Rated Random

void Main()
{
	RandomItem<string> operation = new RandomItem<string>();
	
	operation.Add("A", 0.4d);
	operation.Add("B", 0.5d);
	operation.Add("D");
	
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
	Console.WriteLine(operation.Random().item);
}

// Define other methods and classes here
public class Item<T>
{
	public T item;
	public double rate;
	
	public Item(T item, double rate = 0)
	{
		if(rate > 1) throw new Exception("Rate can't be bigger than 1");
		
		this.item = item;
		this.rate = rate;
	}
}

public class RandomItem<T>
{
	private List<Item<T>> itemList = new List<Item<T>>();
	private double cumulativeRate = 0;
	private int unratedItemCount = 0;
	private bool calculated = false;
	private Random random = new Random();
	
	public void Add(T item, double rate = 0)
	{
		itemList.Add(new Item<T>(item, rate));
		
		if(rate == 0)
		{
			unratedItemCount++;
		}
		else
		{
			cumulativeRate += rate;
		}
		
		
		if(cumulativeRate > 1) throw new Exception("Cumulative rate exceeded 1");
	}
	
	public Item<T> Random()
	{
		CalculateAllRates();
		
		double rand = random.NextDouble();
		Console.WriteLine(rand);
		
		double pivot = 0;
		
		foreach(var item in itemList)
		{
			pivot += item.rate;
			
			if(pivot >= rand)
			{
				return item;
			}
		}
		
		return null;
	}
	
	private void CalculateAllRates()
	{
		if(!calculated)
		{
			double unitRate = (1 - cumulativeRate) / unratedItemCount;
		
			foreach(var item in itemList)
			{
				if(item.rate == 0)
				{
					item.rate = unitRate;
				}
			}
			
			calculated = true;
		}
	}
}