Can a C# class call an interface's default interface method from its own implementation?

问题

If I have a default interface method like this:

public interface IGreeter
{
    void SayHello(string name) => System.Console.WriteLine($"Hello {name}!");
}

Can I have my concrete implementation call that default method?

public class HappyGreeter : IGreeter
{
    public void SayHello(string name)
    {
        // what can I put here to call the default method?
        System.Console.WriteLine("I hope you're doing great!!");
    }
}

So that calling:

var greeter = new HappyGreeter() as IGreeter;
greeter.SayHello("Pippy");

Results in this:

// Hello Pippy!
// I hope you're doing great!!

Indeed Calling C# interface default method from implementing class shows that I can call method that my class does not implement, but as somewhat expected adding call to ((IGreeter)this).SayHello(name); inside HappyGreeter.SaysHello causes stack overflow.

回答1:

As far as I know you can't invoke default interface method implementation in inheriting class (though there were proposals). But you can call it from inheriting interface:

public class HappyGreeter : IGreeter
{
    private interface IWorkAround : IGreeter
    {
        public void SayHello(string name)
        {
            (this as IGreeter).SayHello(name);
            System.Console.WriteLine("I hope you're doing great!!");
        }
    }

    private class WorkAround : IWorkAround {}

    public void SayHello(string name)
    {
        ((IWorkAround)new WorkAround()).SayHello(name);
    }
}

UPD

In my original answer too much wanted to show that you can call base in inheriting interface, but as @Alexei Levenkov suggested in comments cleaner way in this particular case would be something like this:

public class HappyGreeter : IGreeter
{
    private class WorkAround : IGreeter { }
    private static readonly IGreeter _workAround = new WorkAround();

    public void SayHello(string name)
    {
        _workAround.SayHello(name);
        System.Console.WriteLine("I hope you're doing great!!");
    }
}

回答2:

I know that this is not an answer to the question, but the next approach also can be used to emulate base functionality:

public interface IGreeter
{
    void SayHello(string name) => BaseSayHello(name);

    // This static method can be used in implementers of "IGreeter"
    // to emulate "base" functionality.
    protected static void BaseSayHello(string name) => System.Console.WriteLine($"Hello {name}!");
}

public class HappyGreeter : IGreeter
{
    public void SayHello(string name)
    {
        IGreeter.BaseSayHello(name);
        Console.WriteLine("I hope you're doing great!!");
    }
}

来源：https://stackoverflow.com/questions/62334232/can-a-c-sharp-class-call-an-interfaces-default-interface-method-from-its-own-im

标签

c#-8.0

default-interface-member