Base Class and Derived Object Variants

Introduction.

Last week, we explored an example where a Base Class Object was passed through a Set Property Procedure, allowing it to become part of another object in memory. The passed object essentially became an extension or child object of the main object. In that earlier program, we passed the child object to the target object during the instantiation phase of our test program and then assigned values to the child object’s properties later in the code.

In the next example, we will take a slightly different approach.

For those who would like to go through the earlier Articles on the MS-Access Class Module, the links are given below:

• MS-Access Class Module and VBA

• MS-Access VBA Class Object Arrays

• MS-Access Base Class and Derived Objects

• VBA Base Class and Derived Object-2

This time, we will open both objects—ClsArea (the base class) and ClsVolume2 (the target class)—separately in our test program. We will assign values to the ClsArea base class properties before passing it to the ClsVolume2 target class object. Remember, the ClsVolume2 class has only one property, p_Height, and its Volume() method requires the Length and Width values from the base class ClsArea to calculate the volume.

1. Copy and paste the following sample Test Code into a Standard Module.

   The SetNewVol2_2 Procedure.

   Public Sub SetNewVol2_2()
   'Method 2/2
   Dim CA As ClsArea
   Dim Vol As ClsVolume2
   
   Set CA = New ClsArea
   Set Vol = New ClsVolume2
   
   CA.strDesc = "Bed Room"
   CA.dblLength = 90
   CA.dblWidth = 10
   Stop
   
   
   'Here ClsArea class Object CA is passed to the 
   ‘Property procedure Set CArea of ClsVolume2 object Vol
   Set Vol.CArea = CA 'Pass ClsArea obj to ClsVolume2
   
   Vol.dblHeight = 10 'assign height to ClsVolume2
   
   
   Debug.Print "Description", "Length", "Width", "Area", "Height", "Volume"
   With Vol.CArea
     Debug.Print .strDesc, .dblLength, .dblWidth, .Area(), Vol.dblHeight, Vol.Volume()
   End With
   Stop
   
   Set CA = Nothing
   Set Vol = Nothing
   
   End Sub
   

   VBA Code Review.

   In the first Dim statement, CA is declared as a ClsArea object and Vol as a ClsVolume2 object. The next two statements instantiate these objects in memory.

   The following three statements assign values to the properties of the ClsArea object.

   A Stop statement is then used to pause code execution, allowing us to inspect the property values of the object in the Locals window.

   Next, the statement Set Vol.CArea = CA assigns the ClsArea object (CA) as a child object of the ClsVolume2 object (Vol).

   After that, the dblHeight property of the ClsVolume2 object is assigned the value 10.

   The subsequent statements, placed before the next Stop statement, print the property values from memory to the Debug window.

   Finally, the last two Set statements release both objects from memory before the program ends.

   Display the Locals Window.

2. Inspecting the Locals Window

   1. Open the Locals Window
      From the View menu in the VBA Editor, select Locals Window.

   2. Run the Code

      • Click anywhere in the middle of the code window.

      • Press F5 to run the program until it pauses at the Stop statement.

      • Alternatively, press F8 to run the code step by step, which lets you observe the changes in the Locals Window at each step.

   3. Expand the Objects
      Click the [+] symbol next to the object names in the Locals Window to expand and display their properties and current values.

   4. Observe Object References

      • Check the CArea and p_Area object references under the Vol object.

      • At this point, their values will show as Nothing because we have not yet passed the CA object to the Vol object.

   5. Continue Running the Code

      • After reviewing the Locals Window, run the code until it pauses at the next Stop statement.

      • Now, the CArea Set Property procedure assigns the p_Area object reference to the ClsArea object, linking it into the ClsVolume2 object.

   ---

   Next, we will try another variation of this example using the same two classes — ClsArea and ClsVolume2 — to demonstrate a slightly different approach.

New Class Module ClsVolume3.

