using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public void Main()

    {

        //Setup Sample Data

        var data1 = new Dictionary<string, string>();

        data1.Add("KeyA", "ValueA");

        data1.Add("KeyB", "ValueB");

        data1.Add("KeyC", "ValueC");

        var data2 = new Dictionary<string, string>();

        data2.Add("KeyB", "ValueB");

        data2.Add("KeyC", "ValueC");

        data2.Add("KeyD", "ValueD");

        //Second DataType in the Dictionary could be something other than a Tuple

        var result = new Dictionary<string, Tuple<string, string>>();

        //Fill in for items existing only in data1 and in both data1 and data2

        foreach(var item in data1)

        {

            result.Add(item.Key, new Tuple<string, string>(item.Value, data2.FirstOrDefault(x => x.Key == item.Key).Value));

        }

​

        //Fill in remaining items that exist only in data2

        foreach(var item in data2.Where(d2 => !result.Any(x => x.Key == d2.Key )))

        {

            result.Add(item.Key, new Tuple<string, string>(null, item.Value));

        }

        //Demonstrating how to access the data

        var formattedOutput = result.Select(x => string.Format("{0}, {1} (of D1), {2} (of D2)", x.Key, x.Value.Item1 ?? "NoValue", x.Value.Item2 ?? "NoValue"));

        foreach(var line in formattedOutput)

        {

            Console.WriteLine(line);

        }

    }

}