Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;

namespace ConsoleApplication13
{
    class Program
    {
        static void Main(string[] args)
        {
            var dict = new Dictionary<string, object>
            {
                { "123", 123 },
                { "17", "17" },
                { Math.PI.ToString(), Math.PI },
            };

            foreach (var pair in dict)
            {
                Console.WriteLine(pair);
            }

            var someString = String.Join(", ",
                from pair in dict
                where pair.Key.Length > 2
                select String.Format("{0}={1}", pair.Key, pair.Value)
            );
            someString = String.Join(", ",
                dict
                .Where(pair => pair.Key.Length > 2)
                .Select(pair => String.Format("{0}={1}", pair.Key, pair.Value))
            );

            Console.WriteLine(someString);
        }
    }
}