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Both Java and C# offer the same capabilities for creating concrete classes that cannot be instantiated. Declaring at least one constructor ensures that the compiler does not generate a default constructor. Declaring this constructor private renders it inaccessible to other code, so the class cannot be instantiated.
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5. Data Types
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It's not possible to define anonymous or local classes in C#; all classes must be explicitly defined before use. Java anonymous classes are regularly used for event handling, especially with GUI components. See the "Events" section later in this chapter for details of event handling in C#.
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Interfaces
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Interfaces serve the same purpose in C# as Java. There are some differences in the declaration syntax, but the primary difference is that constants cannot be declared within the context of a C# interface.
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Declaration
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A C# interface declaration takes the following form: [attributes] [modifiers] interface identifier [:superinterfaces] {body} For example, the C# code to declare a public interface named MyInterface that extends the superinterfaces IAnInterface and IAnotherInterface follows:
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public MyInterface : IAnInterface, IAnotherInterface { // member declarations }
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C# uses a colon followed by a comma-separated list of superinterfaces to specify inheritance.
Modifiers
The applicability of modifiers to an interface declaration depends on the context in which the interface is declared. Interfaces can be declared as a top-level type, being direct members of an enclosing namespace, or they can be nested within the definition of a class or struct. Table 5-7 summarizes modifier availability.
Table 5-7. Interface Declaration Modifier Availability
Interface Declaration Context Member of Namespace Accessibility public protected private internal protected internal Inheritance new abstract
Member of Class Member of Struct
N/A N/A (default) N/A N/A N/A
(default)
N/A (default) N/A N/A N/A
5. Data Types
sealed virtual override Other readonly volatile static extern
Members
N/A N/A N/A N/A N/A N/A
N/A N/A N/A N/A N/A N/A
N/A N/A N/A N/A N/A N/A
Interfaces can contain the following member types: method, property, event, and indexer. Aside from support for the member types introduced by C#, the major difference from Java is that constants cannot be declared in interfaces. The alternative provided by C# is to use a peer-level enum; however, this does not keep associated constants together with the interface.
More Information
For comprehensive coverage of the members applicable in the context of an interface declaration, see the "Members" section later in this chapter.
Implementing interfaces
C# classes and structs implement interfaces using the same syntax. The following example shows a struct MyStruct and a class MyClass both declaring the implementation of two interfaces IAnInterface and IAnotherInterface:
public struct MyStruct :IAnInterface, IAnotherInterface { // Implementation code } public class MyClass :IAnInterface, IAnotherInterface { // Implementation code }
Explicit interface implementation
C# includes a feature called explicit interface implementation, which gives greater control over interface member implementations. Explicit interface implementation is most commonly used when implementing multiple interfaces that contain members with conflicting names or signatures. The only mandatory use of explicit interface implementation is when implementing an indexer declared in an interface. The following is an example of a class implementing two interfaces with conflicting member declarations. Explicit interface implementation is used to differentiate between the implementation of the interface methods.
5. Data Types public interface IMyInterface { void SomeMethod(); } public interface IMyOtherInterface { void SomeMethod(); } public class MyClass : IMyInterface, IMyOtherInterface { void IMyInterface.SomeMethod() { // Implementation Code } void IMyOtherInterface.SomeMethod() { // Implementation Code } }
Each implementation of SomeMethod is qualified using the name of the interface from which it's derived. Explicit interface implementation provides the following benefits:
An implementation can differentiate between interface members that have the same signature and return type. In Java, and nonexplicit member implementations in C#, a single implementation is used to satisfy all matching interface members. An implementation can differentiate between interface members that have the same signature and different return types. This is not possible in Java. If an interface that derives from a superinterface has hidden an inherited member, explicit interface implementation is used to differentiate between the implementation of the parent and child members.
The use of explicit interface implementation has some consequences worth mentioning:
The members can no longer be accessed through a class instance; they must be accessed through an instance of the interface in which the member is declared. A compile-time error will occur if any access modifiers are applied to the member. The accessibility of an explicitly implemented member is a special case: it is never accessible through the class instance but always accessible through the interface instance. A compile-time error will occur if the abstract, virtual, override, or static modifier is applied to the explicit implementation.
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