We use an interface to get access to an object's data and behavior. The object's data and behaviors are contained within the object, so a client application can treat the object like a black box accessible only through its interface. This is a key object-oriented concept called Encapsulation. The idea is that any programs that make use of this object won't have direct access to the behaviors or data-but rather those programs must make use of our object's interface.
There are three main parts of Object:
1. Interface
2. Implementation or Behavior
3. Member or Instance variables
Interface
The interface is defined as a set of methods (Sub and Function routines), properties (Property routines), events, and fields (variables or attributes) that are declared Public in scope.
Implementation or Behavior
The code inside of a method is called the implementation. Sometimes it is also called behavior since it is this code that actually makes the object do useful work.
Client applications can use our object even if we change the implementation-as long as we don't change the interface. As long as our method name and its parameter list and return data type remain unchanged, we can change the implementation all we want.
So Method Signature depends on:
- Method name
- Data types of parameters
- Either Parameter is passed ByVal or ByRef.
- Return type of method.
Member or Instance Variables
The third key part of an object is its data, or state. Every instance of a class is absolutely identical in terms of its interface and its implementation-the only thing that can vary at all is the data contained within that particular object.
Member variables are those declared so that they are available to all code within our class. Typically member variables are Private in scope-available only to the code in our class itself. They are also sometimes referred to as instance variables or as attributes. The .NET Framework also refers to them as fields.
We shouldn't confuse instance variables with properties. A Property is a type of method that is geared around retrieving and setting values, while an instance variable is a variable within the class that may hold the value exposed by a Property.
Interface looks like a class, but has no implementation.
The only thing it contains is definitions of events, indexers, methods and/or properties. The reason interfaces only provide definitions is because they are inherited by classes and structs, which must provide an implementation for each interface member defined. So, what are interfaces good for if they don't implement functionality? They're great for putting together plug-n-play like architectures where components can be interchanged at will. Since all interchangeable components implement the same interface, they can be used without any extra programming. The interface forces each component to expose specific public members that will be used in a certain way.
Because interfaces must be defined by inheriting classes and structs, they define a contract. For instance, if class foo inherits from the IDisposable interface, it is making a statement that it guarantees it has the Dispose() method, which is the only member of the IDisposable interface. Any code that wishes to use class foo may check to see if class foo inherits IDisposable. When the answer is true, then the code knows that it can call foo.Dispose().
Defining an Interface: MyInterface.c
interface IMyInterface
{void MethodToImplement();
}
Above listing shows defines an interface named IMyInterface. A common naming convention is to prefix all interface names with a capital "I", but this is not mandatory. This interface has a single method named MethodToImplement(). This could have been any type of method declaration with different parameters and return types. Notice that this method does not have an implementation (instructions between curly braces- {}), but instead ends with a semi-colon, ";". This is because the interface only specifies the signature of methods that an inheriting class or struct must implement.
All the methods of Interface are public by default and no access modifiers (like private, public) are allowed with any method of Interface.
Using an Interface: InterfaceImplementer.cs
class InterfaceImplementer : IMyInterface
{public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
The InterfaceImplementer class in above listing implements the IMyInterface interface. Indicating that a class inherits an interface is the same as inheriting a class. In this case, the following syntax is used:
class InterfaceImplementer : IMyInterface
Note that this class inherits the IMyInterface interface; it must implement its all members. While implementing interface methods all those needs to be declared public only. It does this by implementing the MethodToImplement() method. Notice that this method implementation has the exact same signature, parameters and method name, as defined in the IMyInterface interface. Any difference will cause a compiler error. Interfaces may also inherit other interfaces. Following listing shows how inherited interfaces are implemented.
Interface Inheritance: InterfaceInheritance.cs
using System;interface IParentInterface
{void ParentInterfaceMethod();
}interface IMyInterface : IParentInterface
{void MethodToImplement();
}class InterfaceImplementer : IMyInterface
{public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}public void ParentInterfaceMethod()
{
Console.WriteLine("ParentInterfaceMethod() called.");
}
}
The code in above listing contains two interfaces: IMyInterface and the interface it inherits, IParentInterface. When one interface inherits another, any implementing class or struct must implement every interface member in the entire inheritance chain. Since the InterfaceImplementer class in above listing inherits from IMyInterface, it also inherits IParentInterface. Therefore, the InterfaceImplementer class must implement the MethodToImplement() method specified in the IMyInterface interface and the ParentInterfaceMethod() method specified in the IParentInterface interface.
In summary, you can implement an interface and use it in a class. Interfaces may also be inherited by other interface. Any class or struct that inherits an interface must also implement all members in the entire interface inheritance chain.
