li></ol>

<div class="moreinformation_section">

Steps to Reproduce Behavior
Create a class module method:

<ol> Open the sample database Northwind.mdb.</li> On the Insert menu, click Class module.</li>  Type the following line in the Declarations section if it is not already there:

<pre class="fixed_text">     Option Explicit </li>  Type the following procedure:

Function MultiplyByTen(clsVar As Variant) As Variant MultiplyByTen = clsVar * 10 End Function </li> Close and save the class module as MultiplyClass.</li></ol>

Call the class module method from a query:

<ol>  Create a new query based on the Orders table:

<pre class="fixed_text">     Query: ClassTestQuery -     Type: Select Query

Field: Freight Table: Orders Field: Expr1:MultiplyByTen([Freight]) </li> Run the query. Note that you receive the following error message:

Undefined function 'MultiplyByTen' in expression.

</li></ol>

Call the class module method from a form:

<ol>  Create a new form based on the Orders table:

<pre class="fixed_text">     Form: ClassTestForm -     ControlSource: Orders Table

Text box: Name: Freight Caption: Freight ControlSource: Freight Text box: Name: Text1 Caption: Text1 ControlSource: =MultiplyByTen([Freight]) </li> Switch the form to Form view. Note that the error "#Name?" appears in Text1.</li></ol>

Call the class module method from a report:

<ol>  Create a new report based on the Orders table:

<pre class="fixed_text">     Report: ClassTestReport ---     ControlSource: Orders Table

Text box: Name: Freight Caption: Freight ControlSource: Freight Text box: Name: Text1 Caption: Text1 ControlSource: =MultiplyByTen([Freight]) </li> <li>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."</li></ol>

Call the class module method from a macro:

<ol> <li> Create a new macro:

<pre class="fixed_text">     Macro Name            Action ClassTestMacro       MsgBox

ClassTestMacro Action -     MsgBox Message: =MultiplyByTen(5) Beep: Yes Type: None </li> <li>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 you entered in the expression.

</li></ol>

<div class="references_section">