What is concrete implementation?

Question

I saw lot of posts in StackOverflow and elsewhere talking about concrete implementation. While fiddling with WCF i came through the line

Tying your service implementation, or any “service based” class to a concrete implementation is never a good idea.

Can anyone explain what Concrete Implementation is?

Answer 1

It is implementation of something abstract (abstract class, interface). Note that you can instantiate only objects of concrete classes.

Just for example if you have :

abstract class AbstractClass
{
    .......
   // Here you have some abstract methods 
}

class ConcreteClass : AbstractClass
{
.......
}

In case of WCF it wants to say that although it is allowed to mark classes with ServiceContract attribute better to have it on separate Interface and implement that interface in concrete class marked with ServiceBehavior attribute.

Like this :

[ServiceContract(Namespace = "MyNamespaceName")]
interface IMyInterface
{
    [OperationContract]
    int SomeMethod(.....);

   ......    
   ......    
}

[ServiceBehavior(......)]
public class SomethingConcrete : IMyInterface
{
    // implementation of SomeMethod
}

Answer 2

If you have an interface or abstract class, it need to be implemented.

A class that implements such an interface or class is called a concrete implementation (because only such an implemented class can be instantiated).

The principle stated means that you shouldn't code against the concrete implementation directly, as you may want to swap it for another concrete implementation at a later time without changing your code. This means you should use interface and abstract class references instead of concrete implementations.