Programming Visual Basic 2005

Queues

A queue represents a first-in, first-out (FIFO) collection. The classic analogy is to a line (or queue if you are British) at a ticket window. The first person in line ought to be the first person to come off the line to buy a ticket.

A queue is a good collection to use when you are managing a limited resource. For example, you might want to send messages to a resource that can only handle one message at a time. You would then create a message queue so that you can say to your clients: "Your message is important to us. Messages are handled in the order in which they are received."

The Queue class has a number of member methods and properties, as shown in Table 17-3.

Table 17-3. Queue methods and properties

Method or property	Purpose
Count	Public property that gets the number of elements in the Queue
Clear	Removes all objects from the Queue
Contains	Determines if an element is in the Queue
CopyTo	Copies the Queue elements to an existing one-dimensional array
Dequeue	Removes and returns the object at the beginning of the Queue
Enqueue	Adds an object to the end of the Queue
GetEnumerator	Returns an enumerator for the Queue
Peek	Returns the object at the beginning of the Queue without removing it
ToArray	Copies the elements to a new array

Add elements to your queue with the Enqueue command and take them off the queue with Dequeue. You can also see what is next in the queue (without removing it) using Peek. Example 17-6 illustrates.

Example 17-6. Working ...