using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        KeyValuePair<string, double> kv1 = new KeyValuePair<string, double>("a", 5);

        KeyValuePair<string, double> kv2 = new KeyValuePair<string, double>("b", 7);

        var sum = kv1.Add(kv2);

        Console.WriteLine(sum);

    }

}

​

public static class MyKeyValuePairExtensions

{

    public static KeyValuePair<string, double> Add(

        this KeyValuePair<string, double> first,

        KeyValuePair<string, double> second)

    {

        return new KeyValuePair<string, double>(

            first.Key + " + " + second.Key,

            first.Value + second.Value);

    }

}

​