Heterogenous collections require type checking to avoid errors that can occur when retrieving a value or an instance of an incorrect type. To avoid these errors, the property classes use C++ templates for static type checking and runtime checking.
TProperty classes
Two kinds of property classes derive from TProperty:
When you instantiate one of these classes to create a property container, you pass the data type of the property as the template argument--in place of ABasicCType or AClassType.
TPrimitiveProperty<ABasicCType> can contain one of the basic C++ data types, or TDoubleLong. In place of ABasicCType, TPrimitiveProperty<ABasicCType> accepts:
| signed char | unsigned char | TDoubleLong | double |
| signed short | unsigned short | float | signed long |
| unsigned long | signed char | unsigned char | short |
| long |
You can also use any type definition that corresponds to one of these values. For example, TPrimitiveProperty<Boolean> is acceptable.
TExtendedProperty<AClassType> can contain an instance of a class. For extended properties, the instance must be of a class that derives from MCollectible.
NOTE 
The MCollectible hierarchy will change in a future software release. Read Chapter 1, "Collections" to learn more about collections.
TPropertyID classes
TPropertyID has two concrete classes that use templates in the same way as the classes derived from TProperty. Instances TPrimitivePropertyID<ABasicCType> and TExtendedPropertyID<AClassType> provide the identifiers that correspond to instances of TPrimitiveProperty<ABasicCType> and TExtendedProperty<AClassType>.
TExtendedProperty<AClassType> defines a Boolean function called AllowsPolymorphicValues.This member function flags whether or not a program can use the identifier to store and retrieve polymorphic values. The default for AllowsPolymorphicValues does not allow polymorphism.
When evaluating properties, the Properties interface considers two instances of property identifier equal if the name and polymorphism flag are both equal. If either of these components is not equal, the identifiers are not equal. If you add an identifier to a property collection that contains an equal identifier, the Properties interface replaces the original property instance with the new one.
Handling polymorphic class types
Evaluating equality
within
a collection
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