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`Queue`

In a queue, what is requested first is handled first. In C# the Queue type is a FIFO collection. It processes elements in a first-in, first-out order.

Queue handles the elements that it received longest ago first. To further understand Queue, it is helpful to examine some uses of this generic class.

`Enqueue` example

Here we demonstrate the use of the Enqueue method on the Queue. Enqueue adds to the end of a Queue. We often call it multiple times.

Step 1 First we add 4 ints to an instance of Queue. These could correspond to requests.

Step 2 We print out the total count of elements in the Queue. We added 4 ints, so there are 4 elements.

Tip With Enqueue, we add the new requests to the end of the Queue, because they will be dealt with last.

using System;
using System.Collections.Generic;

// Step 1: create a new Queue of integers.
var elements = new Queue<int>();
elements.Enqueue(5);
elements.Enqueue(10);
elements.Enqueue(15);
elements.Enqueue(20);

// Step 2: write count of elements.
Console.WriteLine("QUEUE COUNT: {0}", elements.Count);
QUEUE COUNT: 4

Loop example

Next we loop through the elements in the Queue. Frequently you will need to loop through the elements. We can do this in the standard way.

Tip The foreach-loop calls the Queue's enumerator, and you can use the standard foreach syntax.

Info Behind the scenes, the foreach statement is invoking the Queue's enumerator. The foreach syntax hides this.

using System;
using System.Collections.Generic;

// Add integers to queue.
Queue<int> collection = new Queue<int>();
collection.Enqueue(5);
collection.Enqueue(6);

// Loop through queue.
foreach (int value in collection)
{
    Console.WriteLine(value);
}5
6

Help system

This code could be adapted into a help request processing system. You don't want your company's visitors to end up waiting forever. The Queue ensures this won't happen.

Info This example simulates a system where new help RequestTypes are added to the Queue. New requests are the last to be processed.

Tip The oldest requests, those that have been waiting the longest, are the closest to being acted on. No one is left hanging infinitely.

using System;
using System.Collections.Generic;

class Program
{
    enum RequestType
    {
        MouseProblem,
        TextProblem,
        ScreenProblem,
        ModemProblem
    };

    static void Main()
    {
        // Initialize help request Queue.
        Queue<RequestType> help = new Queue<RequestType>();

        // Website records a Text Problem help request.
        help.Enqueue(RequestType.TextProblem);

        // Website records a Screen Problem help request.
        help.Enqueue(RequestType.ScreenProblem);

        // You become available to take a new request.
        // ... You can't help with Mouse problems.
        if (help.Count > 0 &&
            help.Peek() != RequestType.MouseProblem)
        {
            // You solve the request.
            // ... It was a TextProblem.
            help.Dequeue();
        }

        // Record a Modem Problem help request.
        help.Enqueue(RequestType.ModemProblem);
    }
}

Copy

We can copy Queue elements. We might have a Queue collection but need to loop over the elements in the reverse order. We call CopyTo, and then loop over the array in reverse.

Here We use CopyTo with an int array. We allocate the number of elements to the int array with the same integer as the Queue's Count.

Next We use the for-loop syntax, which gives more power when iterating over the arrays.

Result The array is written to the Console in front-to-back order, and then it is written in the opposite direction.

using System;
using System.Collections.Generic;

// New Queue of integers.
Queue<int> queue = new Queue<int>();
queue.Enqueue(5);
queue.Enqueue(10);
queue.Enqueue(15);
queue.Enqueue(20);

// Create new array with Length equal to Queue's element count.
int[] array = new int[queue.Count];

// Copy the Queue to the int array.
queue.CopyTo(array, 0);

// Loop through and display int[] in order.
Console.WriteLine("Array:");
for (int i = 0; i < array.Length; i++)
{
    Console.WriteLine(array[i]);
}

// Loop through int array in reverse order.
Console.WriteLine("Array reverse order:");
for (int i = array.Length - 1; i >= 0; i--)
{
    Console.WriteLine(array[i]);
}Array:
5
10
15
20
Array reverse order:
20
15
10
5

`Contains`

The Contains() method works in the same way as the List Contains method. It searches iteratively through the queue. The Queue does not use hashing so the search takes linear time.

using System;
using System.Collections.Generic;

var elements = new Queue<string>();
elements.Enqueue("bird");

// Search Queue with Contains.
if (elements.Contains("bird"))
{
    Console.WriteLine("CONTAINS: BIRD");
}
CONTAINS: BIRD

The performance of Queue, and the List.Insert method, should be tested. Using Insert on a List in the wrong way can hurt performance.

In .NET the Queue generic class provides methods to add (enqueue) and remove elements. With Queue we could process requests. This collection is useful for certain situations.

Queue