using System;

public class Program

{

    public static void Main()

    {

        var target = 1;

        var source = 2;

        var mapper = new NumberMapper();

        mapper.Map(source, target);

        //Expected value of target: 2

        Console.WriteLine(target);      

    }

    public interface IMapper<in TSource, in TTarget>

    {

        /// <summary>

        ///     Maps the object in <paramref name="source" /> into the existing <paramref name="target" /> object.

        /// </summary>

        /// <param name="source">The source object.</param>

        /// <param name="target">The target object.</param>

        void Map(TSource source, TTarget target);

    }

    public class NumberMapper : IMapper<int, int>

    {

        public void Map(int source, int target)

        {

            target = source;

        }

    }

}