Programming Excel with VBA and .NET

Things You Can’t Do

If you are familiar with an object-oriented programming language such as Java or Visual Basic .NET you might be waiting for me to discuss constructors, inheritance, and overloading. You’ll have a long wait, because Excel’s Visual Basic can’t do any of those. There are also some limits on things you might assume you can do from looking at the Excel objects. For instance, you can’t create default properties. Table 5-1 lists these language limitations and provides some detail.

Table 5-1. Object-oriented features not available in Excel

Feature	Limitation and workaround
Constructors	Only a default constructor is available. If you want to initialize an object, you must implement an Initialize method or something similar.
Destructors	Only a default destructor is available. If you want to free nonmemory resources used by an object, you must implement a separate Dispose method or something similar.
Collection types	There is only one collection type: Collection. To implement a collection, create a class that “wraps” that type as shown earlier in "Collect Objects.”
Default properties	Not available in custom classes. Properties must be called by name.
Inheritance	Not available. You can’t base one class on another.
Interfaces	Not available. You can’t create a prototype for a class.
Overloaded methods	Not available. Use the Optional keyword to create methods that accept different sets of arguments.

I include Table 5-1 because it’s hard to know what’s missing ...