Default interface not found in c# class

Question

I have this peace of code in a console app .net core 3.1 on VS 16.5.1:

namespace DefaultInterfaceTest
{
    class Program
    {
        static void Main(string[] args)
        {   
            var person = new Person();
            person.GetName();//error here
        }
    }

    public interface IPerson
    {
        string GetName()
        {
            return "Jonny";
        }
    }

    public class Person: IPerson
    {

    }
}

I was hopping I could access the default implementation oif GetName from the person itself, as it's a public method but it yields this error:

'Person' does not contain a definition for 'GetName' and no accessible extension method 'GetName' accepting a first argument of type 'Person' could be found (are you missing a using directive or an assembly reference?)

How can I access the default implementation of an interface from outside code or from the Person class itself? Thanks!

Answer 1

You can only access default implementation methods by calling via an interface reference (think of them as explicitly implemented methods).

For example:

// This works
IPerson person = new Person();
person.GetName();

But:

// Doesn't works
Person person = new Person();
person.GetName();

If you want to call the default interface method from within your class then you'll need to cast this to an IPerson in order to do so:

private string SomeMethod()
{
  IPerson self = this;
  return self.GetName();
}

There's no way around the case if you are using interfaces. If you really want this sort of behavior then you'll need to use an abstract class where GetName is a virtual method.

abstract class PersonBase
{
  public virtual string GetName()
  {
    return "Jonny";
  }
}

Answer 2

is casting something you can use in your situation?

using System;

namespace DefaultInterfaceTest
{
    class Program
    {
        static void Main(string[] args)
        {   
            IPerson person = new Person();

            Person fooPerson = (Person) person;
            Console.WriteLine(person.GetName());
            Console.WriteLine(fooPerson.Foo());
        }
    }

    public interface IPerson
    {
        public string GetName()
        {
            return "Jonny";
        }
    }

    public class Person: IPerson
    {
      public string Foo()
      {
           return "Hello";
      }
    }
}