TheAlgorithms/C-Sharp · error · InvalidOperationException

There are no items in the queue.

Error message

There are no items in the queue.

What it means

ArrayBasedQueue.Dequeue throws InvalidOperationException('There are no items in the queue.') when IsEmpty() is true. Dequeuing from an empty queue is an invalid state operation; the error is thrown before any index math runs.

Solutions

1. Check IsEmpty() (or Count) before each Dequeue.
2. Catch InvalidOperationException around Dequeue when drain-to-empty is normal.
3. Fix producer logic so enqueue happens before consumer dequeue attempts.

Example fix

// before
var item = queue.Dequeue(); // throws when empty
// after
if (!queue.IsEmpty())
    var item = queue.Dequeue();

if (!queue.IsEmpty())
    var item = queue.Dequeue();

try { var item = queue.Dequeue(); }
catch (InvalidOperationException) { /* queue empty */ }

When it happens

Trigger: Calling Dequeue() on a fresh queue or after all items were dequeued, e.g. consumer loops that dequeue more times than producers enqueued.

Common situations: BFS/producer-consumer code where the queue drains faster than expected, or missing the enqueue step on an early-exit path.

Related errors

• Cannot prune empty list
• There are no items in the queue.
• The queue contains no items.
• No training data available.
• Model must be fitted before prediction.

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/a379c0a832e72d81. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Queue/ArrayBasedQueue.cs:43

    ///     Clears the queue.
    /// </summary>
    public void Clear()
    {
        startIndex = 0;
        endIndex = 0;
        isEmpty = true;
        isFull = false;
    }

    /// <summary>
    ///     Returns the first item in the queue and removes it from the queue.
    /// </summary>
    /// <exception cref="InvalidOperationException">Thrown if the queue is empty.</exception>
    public T Dequeue()
    {
        if (IsEmpty())
        {
            throw new InvalidOperationException("There are no items in the queue.");
        }

        var dequeueIndex = endIndex;
        endIndex++;
        if (endIndex >= queue.Length)
        {
            endIndex = 0;
        }

        isFull = false;
        isEmpty = startIndex == endIndex;

        return queue[dequeueIndex];
    }

    /// <summary>
    ///     Returns a boolean indicating whether the queue is empty.
    /// </summary>

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