Intro

Queue<T> is a FIFO (first in, first out) collection. The earliest enqueued item is processed first. Use it for buffering, breadth-first traversal, and producer-consumer style pipelines.

Queue<T> is implemented as a circular buffer in .NET:

• Enqueue adds at the tail; Dequeue removes from the head.
• Head/tail indices wrap around instead of shifting elements.
• Operations are O(1) on average, with occasional O(n) resize copies.

Because head and tail indices advance modulo the array length, neither Enqueue nor Dequeue shifts elements — only index arithmetic occurs. When the buffer fills completely, the queue copies all elements to a larger array with the head reset to index 0, then resumes the circular layout. This one-time O(n) resize is amortized over many operations so steady-state throughput stays O(1).

Structure

graph LR
    F[front] --> A[item one] --> B[item two] --> C[item three] --> T[back]

Example

var jobs = new Queue<string>();
jobs.Enqueue("job-1");
jobs.Enqueue("job-2");

Console.WriteLine(jobs.Dequeue()); // job-1
Console.WriteLine(jobs.Peek());    // job-2

Pitfalls

• Dequeue/Peek on an empty queue throws InvalidOperationException. Guard with Count when queue emptiness is expected.
• Using a queue where priority matters can delay urgent work. Switch to PriorityQueue<TElement, TPriority> when ordering by priority is required.
• Unbounded enqueues can grow memory silently in bursty systems. Apply backpressure or capacity policies at architecture boundaries.

Tradeoffs

• Queue<T> vs Stack<T>: queue preserves arrival order, stack prioritizes newest items.
• Queue<T> vs Channel<T>: queue is simple in-memory buffering, channels provide richer async coordination for concurrent producers/consumers.

Questions

Links

• Queue<T> class — API reference covering Enqueue, Dequeue, Peek, and circular buffer internals.
• PriorityQueue<TElement, TPriority> class — use when ordering by priority rather than arrival time is required.
• Collections in .NET — overview of all collection types with complexity and usage guidance.
• System.Threading.Channels library — async producer-consumer channels; the right upgrade path when Queue<T> needs concurrent access.
• Queue implementation in dotnet runtime — source code showing the circular buffer and resize logic.