using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Dictionary<string, string> dico = new Dictionary<string, string>() {

            { "test_key", "test_value" },

        };

        Console.WriteLine("value is: "+dico.GetValueOrDefault("test_key"));

        Console.WriteLine("value is: "+dico.GetValueOrDefault("missing_key"));

        Console.WriteLine("value is: "+dico.GetValueOrDefault("missing_key", "default value"));

    }

}

​