An example of Double Dispatch for the Multiple Dispatch (Multimethods) article.

DoubleDispatchMultimethods.cs

using System;

namespace CSharpDoubleDispatch
{
    class Program
    {
        static void Main(string[] args)
        {
            var op1 = new MyInt(5);
            var op2 = new MyRational(2, 3);
            var res = Adder(op1, op2);
            Console.WriteLine(res);
        }

        private static INumber Adder(INumber op1, INumber op2) => op1.Add(op2);
    }

    public interface INumber
    {
        INumber Add(INumber number);
        INumber Add(MyInt number);
        INumber Add(MyRational value);
    }
    
    public class MyInt : INumber
    {
        public int Val { get; }

        public MyInt(int val)
        {
            Val = val;
        }

        public INumber Add(INumber number) => number.Add(this);
        public INumber Add(MyInt number) => new MyInt(number.Val);
        public INumber Add(MyRational number) => new MyRational(Val * number.Den + number.Num, number.Den);
        
        public override string ToString() => Val.ToString();
    }
    
    public class MyRational : INumber
    {
        public int Num { get; }
        public int Den { get; }

        public MyRational(int num, int den)
        {
            Num = num;
            Den = den;
        }

        public INumber Add(INumber number) => number.Add(this);
        public INumber Add(MyInt number) => number.Add(this);
        public INumber Add(MyRational number) => 
            new MyRational(Num * number.Den + number.Num * Den, Den * number.Den);

        public override string ToString() => $"{Num}/{Den}";
    }
}