Retos 5 MSDN: http://blogs.msdn.com/b/esmsdn/archive/2014/10/31/retosmsdn-reto-5.aspx

DictionaryPlus.cs

using System.Collections.Generic;
using System.Linq;

namespace Reto5
{
    public class DictionaryPlus<TK, TV> : Dictionary<TK, TV>
    {
        public IEnumerable<TV> this[params TK[] keys] {
            get {
                return keys.Select(key => base[key]);
            }
        }
    }
}

Duration.cs

using System;

namespace Reto5
{
    public class Duration
    {

        private readonly Func<DateTime, DateTime> func;

        private Duration(Func<DateTime, DateTime> func = null) {       
             this.func = func;
        }

        public DateTime From(DateTime date) {
            if (func == null) 
                throw new ArgumentOutOfRangeException();
            return func(date);
        }


        public static explicit operator Duration(int num) {
            return new Duration();
        }


        public static Duration Day {
            get {
                return new Duration(t => t.AddDays(1));
            }
        }

        public static Duration Week {
          get {
                return new Duration(t => t.AddDays(7));
          }
        }

        public static Duration Month {
            get {
                return new Duration(t => t.AddMonths(1));
            }
        }
    }
}

NotNullExtension.cs

using System;
using System.Runtime.CompilerServices;

namespace Reto5
{
    public static class NotNullExtension
    {
        public static T NotNull<T>(this T obj, string name = null, [CallerMemberName]string caller = "") where T : class
        {
            if (obj != null) return obj;
            var msg = String.Format("{0} was called with null parameter", caller);
            throw new ArgumentNullException(name, msg);
        }
    }
}

ToUpperNoCopyExtension.cs

using System;

namespace Reto5
{
    // MUST BE COMPILED WITH: /unsafe 
    
    public static class ToUpperNoCopyExtension
    {
        public unsafe static void ToUpperNoCopy(this string obj) {
            if (obj == null) throw new ArgumentNullException();
            fixed (char* p = obj) {
                for (int i = 0; i < obj.Length; i++) {
                    p[i] = Char.ToUpper(p[i]);
                }
            }
        }
    }
}