Your MDI application can also store per-document data by using window properties. Properties are different from extra space in the window structure in that you need not allocate extra space when registering the window class. A window can have any number of properties. Also, where offsets are used to access the extra space in window structures, properties are referred to by string names.
Associated with each property is a handle. For example, Multipad could have used a property called EditCntl to store the edit control window handle discussed previously. The handle could be any 2-byte value and could be a handle of a data structure. Properties are often more convenient than extra space in the window structure. This is because, when using properties, the application is not required to reserve extra space in advance or to calculate offsets to variables. On the other hand, accessing extra space by offset is generally faster than accessing properties.