using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var d1 =  new Dictionary<string,string>{

            {"ciao","mondo"}

        };

        var d2 =  new Dictionary<string,string>{

            {"hello","World"},

            {"ciao","scemo"}

​

        };  

        d1 = d1.Union(d2, new KeyEqualityComparer<string,string>()).ToDictionary(a => a.Key, b => b.Value);

        d1.Dump();

    }

    public class KeyEqualityComparer<T1, T2> : IEqualityComparer<KeyValuePair<T1, T2>>

    {

        public bool Equals(KeyValuePair<T1, T2> x, KeyValuePair<T1, T2> y)

        {

            return x.Key.Equals(y.Key);

        }

​

        public int GetHashCode(KeyValuePair<T1, T2> obj)

        {

            return obj.Key.GetHashCode();

        }

    }

}