A component is an implementation of a set of interfaces. A component is what you mark in your IDL file (or type library) with CoClass or a class in C#.

An object is an instance of a component. You can create objects by calling CoCreateInstance( ) in C++, specifying the class ID (the type) of the object you want to create. If you use Visual Basic 6.0, you can create objects using new or CreateObject( ). A C# client uses new to create a new instance of a component.

I use the following terms in the book: CoCreating refers to calling CoCreateInstance() in C++, or new or CreateObject( ) in Visual Basic. Querying an object for an interface refers to calling IUnknown::QueryInterface( ) on the object. Releasing an object refers to calling IUnknown::Release( ) on the object.

The graphical notations in Figure P-1 are used in almost every design diagram in the book. The “lollipop” denotes an interface, and a method call on an interface is represented by an arrow beginning with a full circle.

Error handling in the code samples is rudimentary. The code samples serve to demonstrate a design or a technical point, and cluttering them with too much error handing would miss the point. In a production environment, you should verify the returned HRESULT of every COM call, catch and handle exceptions in C#, and assert every assumption.

Italic is used for new terms, citations, online links, filenames, directories, and pathnames.

Constant width is used to indicate command-line computer output and code examples, as well as classes, constants, functions, interfaces, methods, variables, and flow-controlled statements.

Constant-width bold is used for code emphasis and user input.

Constant-width italic is used to indicate replaceable elements in code statements.