Figure 2.4
Output of Listing 2.1.4 when viewed through a browser.

Adding Elements to a Queue
In Listing 2.1.4, we begin by creating an instance of the Queue class, qTasks (line 5). In line 7 through 14, we add eight new elements to qTasks using the Enqueue method. Recall that a queue supports First In, First Out ordering, so when we get ready to remove these elements, the first element to be removed will be "Wake Up", which was the first element added.

To quickly check if a particular element is an element of the queue, you can use the Contains method. Line 18 demonstrates usage of the Contains method. Note that it takes a single parameter, the element to search for, and returns True if the element is found in the queue and False otherwise.

Removing Elements from a Queue
With a Queue, you can only remove the element at the head. With such a constraint, it's no wonder that the Queue class only has a single member to remove an element: Dequeue. Dequeue not only removes the element at the head of the queue, but it also returns the element just removed.

If you attempt to remove an element from an empty Queue, the InvalidOperationException exception will be thrown and you will receive an error. Therefore, to prevent producing a runtime error in your ASP.NET page, be sure to either place the Dequeue statement in a Try ... Catch ... Finally block or ensure that the Count property is greater than zero (0) before using Dequeue. (For more information on Try ... Catch ... Finally blocks, refer to Chapter 9, "ASP.NET Error Handling." For an example of checking the Count property prior to using Dequeue, see lines 28 through 32 in Listing 2.1.4.) As with all the other collection types, you can remove all the Queue elements with a single call to the Clear method (line 36).

There might be times when you want to access the element at the head of the Queue without removing it from the Queue. This is possible via the Peek method, which returns the element at the head of the Queue without removing it. As with the Dequeue method, if you try to Peek an empty Queue, an InvalidOperationException exception will be thrown.

Iterating Through the Elements of a Queue
One way to iterate through the elements of a Queue is to simply use Dequeue to successively grab each item off the head. This approach can be seen in lines 27 through 32 in Listing 2.1.4. The major disadvantage of this approach is that, after iteration is complete, the Queue is empty!

As with every other collection type, the Queue can be iterated via a For Each ... Next loop or through the use of an enumerator. The following code snippet illustrates using the C# foreach statement to iterate through all the elements of a Queue without affecting the structure:

Queue qMyQueue = new Queue();  // Create a Queue

qMyQueue.Enqueue(5);
qMyQueue.Enqueue(62);  // Add some elements to the Queue
qMyQueue.Enqueue(-7);

// Iterate through each element of the Queue, displaying it
foreach (int i in qMyQueue)
Response.Write("Visiting Queue Element with Value: " + i + "<br>");
Working with the Stack Class
A stack is a data structure similar to a queue in that it supports only sequential access. However, a stack does bear one major difference from a queue: Rather than storing elements with a First In, First Out (FIFO) semantic, a stack uses Last In, First Out (LIFO). A crowded elevator behaves similar to a stack: The first person who enters the crowded elevator is the last person to leave, whereas the last person to board the elevator is the first out when it reaches its destination.

Adding, Removing, and Accessing Elements in a Stack