Abstract Factory vs Factory method: Composition vs

Question

This question already has an answer here:

• Factory, Abstract Factory and Factory Method 3 answers

I have read a lot of posts about different between Abstract Factory and Factory method, but there are a problem I can't understand.

One difference between the two is that with the Abstract Factory pattern, a class delegates the responsibility of object instantiation to another object via composition whereas the Factory Method pattern uses inheritance and relies on a subclass to handle the desired object instantiation

Maybe I know why Abstract Factory pattern use composition and delegates to create object, but I can't understand why Factory Method pattern uses inheritance to create concrete class objects.

Answer 1

Abstract Factory

public interface IMyFactory
{
    IMyClass CreateMyClass(int someParameter);
}

Usage:

public class SomeOtherClass
{
    private readonly IMyFactory factory;

    public SomeOtherClass(IMyFactory factory)
    {
        this.factory = factory;
    }

    public void DoSomethingInteresting()
    {
        var mc = this.factory.CreateMyClass(42);
        // Do something interesting here
    }
}

Notice that SomeOtherClass relies on Composition to be composed with an IMyFactory instance.

Factory Method

public abstract class SomeOtherClassBase
{
    public void DoSomethingInteresting()
    {
        var mc = this.CreateMyClass(42);
        // Do something interesting here
    }

    protected abstract IMyClass CreateMyClass(int someParameter)
}

Usage:

public class SomeOtherClass2 : SomeOtherClassBase
{   
    protected override IMyClass CreateMyClass(int someParameter)
    {
        // Return an IMyClass instance from here
    }
}

Notice that this example relies on inheritance to work.