ShuffleBag.cs

using System;
using System.Collections.Generic;

public class ShuffleBag<T>
{
    public ShuffleBag() : this(-1) { }
    
    public ShuffleBag(Int32 seed)
    {
        if(seed < 0)
            random = new Random();
        else
            random = new Random(seed);

        activeList = new List<T>();
        inactiveList = new List<T>();
    }

    public void Reset()
    {
        activeList.AddRange(inactiveList);
        inactiveList.Clear();
    }

    public void Add(T t, int copies = 1)
    {
        for(int i = 0; i < copies; i++)
        {
            activeList.Add(t);
        }
    }

    public void Add(IEnumerable<T> enumerable)
    {
        foreach(var t in enumerable)
        {
            Add(t);
        }
    }

    public T Next()
    {
        int selectedIndex = random.Next(activeList.Count);
        T t = activeList[selectedIndex];
        activeList.RemoveAt(selectedIndex);
        inactiveList.Add(t);
        return t;
    }

    Random random;
    List<T> activeList;
    List<T> inactiveList;
}