When to use a Class in VBA?

Question

When is it appropriate to use a class in Visual Basic for Applications (VBA)?

I\'m assuming the accelerated development and reduction of introducing bugs is a common

Answer 1

I use classes if I want to create an self-encapsulated package of code that I will use across many VBA projects that come across for various clients.

Answer 2

I think the criteria is the same as other languages

If you need to tie together several pieces of data and some methods and also specifically handle what happens when the object is created/terminated, classes are ideal

say if you have a few procedures which fire when you open a form and one of them is taking a long time, you might decide you want to time each stage......

You could create a stopwatch class with methods for the obvious functions for starting and stopping, you could then add a function to retrieve the time so far and report it in a text file, using an argument representing the name of the process being timed. You could write logic to log only the slowest performances for investigation.

You could then add a progress bar object with methods to open and close it and to display the names of the current action, along with times in ms and probable time remaining based on previous stored reports etc

Another example might be if you dont like Access's user group rubbish, you can create your own User class with methods for loging in and out and features for group-level user access control/auditing/logging certain actions/tracking errors etc

Of course you could do this using a set of unrelated methods and lots of variable passing, but to have it all encapsulated in a class just seems better to me.

You do sooner or later come near to the limits of VBA, but its quite a powerful language and if your company ties you to it you can actually get some good, complex solutions out of it.

Answer 3

For data recursion (a.k.a. BOM handling), a custom class is critically helpful and I think sometimes indispensable. You can make a recursive function without a class module, but a lot of data issues can't be addressed effectively.

(I don't know why people aren't out peddling BOM library-sets for VBA. Maybe the XML tools have made a difference.)

Multiple form instances is the common application of a class (many automation problems are otherwise unsolvable), I assume the question is about custom classes.

Answer 4

You can also reuse VBA code without using actual classes. For example, if you have a called, VBACode. You can access any function or sub in any module with the following syntax:

VBCode.mysub(param1, param2)

If you create a reference to a template/doc (as you would a dll), you can reference code from other projects in the same way.

Answer 5

I don't see why the criteria for VBA would be any different from another language, particularly if you are referring to VB.NET.

Answer 6

Classes are extremely useful when dealing with the more complex API functions, and particularly when they require a data structure.

For example, the GetOpenFileName() and GetSaveFileName() functions take an OPENFILENAME stucture with many members. you might not need to take advantage of all of them but they are there and should be initialized.

I like to wrap the structure (UDT) and the API function declarations into a CfileDialog class. The Class_Initialize event sets up the default values of the structure's members, so that when I use the class, I only need to set the members I want to change (through Property procedures). Flag constants are implemented as an Enum. So, for example, to choose a spreadsheet to open, my code might look like this:

Dim strFileName As String
Dim dlgXLS As New CFileDialog

With dlgXLS
  .Title = "Choose a Spreadsheet"
  .Filter = "Excel (*.xls)|*.xls|All Files (*.*)|*.*"
  .Flags = ofnFileMustExist OR ofnExplorer

  If OpenFileDialog() Then
    strFileName = .FileName
  End If
End With
Set dlgXLS = Nothing

The class sets the default directory to My Documents, though if I wanted to I could change it with the InitDir property.

This is just one example of how a class can be hugely beneficial in a VBA application.