using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var dict = new Dictionary<string, int[]>();

        dict.Add("test", new[] { 1, 2 });

        Console.Write("До: ");

        Console.WriteLine(dict["test"][0]);

        dict["test"][0]++; //!!

        Console.Write("После: ");

        Console.WriteLine(dict["test"][0]);

    }

}