1.  Insert a new Class Module and change its name Property Value to ClsVolume3.

2.  Copy and Paste the following VBA Code into the ClsVolume3 Class Module:

Option Compare Database
Option Explicit
'Method three 
Private p_Height As Double
Public p_Area As ClsArea

Public Property Get dblHeight() As Double
    dblHeight = p_Height
End Property

Public Property Let dblHeight(ByVal dblNewValue As Double)
    p_Height = dblNewValue
End Property

Public Function Volume() As Double
    Volume = p_Area.dblLength * p_Area.dblWidth * Me.dblHeight
End Function

Private Sub Class_Initialize()
    Set p_Area = New ClsArea
End Sub

Private Sub Class_Terminate()
    Set p_Area = Nothing
End Sub

In the code, p_Height is declared as private property, while p_Area is declared as a public ClsArea object within the ClsVolume3 class. This means p_Area appears as a property of the ClsVolume3 class, with its own accessible properties and methods for direct Get/Let operations in the user program (from a standard module). Although the ClsArea object is exposed as a public property of ClsVolume3, its internal properties and methods remain encapsulated within the ClsArea class itself.

It is important to ensure that the ClsArea class is fully developed and free of errors before using it inside other classes.

The Class_Initialize() and Class_Terminate() routines handle the lifecycle of the embedded object: The ClsArea object is instantiated in Class_Initialize() when a ClsVolume3 object is created, and released from memory in Class_Terminate() When the user program ends.

The Testing Program.

The sample Test VBA Code is given below.

Copy and paste the code into the Standard Module.

Public Sub SNewVol3()
'Here ClsArea class is declared as a Public Property of ClsVolume3
Dim volm As ClsVolume3

Set volm = New ClsVolume3

volm.p_Area.strDesc = "Bed Room"
volm.p_Area.dblLength = 15 'assign length
volm.p_Area.dblWidth = 10 'assign width in clsArea
volm.dblHeight = 10 'assign height to ClsVolume2

Debug.Print "Description", "Length", "Width", "Area", "Height", "Volume"
With volm.p_Area
   Debug.Print .strDesc, .dblLength, .dblWidth, .Area, volm.dblHeight, volm.Volume
End With
Set volm = Nothing

End Sub

Display the Locals Window (View -> Locals Window), if it is not already open.

Click somewhere in the middle of the code and press F8 to execute the VBA Code one line at a time and watch the Local Window to track what happens at each step.

All the above variants of the ClsVolume Class have been written with less Code, except the first example of the ClsVolume Class.  

Working with the Recordset Object.

Next week, we will work with a built-in Object 'DAO.Recordset' and build a Class Module to:

1. Calculate and update a Field,

2. Sort the Data,

3. Print the sorted data in the Debug Window,

4. And create a Clone of the Table with sorted data.

That is a lot of action next week.

List of All the Links on this Topic.

Earlier Post Link References:

1. MS-Access Class Module and VBA
2. MS-Access VBA Class Object Arrays
3. MS-Access Base Class and Derived Objects
4. VBA Base Class and Derived Objects-2
5. Base Class and Derived Object Variants
6. Ms-Access Recordset and Class Module
7. Access Class Module and Wrapper Classes
8. Wrapper Class Functionality Transformation
9. Ms-Access and Collection Object Basics
10. Ms-Access Class Module and Collection Object
11. Table Records in Collection Object and Form
12. Dictionary Object Basics
13. Dictionary Object Basics-2
14. Sorting Dictionary Object Keys and Items
15. Display Records from Dictionary to Form
16. Add Class Objects as Dictionary Items
17. Update Class Object Dictionary Item on Form

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Opening Multiple Instances of Form in Memory

Introduction.

For the last few weeks, we have been through learning the usage of dot (.) separator and exclamation symbol (!) in VBA object references.  Now, we will explore some interesting tricks with Forms in VBA.  How to Call a Function Procedure embedded in a Form Module (Class Module), from a program outside the Form?

We will explore two different aspects of this particular topic.

1. How do we open several instances of a single Microsoft Access Form in memory, displaying different information on each of them?

   A sample screenshot of two instances of the Employees Form is given below for reference.  The first form is behind the second instance of the form, displaying employee details of ID: 4 & 5.   Click on the image to enlarge the picture. 

   
   
   

   Calling the Form's Class Module Public Function.

2. How to call a Function Procedure on the Form's Class Module, from outside the Form?

   Call the Function from a Standard Module,  from the Module of another form, or from the VBA Debug Window (Immediate Window).  The target form must be opened in memory in order to call the function procedure of the form from outside.

Function Procedures in a Form module are helpful to avoid duplication of code. It can be called from subroutines in different locations on the same Form, from a command button click, or from some other Event Procedures of the Form.  The function procedure in a Form Module can be anything that does some calculation, validation check, updating the information, or a stand-alone search operation procedure, like the one we are going to use on our sample Employees Form.

All the Event Procedures on a Form Module are automatically declared as Private Subroutines and they all will start with the beginning and end Statements, like the sample statements given below.   Our own VBA code that does something will go within this block of codes:

Private Sub Command8_Click()
.
.
End Sub

The scope of Private declared Subroutine/Function stays within that module and cannot be called from outside.  The private declaration is absolutely necessary to avoid a procedure name clash with the same name in another Class Module or Standard Module.  The Form's Function Procedure must be declared as Public in order to call it from outside the Form.

To perform a trial run of the above trick you need the Employees Table and a Form.

1. Import Employees Table from Northwind sample database.
2. Click on the Employees Table to select it.
3. Click on Create Ribbon.
4. Select the Form option and create a Form, for Employees Table, in the format shown above.
5. Save the Form with the name frmEmployees.
6. Open the frmEmployees Form in Design View.  Set the Form Property Has Module Value to Yes.
7. Select the Design Menu and select VBA Code from the Tools button group, to open the Form Module.

8. Copy the following VBA code and Paste them into the VBA Module of the Form.

   Public Function in Form ClassModue.

   Public Function GetFirstName(ByVal EmpID As Integer) As String
   Dim rst As Recordset, crit As String
   Dim empCount As Integer
   
   'get total count of employees in the Table
   empCount = DCount("*", "Employees")
   
   'validate employee code
   If EmpID > 0 And EmpID <= empCount Then
   
       crit = "ID = " & EmpID
       Set rst = Me.RecordsetClone
       rst.FindFirst crit
       
       If Not rst.NoMatch Then
             
        	Me.Bookmark = rst.Bookmark
           GetFirstName = rst![First Name]
       End If
       
          rst.close
          Set rst = Nothing
       
     Else
       MsgBox "Valid Employee IDs: 1 to " & empCount
   End If
   
   End Function
   

9. Save and Close the Form.

Have you noticed the starting line of the above Function, which is declared as Public?

The Function GetFirstName() accepts a parameter EmployeeID value, finds that record and will make that record current on the form. The Function returns the First Name of the Employee to the calling program if the search was successful.  If the search operation fails, then it gives a warning message, saying that the employee ID code passed to the function is not within the range of ID codes available in the Employees table.

Now, we need another program, in the Standard Module, to run the search function GetFirstName() from the frmEmployees Form Module.  Besides that this program demonstrates how to create more than one instance of a Microsoft Access Form and open them in memory, to access their properties, methods, or control contents.

1. Open VBA Editing Window (Alt+F11).
2. Select the Module option from Insert Menu and add a new Standard Module.

3. Copy and paste the following VBA Function code into the new Module.

   Call GetFirstName() from Standard Module.

   Public Function frmInstanceTest()
   
         Dim frm As New Form_frmEmployees '1st Form instance
   
         Dim frm2 As New Form_frmEmployees '2nd instance declaration
   
         Dim Name1 As String, Name2 As String
   
     frm.Visible = True 'make the instance visible in Application Window
     frm2.Visible = True '2nd instance visible
   
     Name1 = frm.GetFirstName(4) 'Call the GetFirstName of Employee ID 4
     
     Name2 = frm2.GetFirstName(5) ''Call the GetFirstName of Employee ID 5
   
   'pause execution of this code to view
   'the Employees Form instances in Application Window.
   
   Stop
   
     MsgBox "Employees " & Name1 & ", " & Name2
     
   End Function

Trial Run of Function frmInstanceTest()

Let us run the code and view the result in Application Window.

1. Click somewhere within the body of the frmInstanceTest() function and press the F5 key to run the code.

   The program will pause at the Stop statement and this will facilitate the viewing of the Application window, where the frmEmployees Form instances are open in normal view mode, one overlapping the other.

2. Press Alt+F11 to display the Application Window displaying both instances of the Form, the second form overlapping the first one.
3. Click and hold on to the title bar area of the top form and drag it to the right, to make part of the form behind visible.

   Check the employee records on both forms, they are different, one with employee code 4 and the other is 5.  Check the title area of the forms, both are showing frmEmployees titles.  Now, let us come back to the program and continue running the code to complete the task.

4. Press Alt+F11 again to switch back to the VBA Window and press the F5 key one more time to continue executing the remaining lines of code.

   The Message Box appears in the Application Window displaying the Employee names Mariya and Steven together.  When you click the OK MsgBox Button, the frmEmployee form instances disappear from the Application Window.

