ConcurrentQueue

C# course · Expert · Lesson 26/47

Learn the thread-safe queue with ConcurrentQueue.

ConcurrentQueue

ConcurrentQueue<T> is a thread-safe first-in-first-out (FIFO) queue. Many threads can call Enqueue and TryDequeue at once; the contents stay consistent without locking.

It uses a lock-free linked list internally. In producer-consumer scenarios, threads add tasks to the queue and drain them from a pool.

TryDequeue and TryPeek

TryDequeue(out T item) removes the front item and returns true; if the queue is empty it returns false instead of throwing. This is the safe way to poll an empty queue.

TryPeek(out T item) looks at the front without removing it. In a loop, while (queue.TryDequeue(out var x)) drains every item in order.

Exercise Enqueue and drain the queue

The queue below is empty. Use Enqueue to add three items: "a", "b", "c". Then loop with while (queue.TryDequeue(out var item)) and print each item with Console.WriteLine(item).

Show example solution

queue.Enqueue("a"); queue.Enqueue("b"); queue.Enqueue("c"); Then while (queue.TryDequeue(out var item)) Console.WriteLine(item);
Example solution:
using System;
using System.Collections.Concurrent;

class Program
{
    static void Main()
    {
        var queue = new ConcurrentQueue<string>();
        queue.Enqueue("a");
        queue.Enqueue("b");
        queue.Enqueue("c");
        while (queue.TryDequeue(out var item))
        {
            Console.WriteLine(item);
        }
    }
}

Test yourself

Complete the Enqueue call
```
queue.____;
```
Show answer
```
queue.Enqueue("task");
```
Which method removes an item from a ConcurrentQueue?
- Pop
- Remove
- Take
- TryDequeue
Show answer

TryDequeue — Correct. TryDequeue safely removes the front item.
Match each queue method

Enqueue

Add to the back
TryDequeue

Remove from front
TryPeek

Look at front without removing

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ConcurrentQueue