Abstract Factory vs Factory method: Composition vs Inheritance?

Question

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.

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This question is not about what abstract factory is or what factory method is and hence does not have answer here

It is about why factory method seems to use inheritance when a client can call factory method directly too. Please unmark it as duplicate.

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.