using System.Collections.Generic; //For Dictionary

using System.Collections.ObjectModel; //For ReadOnlyDictionary

public class Program

{

    private static readonly Dictionary<string, int> testDict1 = new Dictionary<string, int>

    {

        {"One", 1},

        {"Two", 2}

    };

    private static ReadOnlyDictionary<string, int> testDict2 = new ReadOnlyDictionary<string, int>(

        new Dictionary<string, int>

        {

            {"One", 1},

            {"Two", 2}

        });

    public static void Main()

    {

        //In a normal dictionary, the values can still be changed if it's declared as readonly

        //readonly only protects the reference. So I can do this:

        testDict1["One"] = -99; //Totally fine.

​

        //But I can't do this:

        //testDict1 = new Dictionary<string, int>();

​

        //A ReadOnlyDictionary is the exact opposite in this regard.

        //I can't change values:

        //testDict2["One"] = -99; //Error

​

        //But I can change the reference to a different object:

        testDict2 = new ReadOnlyDictionary<string, int>(

            new Dictionary<string, int>

            {

                {"One", -99},

                {"Two", -99}

            });

    }

}