C# call interface method with default implementation

Question

I have the following interface and class:

interface IMyInterface
{
    void A();
   
    void B() 
    { 
        Console.WriteLine("B"); 
    } 
}

class MyClass : IMyInterface
{
    public void A()
    {
        Console.WriteLine("A");
    }
}

I'd like to instantiate MyClass and call B() like so:

MyClass x = new MyClass();
x.B();

However, this doesn't work since MyClass does not contain a definition for 'B'. Is there anything I can add to MyClass so this code calls the default implementation of B() in IMyInterface?

I understand the code below works, but I don't want to change the data type from MyClass to IMyInterface.

IMyInterface x = new MyClass();
x.B();

No, there is nothing you can do. That's not how default implementations work. They don't add anything to the class. You can inherit from a base class that provides that method, but the interface doesn't make that method available through a class reference. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Sep 29, 2021 at 21:17
@LasseV.Karlsen That's really unfortunate. This is how default implementations work in other languages like Kotlin. — tronman
– tronman, Commented Sep 29, 2021 at 21:40

Rubix Cube · Accepted Answer · 2021-10-29 16:25:45Z

3

You can call it like:

MyClass x = new MyClass();
(x as IMyInterface).B();

answered Oct 29, 2021 at 16:25

Rubix Cube

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ZeHasNoName · Accepted Answer · 2021-10-29 13:39:19Z

3

Maybe this is a bit of a hack, but you could use a class extension to add the B() method to every class that implements IMyInterface.

static class ImplIMyInterface
{
    public static void B(this IMyInterface obj) => obj.B();
}

Now you can call B() on an instance of MyClass.

answered Oct 29, 2021 at 13:39

ZeHasNoName

312 bronze badges

Comments

Shohreh Mortazavi · Accepted Answer · 2021-09-29 21:34:14Z

2

you can't do that. you must implement the interface that your class has inherited from it. but you can use abstract class instead of interface. like this:

 abstract class MyAbstract
{
    public void A() { }

   public void B()
    {
        Console.WriteLine("B");
    }
}

class MyClass : MyAbstract
{
    public new void A()
    {
        Console.WriteLine("A");
    }
}

var x = new MyClass();
    x.B();

answered Sep 29, 2021 at 21:34

Shohreh Mortazavi

2243 silver badges12 bronze badges

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