TheAlgorithms/C-Sharp · error · InvalidOperationException

Deque is empty.

Error message

Deque is empty.

What it means

RemoveFront() throws InvalidOperationException when the deque has no elements. The library validates IsEmpty before accessing items[front] because a circular-array deque has no sentinel object to return when Count == 0. It is a deliberate fail-fast contract: removing from an empty deque is a caller bug, not a recoverable condition.

Solutions

1. Check the IsEmpty property (or Count == 0) before calling RemoveFront()
2. Wrap the call in try-catch for InvalidOperationException if emptiness is expected and recoverable
3. Use PeekFront() behind an IsEmpty check to inspect without removing
4. Fix producer/consumer balance so consumers never out-drain producers

Example fix

// before
T item = deque.RemoveFront();
// after
if (deque.IsEmpty) return;
T item = deque.RemoveFront();

if (deque == null) throw new ArgumentNullException(nameof(deque));
if (deque.IsEmpty)
    return; // or default(T) / Optional<T>.None
T item = deque.RemoveFront();

bool CanRemoveFront<T>(Deque<T> d) => d != null && !d.IsEmpty;

try
{
    T item = deque.RemoveFront();
    Process(item);
}
catch (InvalidOperationException ex) when (ex.Message == "Deque is empty.")
{
    // deque drained; handle empty case
}

When it happens

Trigger: Calling RemoveFront() when Count == 0, i.e. on a newly constructed Deque<T>, after removing all elements (front and rear drained to empty), or calling it a second time after a prior RemoveFront/RemoveRear emptied the deque.

Common situations: Looping 'while' over a deque draining elements without checking Count/IsEmpty; off-by-one bookkeeping where the consumer removes one more item than was added; shared deque consumed by multiple callers where another consumer emptied it first.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

• Tree is empty!
• Capacity must be at least 1.
• There are no items in the queue.
• The queue contains no items.
• Tree is empty!

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/63cc5b6a1ddd686e. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Deque/Deque.cs:139

        count++;
    }

    /// <summary>
    ///     Removes and returns the element at the front of the deque.
    ///     This operation is O(1) time complexity.
    /// </summary>
    /// <returns>The element at the front of the deque.</returns>
    /// <exception cref="InvalidOperationException">Thrown when the deque is empty.</exception>
    /// <example>
    ///     // Deque: [3, 5, 7].
    ///     int value = deque.RemoveFront();  // Returns 3, Deque: [5, 7].
    /// </example>
    public T RemoveFront()
    {
        // Validate that deque is not empty
        if (IsEmpty)
        {
            throw new InvalidOperationException("Deque is empty.");
        }

        // Retrieve the front element
        T item = items[front];

        // Clear the reference to help garbage collection
        items[front] = default!;

        // Move front pointer forward in circular fashion
        front = (front + 1) % items.Length;
        count--;

        return item;
    }

    /// <summary>
    ///     Removes and returns the element at the rear of the deque.
    ///     This operation is O(1) time complexity.

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