using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    private static Dictionary<Item, int> results;

    private static List<Item> items;

    private static int maxChance;

    private static Random random = new Random();

    public static void Main()

    {       

        var a = new Item("a", 1);

        var b = new Item("b", 1);

        var c = new Item("c", 1);

        var d = new Item("d", 1);

        results = new Dictionary<Item, int>();

        results.Add(a, 0);

        results.Add(b, 0);

        results.Add(c, 0);

        results.Add(d, 0);

        items = new List<Item>();

        items.Add(a);

        items.Add(b);

        items.Add(c);

        items.Add(d);

        maxChance = items.Select(i => i.Chance).Sum();

        for(int i = 0; i < 1000000; ++i)

        {

            var item = GetItem();

            results[item]++;

        }

        Print(results);

    }

    private static Item GetItem()

    {

        var r = random.Next(0, maxChance + 1);

        var c = 0;

        for(int i = 0; i < items.Count; ++i)

        {

            c += items[i].Chance;

            if(c >= r) return items[i];

        }

        throw new ApplicationException("Your algorythm is suck! (" + r +"/" +  maxChance +")");

    }

    private static void Print<K,V>(Dictionary<K, V> dictionary)

    {

        foreach(var k in dictionary.Keys)

        {

            Console.WriteLine(k.ToString() + " = " + dictionary[k].ToString()); 

        }

    }

}

​

public class Item

{

    public string Name;

    public int Chance;

    public Item(string name, int chance)

    {

        Name = name;

        Chance = chance;

    }

    public override string ToString()

    {

        return Name;

    }

}