using System.Collections.Generic;

using System.Linq;

using System;

​

public class Program

{

    public static void Main()

    {

        var list = new List<KeyValuePair<string, string>>();

        list.Add(new KeyValuePair<string, string>("1", "1"));

        list.Add(new KeyValuePair<string, string>("2", "2"));

        list.Add(new KeyValuePair<string, string>("3", "3"));

        Console.WriteLine(list.Select(kvp => kvp.Key).Contains("2"));

        Console.WriteLine(list.Select(kvp => kvp.Key).Contains("5"));

    }

}