Priority queue simple implementation in C#

priority_queue.cs

public class PrioQueue<T>
{
    int total_size;
    SortedDictionary<int, Queue<T>> storage;

    public PrioQueue()
    {
        this.storage = new SortedDictionary<int, Queue<T>>();
        this.total_size = 0;
    }

    public bool IsEmpty()
    {
        return (total_size == 0);
    }

    public object Dequeue()
    {
        if (IsEmpty())
        {
            throw new Exception("Please check that priorityQueue is not empty before dequeing");
        }
        else
            foreach (Queue<T> q in storage.Values)
            {
                // we use a sorted dictionary
                if (q.Count > 0)
                {
                    total_size--;
                    return q.Dequeue();
                }
            }

        Debug.Assert(false, "not supposed to reach here. problem with changing total_size");

        return null; // not supposed to reach here.
    }

    // same as above, except for peek.

    public object Peek()
    {
        if (IsEmpty())
            throw new Exception("Please check that priorityQueue is not empty before peeking");
        else
            foreach (Queue<T> q in storage.Values)
            {
                if (q.Count > 0)
                    return q.Peek();
            }

        Debug.Assert(false, "not supposed to reach here. problem with changing total_size");

        return null; // not supposed to reach here.
    }

    public object Dequeue(int prio)
    {
        total_size--;
        return storage[prio].Dequeue();
    }

    public void Enqueue(T item, int prio)
    {
        if (!storage.ContainsKey(prio))
        {
            storage.Add(prio, new Queue<T>());
        }
        storage[prio].Enqueue(item);
        total_size++;
    }