TheAlgorithms/C-Sharp · error · InvalidOperationException

Stack is empty

Error message

Stack is empty

What it means

ListBasedStack<T>.Peek (backed by LinkedList<T>) returns the first element as the stack top. When the underlying list is empty (First is null) it throws InvalidOperationException with 'Stack is empty'.

Solutions

1. Check stack.Count (or IsEmpty) before Peek.
2. Use try/catch around InvalidOperationException when empty is expected.
3. Restructure the loop so Peek is only called after at least one Push.

Example fix

// before
var top = listStack.Peek();
// after
var top = listStack.Count > 0 ? listStack.Peek() : default;

if (listStack.Count == 0) return default;

static bool TryPeek<T>(ListBasedStack<T> stack, out T value) { if (stack.Count > 0) { value = stack.Peek(); return true; } value = default; return false; }

try { var top = listStack.Peek(); } catch (InvalidOperationException) { top = default; }

When it happens

Trigger: Calling Peek on an empty ListBasedStack — before any Push, or after all items were popped.

Common situations: Test code checking Peek on a fresh stack, iterative algorithms peeking between phases when the stack may have been fully consumed.

Related errors

• List is empty.
• Stack is empty
• The stack contains no items.
• Array is empty.
• Queue is empty.

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/187ce1f01319903d. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Stack/ListBasedStack.cs:64

    /// </summary>
    public void Clear() => stack.Clear();

    /// <summary>
    ///     Determines whether an element is in the <see cref="ListBasedStack{T}" />.
    /// </summary>
    /// <param name="item">The item to locate in the <see cref="ListBasedStack{T}" />.</param>
    /// <returns>True, if the item is in the stack.</returns>
    public bool Contains(T item) => stack.Contains(item);

    /// <summary>
    ///     Returns the item at the top of the <see cref="ListBasedStack{T}" /> without removing it.
    /// </summary>
    /// <returns>The item at the top of the <see cref="ListBasedStack{T}" />.</returns>
    public T Peek()
    {
        if (stack.First is null)
        {
            throw new InvalidOperationException("Stack is empty");
        }

        return stack.First.Value;
    }

    /// <summary>
    ///     Removes and returns the item at the top of the <see cref="ListBasedStack{T}" />.
    /// </summary>
    /// <returns>The item removed from the top of the <see cref="ListBasedStack{T}" />.</returns>
    public T Pop()
    {
        if (stack.First is null)
        {
            throw new InvalidOperationException("Stack is empty");
        }

        var item = stack.First.Value;
        stack.RemoveFirst();

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