An abstract class may contain complete or incomplete methods. Interfaces can contain only the signature of a method but no body. Thus an abstract class can implement methods but an interface can not implement methods.

An abstract class can contain fields, constructors, or destructors and implement properties. An interface can not contain fields, constructors, or destructors and it has only the property’s signature but no implementation.

An abstract class cannot support multiple inheritance, but an interface can support multiple inheritance. Thus a class may inherit several interfaces but only one abstract class.

A class implementing an interface has to implement all the methods of the interface, but the same is not required in the case of an abstract Class.

Various access modifiers such as abstract, protected, internal, public, virtual, etc. are useful in abstract Classes but not in interfaces.

interface just declaration.it provide standared for programming.it has no code.on the other hand abstaract class is one which can not make object.it is always inherited.

public abstarct class clscon
{
 protected sqlconnection con= new sqlconnection()
 public clscon()
 {
        con.connectionstring=configurationmanager.
            connectionstring("cn").connectionstring;
 }
}//this is abstract class

interface

public interface intemp
{
 get;
 set;
}

• An interface cannot provide any code; just the signature means all methods must be abstract. But an abstract class can provide complete, default code and/or just the details but at least at least one method must be abstract.
• A class may inherit several interfaces. But a class may inherit only one abstract class.
• If the various implementations only share method signatures then it is better to use Interface. If the various implementations are of the same kind and use common behavior or status then abstract class is better to use.
• 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. But 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.

What is an Interface?

An interface is a contract, a specification that concrete classes MUST follow. It defines method signatures but cannot have any implementations; the latter must be provided by the classes that implement the interface.

C# differs from C++ in this regard because C++ lacks native language support for interfaces. As a C++ programmers you have to create an interface by defining an abstract class with pure virtual methods.

what is an abstract class?

An Abstract class lets you define some behaviors and force your subclasses to provide others.
For example, if you have an application framework, an abstract class may provide default services such as event and message handling. Those services allow your application to plug in to your application framework. However, there is some application-specific functionality that only your application can perform. So instead of trying to define these behaviors, the abstract class can declare abstract methods.

Differences between Interfaces and Abstract classes Which we use ?

1. multiple inheritance
   A class may implement several interfaces but can only extend one abstract class.

2. default implementation
   An interface cannot provide any code at all, much less default code. An abstract class can provide complete code, default code, and/or just stubs that have to be overridden.

3. adding functionality
   If you add a new method to an interface, you must track down all implementations of that interface in the universe and provide them with a concrete implementation of that method.

   If you add a new method to an abstract class, you have the option of providing a default implementation of it. Then all existing code will continue to work without change.

4. is-a vs -able or can-do
   Interfaces are often used to describe the abilities of a class, not its central identity, e.g. an Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.

   An abstract class defines the core identity of its descendants.