Class Module(.cls) vs Module(.bas) in Visual Basic

Question

What is the difference between Class Module(.cls) and . Module(.bas) in Visual Basic ?

Answer 1

A Module(.bas) has methods and variables that can be used globally in your program and there is only a single instance of the data (similar to a Static method or field in C#). A Class Module(.cls) has properties and methods that usually can only be accessed when the object is instantiated, but can have multiple copies, each with differing data.

From MSDN: Visual Basic Concepts:

Classes differ from standard modules in the way their data is stored. There is never more than one copy of a standard module’s data. This means that when one part of your program changes a public variable in a standard module, and another part of your program subsequently reads that variable, it will get the same value.

Class module data, on the other hand, exists separately for each instance of the class.

And from Devx.com: Class Module(.cls) vs. Module(.bas):

Deciding between a standard module and a class module is not a decision based on performance, but one of design. The main difference between the two is in the way that they handle data. A standard module stores only one copy of the data. A class module encapsulates the data within each instance of the class. That is, for each instance of the class, the data exists separately.

The other main difference is the scope of variables and procedures within the module. In general, any variables and procedures declared as Public within a standard module are visible anywhere in the project or external programs if the standard module is in a component. Variables and procedures declared as Public within a class module can only be seen through a reference to an instance of the class module.

The lifetime of data and procedures stored within a module is affected by which type of module is used. The lifetime of the data and procedures in a class module is defined by the lifetime of the object. So data and procedures are available only if a reference to the object exists. Data and procedures declared within standard modules are available for the lifetime of the program.

Therefore, to answer your question, if you are writing a function that you want available throughout the lifetime of the program and visible to all code in the application, then place the function within a standard module.