Language Contrast: C++ Multiple Inheritance in C#.NET

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Language Contrast: C++ Multiple Inheritance
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C# s single inheritance is one of its major differences from C++ It makes for a significant migration path from programming libraries such as Active Template Library (ATL), whose entire approach relies on multiple inheritance
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For the rare cases that require a multiple-inheritance class structure, one solution is to use aggregation; instead of inheriting the second class, the class contains an instance of the class Figure 62 shows an example of this class structure Aggregation occurs when the association relationship defines a core part of the containing object For multiple inheritance, this involves picking one class as the primary base class (PdaItem) and deriving a new class (Contact) from that The second desired base class (Person) is added as a field in the derived class (Contact) Next, all the nonprivate members on the field (Person) are redefined on the derived class (Contact) which then delegates the calls out to the field (Person) Some code duplication occurs because methods are redeclared; however, this is minimal, since the real method body is implemented only within the aggregated class (Person) In Figure 62, Contact contains a private property called InternalPerson that is drawn as an association to the Person class Contact also contains the FirstName and LastName properties but with no corresponding fields Instead, the FirstName and LastName properties simply delegate their calls out to InternalPersonFirstName and InternalPersonLastName, respectively Listing 68 shows the resultant code
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Listing 68: Working around Single Inheritance Using Aggregation
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public class PdaItem { // } public class Person
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Derivation
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Figure 62: Working around Multiple Inheritance Using Aggregation
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{ // } public class Contact : PdaItem { private Person InternalPerson { get; set; } public string FirstName { get { return InternalPersonFirstName; } set { InternalPersonFirstName = value; } } public string LastName {
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6: Inheritance
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get { return InternalPersonLastName; } set { InternalPersonLastName = value; } } // }
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Besides the added complexity of delegation, another drawback is that any methods added to the field class (Person) will require manual addition to the derived class (Contact); otherwise, Contact will not expose the added functionality
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Sealed Classes To design a class correctly that others can extend via derivation can be a tricky task that requires testing with examples to verify the derivation will work successfully Furthermore, a slight performance overhead is associated with derived classes To avoid some of these concerns, you can mark classes as sealed (see Listing 69)
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Listing 69: Preventing Derivation with Sealed Classes
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public sealed class CommandLineParser { // }
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// ERROR: Sealed classes cannot be derived from public sealed class DerivedCommandLineParser : CommandLineParser { // }
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Sealed classes include the sealed modifier, and the result is that they cannot be derived The string type is an example of a type that uses the sealed modifier to prevent derivation
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Overriding the Base Class
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All public and protected members of a base class are inherited in the derived class However, sometimes the base class does not have the optimal
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implementation of a particular member Consider the Name property on PdaItem, for example The implementation is probably acceptable when inherited by the Appointment class For the Contact class, however, the Name property should return the FirstName and LastName properties combined Similarly, when Name is assigned, it should be split across FirstName and LastName In other words, the base class property declaration is appropriate for the derived class, but the implementation is not always valid There needs to be a mechanism for overriding the base class implementation with a custom implementation in the derived class
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virtual Modifier C# supports overriding on instance methods and properties but not on fields or any static members It requires an explicit action within both the base class and the derived class The base class must mark each member for which it allows overriding as virtual If public or protected members do not include the virtual modifier, subclasses will not be able to override those members