Two interfaces have same method

Question

Interface A
{
    int Add(int a,int b);
}

Interface B
{
    int Add(int a,int b);
}

Class D : A, B
{
    int Add(int a,int b)
    {
        return a+b;
    }
}

Code works fine and didn't produce any error. Class D is using which interface's method?

Answer 1

Neither, since neither interface HAS a method, merely a method signature. Your method in D implements the signature provided by both interfaces, so it works.

Remember, an interface merely specifies the signatures of methods that must exist in an implementation.

Answer 2

Since method signatures are the same on both interfaces, and class D implements those methods (with a single function), then it doesn't really matter which interface that function implements, and thus compiler is happy.

However, you can have two different implementations specific for each interface by declaring functions as

class D : A, B
{
    int A.Add(int a, int b)
    {
    }

    int B.Add(int a, int b)
    {
    }
}