using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        KeyValuePair<string, string> kvp = new KeyValuePair<string, string>("1", "1");

        KeyValuePair<string, string> kvp2 = new KeyValuePair<string, string>("2", "2");

        var list = new List<KeyValuePair<string, string>>{kvp, kvp2};

        var result = list.FirstOrDefault(x => x.Key == "3");

        Console.WriteLine(result);

        Console.WriteLine(result.Key is null);

        Console.WriteLine(result.Value is null);

        Console.WriteLine(result.GetType());

        Console.WriteLine(default(KeyValuePair<int, int>));

    }

}

​

​