[Concept Entry]: C# Queue

content/c-sharp/concepts/queue/queue.md

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+---
+Title: 'Queue'
+Description: 'Queue is a first-in, first-out (FIFO) data structure that allows insertion of elements to the rear and removal of elements from the front.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Collections'
+  - 'Data Structures'
+  - 'Queues'
+CatalogContent:
+  - 'learn-c-sharp'
+  - 'paths/computer-science'
+---
+
+**`Queue`** is a [data structure](https://www.codecademy.com/resources/docs/general/data-structures) representing a first-in, first-out (FIFO) collection of elements that allows insertion of new elements to the rear, and removal of elements from the front.
+
+## Syntax
+
+The syntax to create a new queue is:
+
+```pseudo
+Queue<T> queueName = new Queue<T>();
+```
+
+**Parameters:**
+
+- `T`: The type of elements the queue will store.
+
+**Return value:**
+
+Creating a queue with new `Queue<T>()` returns a new empty `Queue<T>` instance.
+
+## Queue Operations
+
+`Queue<T>` provides three primary operations:
+
+- `Enqueue` adds an element to the end of the `Queue<T>` instance. `null` is an acceptable value for reference type elements.
+- `Dequeue` returns and removes an element from the front of the `Queue<T>` instance.
+- `Peek` returns the front-most element in the `Queue<T>` instance without removing it.
+
+## Key Behaviors of a Queue
+
+1. `Queue<T>` accepts `null` as a valid value for reference types, and allows insertion of duplicate elements.
+2. The `Peek()` and `Dequeue()` methods will throw an `InvalidOperationException` if the queue is empty.
+3. `Enqueue()`, `Dequeue()`, and `Peek()` are O(1) amortized.
+4. `Queue<T>` is not thread-safe. Consider using `ConcurrentQueue<T>` for concurrent access.
+5. The `Capacity` property returns the maximum number of elements the `Queue<T>` instance can currently hold. This value is increased automatically as elements are added to the collection, and can be reduced by calling the `TrimExcess()` method.
+
+## Example
+
+In this example a queue is created, populated with strings, and processed using `Peek()` and `Dequeue()`:
+
+```cs
+using System;
+using System.Collections.Generic;
+
+class Program
+{
+  static void Main()
+  {
+    Queue<string> queue = new Queue<string>();
+
+    // Adding elements to the back of the queue
+    queue.Enqueue("one");
+    queue.Enqueue("two");
+    queue.Enqueue("three");
+    queue.Enqueue("four");
+    queue.Enqueue("five");
+
+    // Peeking the front-most element
+    string peek = queue.Peek();
+    Console.WriteLine($"Peek: {peek}");
+
+    // Removing elements from the front of the queue
+    while (queue.Count > 0)
+    {
+      string front = queue.Dequeue();
+      Console.WriteLine($"Dequeue: {front}");
+    }
+  }
+}
+```
+
+Output:
+
+```shell
+Peek: one
+Dequeue: one
+Dequeue: two
+Dequeue: three
+Dequeue: four
+Dequeue: five
+```
+
+## Codebyte Example
+
+In this example a queue of integers is used to demonstrate peeking at the front element and removing items in FIFO order:
+
+```codebyte/csharp
+using System;
+using System.Collections.Generic;
+
+public class Example
+{
+  public static void Main()
+  {
+    Queue<int> queue = new Queue<int>();
+
+    queue.Enqueue(1);
+    queue.Enqueue(2);
+
+    Console.WriteLine("Peek: " + queue.Peek());
+
+    int first = queue.Dequeue();
+    int second = queue.Dequeue();
+
+    Console.WriteLine("Dequeue 1: " + first);
+    Console.WriteLine("Dequeue 2: " + second);
+  }
+}
+```