The distinction between a property and a method is somewhat artificial. Basically, a property is a value that can be both set and retrieved. You can, for example, set and get the Selection property for an Excel application. Another example is Excel's Width property, which applies to many object types. Some Excel properties are read-only; most are read/write.
Properties don't officially have parameters, but some properties
. The property index acts a lot like a parameter.
It doesn't have to be an integer, and it can have more than one
element (row and column, for example). You'll find many indexed
properties in Excel's object model, and Excel VBA can handle
indexed properties in Automation
Methods are more flexible than properties. They can have zero or many parameters, and they can either set or retrieve object data. Most frequently they perform some action, such as showing a window. Excel's Select method is an example of an action method.
The Excel object model supports collection objects. If you use the Worksheets property of the Application object, you get back a Sheets collection object, which represents all the worksheets in the active workbook. You can use the Item property (with an integer index) to get a specific Worksheet object from a Sheets collection, or you can use an integer index directly on the collection.
You've already learned that a COM interface is the ideal way for
Windows programs to communicate with one another, but you've also
learned that designing your own COM interfaces is mostly
You can write COM interfaces that include functions with any
parameter types and return values you specify.
, created in Chapter 24, are some examples. If you're
going to let VBA programmers in, however, you can't be fast and
loose anymore. You can solve the communication problem with one
interface that has a member function smart enough to accommodate
methods and properties as defined by VBA. Needless to say,
has such a function:
. You use
for COM objects that can be
Now you're beginning to see what Automation does. It funnels all
intermodule communication through the
function. How does a client first connect to its component? Because
is merely another COM interface, all the
registration logic supported by COM comes into play. Automation
IDispatch is the heart of Automation. It's fully supported by COM marshaling (that is, Microsoft has already marshaled it for you), as are all the other standard COM interfaces, and it's supported well by the MFC library. At the component end, you need a COM class with an IDispatch interface (plus the prerequisite class factory, of course). At the client end, you use standard COM techniques to obtain an IDispatch pointer. (As you'll see, the MFC library and the wizards take care of a lot of these details for you.)
is the principal member function of
. If you
isn't the only
Another function your controller might call is
. From the VBA programmer's point of view,