When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.
When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is same as an interface. The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.
If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
Fields and Constants No fields can be defined in interfaces.
An abstract class can have fields and constrants defined.
An interface is defined as a syntactical contract that all the classes inheriting the interface should follow.
The interface defines the 'what' part of the syntactical contract and the deriving classes define the 'how' part of the syntactical contract.
Interfaces define properties, methods and events, which are the members of the interface. Interfaces contain only the declaration of the members.
It is the responsibility of the deriving class to define the members. It often helps in providing a standard structure
that the deriving classes would follow.
Abstract classes to some extent serve the same purpose, however, they are mostly used when only few methods are to be declared by
the base class and the deriving class implements the functionalities.
Declaring Interfaces
Interfaces are declared using the interface keyword. It is similar to class declaration. Interface statements are public by default.
Following is an example of an interface declaration:
public interface ITransactions
{
// interface members
void showTransaction();
double getAmount();
}
Example
The following example demonstrates implementation of the above interface:
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace InterfaceApplication
{
public interface ITransactions
{
// interface members
void showTransaction();
double getAmount();
}
public class Transaction : ITransactions
{
private string tCode;
private string date;
private double amount;
public Transaction()
{
tCode = " ";
date = " ";
amount = 0.0;
}
public Transaction(string c, string d, double a)
{
tCode = c;
date = d;
amount = a;
}
public double getAmount()
{
return amount;
}
public void showTransaction()
{
Console.WriteLine("Transaction: {0}", tCode);
Console.WriteLine("Date: {0}", date);
Console.WriteLine("Amount: {0}", getAmount());
}
}
class Tester
{
static void Main(string[] args)
{
Transaction t1 = new Transaction("001", "8/10/2012", 78900.00);
Transaction t2 = new Transaction("002", "9/10/2012", 451900.00);
t1.showTransaction();
t2.showTransaction();
Console.ReadKey();
}
}
}
When the above code is compiled and executed, it produces the following result:
Transaction: 001
Date: 8/10/2012
Amount: 78900
Transaction: 002
Date: 9/10/2012
Amount: 451900
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There are some similarities and differences between an interface and an abstract class that I have arranged in a table for easier comparison:
Multiple inheritance
A class may inherit several interfaces.
A class may inherit only one abstract class.
Default implementation
An interface cannot provide any code, just the signature.
An abstract class can provide complete, default code and/or just the details that have to be overridden.
Access Modfiers
An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public
An abstract class can contain access modifiers for the subs, functions, properties
Core VS Peripheral
Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovable interface.
An abstract class defines the core identity of a class and there it is used for objects of the same type.
Homogeneity
If various implementations only share method signatures then it is better to use Interfaces.
If various implementations are of the same kind and use common behaviour or status then abstract class is better to use.
Speed
Interface Requires more time to find the actual method in the corresponding classes.
Abstract is Fast
Adding functionality (Versioning)
If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
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When to use Abstract class:-
When to use Abstract class:-
Abstract Classes
–> When you have a requirement where your base class should provide default implementation of certain methods whereas other methods should be open to being overridden by child classes use abstract classes.
For e.g. again take the example of the Vehicle class above. If we want all classes deriving from Vehicle to implement the Drive() method in a fixed way whereas the other methods can be overridden by child classes. In such a scenario we implement the Vehicle class as an abstract class with an implementation of Drive while leave the other methods / properties as abstract so they could be overridden by child classes.
–> The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share.
For example a class library may define an abstract class that is used as a parameter to many of its functions and require programmers using that library to provide their own implementation of the class by creating a derived class.
Use an abstract class
When creating a class library which will be widely distributed or reused—especially to clients, use an abstract class in preference to an interface; because, it simplifies versioning. This is the practice used by the Microsoft team which developed the Base Class Library. ( COM was designed around interfaces.) Use an abstract class to define a common base class for a family of types. Use an abstract class to provide default behavior. Subclass only a base class in a hierarchy to which the class logically belongs.
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