using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

            Dictionary<string, string> first = new Dictionary<string, string>();

            Dictionary<string, string> second = new Dictionary<string, string>();

​

​

            first.Add("NAME", "NAME");

            first.Add("TYPE", "TYPE");

            first.Add("REF", "REF");

            first.Add("NUMBER", "NUMBER");

            first.Add("DATE", "DATE");

            first.Add("SOURCE", "SOURCE");

​

            second.Add("TYPE", "T1");

            second.Add("REF", "A1234");

            second.Add("NUMBER", "B456");

            second.Add("DATE", "D123455");

            second.Add("NAME", "NAME");

            second.Add("SOURCE", "SOURCE");

​

            var result = first.Union(second).GroupBy(x => x.Key).Select(x => new { Key = x.Key, Value = x.Last().Value }).ToDictionary(x => x.Key, x => x.Value);

        foreach(var item in result)

        {

            Console.WriteLine(item.Key + ": " + item.Value);

        }

​

    }

}