5. Click the OK button on the MsgBox.

Note: I would like to draw your attention to the Stop statement above the MsgBox() function, at the end part of the code. The Stop statement pauses the execution of the VBA code on that statement.  Normally, this statement is used in a program for debugging code, to trace logical errors and corrections.  Here, it is required to pause the execution of code so that we can go to the Application Window and view both instances of the frmEmployees Form there.  The MsgBox() will pause the code, but we will see only the topmost instance of the form. We cannot drag the top form to the right side while the MsgBox is displaced.

If we don't create a pause in the code execution, both instances of the form are closed immediately, when the program ends.  In that case, we will not be able to view the forms.  Since it is a trial run we would like to know what is happening in the program. It is not necessary to make the Form instances visible, before calling the Function GetFirtName().

The VBA Code Line by Line.

Let us take a closer look at each line of code of the frmInstanceTest() function.  Even though hints are given on each line of code, explaining a few things here will make them more clear to you.  We will start with the first two Dim Statements.

Dim frm As New Form_frmEmployees
Dim frm2 As New Form_frmEmployees

In the above Dim statement, you can see that the New keyword is followed by the object reference. The object name is our frmEmployees prefixed by the direct Object Class name FORM followed by an underscore character separation (Form_) to the frmEmployees Form name (Form_frmEmployees).  These Dim statements themselves open two instances of the frmEmployees in memory.   Form instances opened in this way are not immediately visible in the Application Window.  If we need them to be visible, then make them visible with another statement.

Next, we declared two String Variables: Name1 & Name2 to hold the names returned by the GetFirstName() method.

Next two statements: frm.Visible=True and frm2.Visible=True, makes both instances of the frmEmployees Form visible in the Application Window, for information purposes only.

In the next two lines of code, we are calling the GetFirstName() method of the first and second instances of the frmEmployees to search, find and return the First Names of employee codes 4 and 5.

Default Instance and Other Instances.

The default instance of a Form is opened, in the following manner in programs, for accessing their Properties, Methods, and Controls.  These styles of statements are always used to open a form in programs. The default instance of the Form will be automatically visible, in the Application Window.

Dim frm as Form 'define a Form class object
DoCmd.OpenForm "frmEmployees", vbNormal 'open frmEmployees in Memory
Set frm3 = Forms!frmEmployees ' attach it to the frm3 object

Assume that we have opened frm & frm2 instances first in memory before the default instance through the above code.  How do we address those three instances in a program to do something?  Let us forget about the frm, frm2, frm3 object references, for now, we will go with the straight method, like the one given below:

name3 = Forms![frmEmployees].GetFirstName(5) 'target form in memory is the default instance
'OR
name3 = Forms("frmEmployees").GetFirstName(5) 
'OR
name3 = Forms(2).GetFirstName(5) ' this is the third and default instance

The other two instances in memory cannot be referenced like the first two default methods, using the name of the form. You have to use only the index number of the Forms collection to address the other two instances.

name1 = Forms(0).GetFirstName(3)
name2 = Forms(1).GetFirstName(6)

A Shortcut Method.

There is a shortcut method you can use to run the GetFirstName() Method of the frmEmployees Form from the debug window (Ctrl+G).  Type the following command on the Debug Window and press Enter Key:

? form_frmEmployees.GetFirstName(5)
'Result: Steven
'OR
X = form_frmEmployees.GetFirstName(5)

What happens when we execute the above command?  It opens an instance of the frmEmployees in memory, Calls the Function GetFirstName() with the employee Code 5. The GetFirstName() runs and finds the record and returns the First Name of the employee and closes the form.

Tip: Even after closing the Form, after the execution of the above command, the current record, of Employee ID 5, remains as current on the closed Form.

You can check this by executing the following shortcut command by typing it in the debug window and pressing Enter Key.

? form_frmEmployees![First Name]
'Result: Steven

A Fancy Approach.

In the above command, we didn't run the GetFirstName() method, but the current record's First Name field value is printed. If you want to get a little fancy with the command, then try this by typing it in the debug window and pressing the Enter Key:

MsgBox "First Name: " & form_frmEmployees.GetFirstName(8)
'OR
MsgBox "First Name: " & form_frmEmployees![First Name]

Or try the above command from a Command Button Click Event Procedure from another Form's Module, as given below.

Private Sub Command8_Click()
  MsgBox "First Name: " & Form_frmEmployees.GetFirstName(8)

End Sub

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