The Queue Collections.Generic in Visual Basic

The Queue is a bit particular as it’s based on the First In, First out (or first come first served) principle. This is exactly like when you’re in line to get your favorite coffee: the first customer that arrived is served first and so on. Here is an image to better illustrate the situation.

It’s the same thing in a Queue, the first element that you insert will be the first one to be removed.

The Queue is a Generic collection, that means it can take any type of information of a type specified at its declaration and has been specifically created to wrap the First come, first served principle. The collection is really convenient to use for anything ranging from email delivery queue to thread pool for example. There are several situations in which you will want to use a Queue instead of a List, again, the important part is that you know this type of collection exists so you can figure out when to use it!

Here is how to declare a Queue that can contain elements of type String. In this example I am going to simulate a car production Queue.

Dim CarProductionQueue = New Queue(Of String)

Enqueue(T Element)

To add items to a Queue, you need to use the Enqueue(T Element) method instead of add(T Element). That method name makes a bit more sense as well! Here is an example.

'Enqueue new elements
CarProductionQueue.Enqueue("Toyota GT86")
CarProductionQueue.Enqueue("Scion FRS")
CarProductionQueue.Enqueue("Subaru BRZ")

In the code above, I am adding 3 cars in the car production Queue (String because its only the car brand and name). That’s why I am calling the Enqueue(String CarToEnqueue) method three times.

Dequeue()

Instead of Remove(), we’re using Dequeue() to retrieve the next element. It then returns the Element that has been dequeued and you need to retrieve that element if you wish to use it in some way. In the following example, I’m using the element directly without storing it in a variable in between the steps. Remember that once the element is dequeued, it is not contained in the Queue anymore!

'Dequeue an element from the ProductionQueue
Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())

I am calling the Dequeue() method three times because there are three elements in the Queue, if you try to call the method when no elements are left in the collection, it’s going to throw an exception and your code will crash. Make sure that there is at least one element in the Queue before you Dequeue!

Count

Here is a way to output all the elements while making sure the Queue is not empty! But you can surely find other ways to do the same thing!

While CarProductionQueue.Count > 0
     Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
End While

The Count parameter will give you the number of elements in the Queue at the current time and can be useful to avoid a code crash by checking how many elements are left in the collection.

Complete Code

Here is the complete code containing three ways you can dequeue elements from your Queue.

Module Program

    Sub Main()
        Dim CarProductionQueue = New Queue(Of String)

        'Enqueue new elements in the ProductionQueue
        CarProductionQueue.Enqueue("Toyota GT86")
        CarProductionQueue.Enqueue("Scion FRS")
        CarProductionQueue.Enqueue("Subaru BRZ")

        ''Dequeue elements from the ProductionQueue (one by one)
        'Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
        'Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
        'Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())

        'Dequeue with a While loop
        While CarProductionQueue.Count > 0
            Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
        End While

        ''Dequeue with a For loop
        'Dim NumberOfCarsInQueue = CarProductionQueue.Count

        'For Index As Integer = 1 To NumberOfCarsInQueue Step 1
        '    Console.WriteLine("Production complete for " & CarProductionQueue.Dequeue())
        'Next

        Console.ReadKey()
    End Sub

End Module

There is something important to remember about collections like we’re seeing right now: they are not thread safe! What is a thread? we will dive into that topic later but that’s still something to remember.

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