Introduction
For one of my projects I needed to show the 30 last messages to the user. I was not interested in keeping these messages. So I decided to use a queue. The messages come from different sources and different threads.
And the different threads bit is important.
The queue
Here is what can happen if you use the not threadsafe queue.
Let’s take this little example.
Imports System.Threading
Module Module1
Private _queue As Queue(Of String)
Private _count As Integer
Sub Main()
Console.WriteLine("Not concurrent threads")
_count = 0
Dim t As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessage(x))))
Dim t2 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessage(x))))
Dim t3 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessage(x))))
Dim t4 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessage(x))))
t.Start()
t2.Start()
t3.Start()
t4.Start()
Do While _count < 4
Loop
_count = 1
For Each q In _queue.Reverse.ToList
Console.WriteLine(_count & ". " & q)
_count += 1
Next
Console.ReadLine()
End Sub
Public Sub Addmessages(ByVal action As Action(Of String))
For x = 1 To 1000000
action.Invoke("test")
Next
_count += 1
End Sub
Public Sub AddMessage(message As String)
If _Queue Is Nothing Then
_queue = New Queue(Of String)(1000000)
End If
If Not String.IsNullOrEmpty(message) Then
_Queue.Enqueue(message)
End If
Dim result As String
If _queue.Count > 10 Then
_queue.Dequeue()
End If
End Sub
End Module```
That is 4 threads that add 1000000 messages to the queue at an alarming rate. And I only want to keep the last 10 messages in my queue.
The result.
> 128683. test
> 128684. test
> 128685. test
> 128686. test
> 128687. test
> 128688. test
> 128689. test
> 128690.
> 128691. test
> 128692. test
> 128693. test
> 128694. test
> 128695. test
> 128696. test
> 128697. test
> 128698. test
> 128699. test
> 128700. test
> 128701. test
> 128702. test
> 128703. test
> 128704. test
Those are only the last ones. There are 128704 elements in our queue and some are empty. Not sure how I get the empty ones but I guess it must be somewhere between an enqueue and a dequeue.
Anyway the results are not what we desire.
And of course now I could start adding blocks here and there to make sure the enqueue and dequeue are atomic operations and what not but Since .Net 4.0 there are Concurent collections and they are threadsafe by design. Lucky us.
## The concurrent queue
We can simply change our code to this.
```vbnet
Imports System.Collections.Concurrent
Imports System.Threading
Module Module1
Private _concurrentQueue As ConcurrentQueue(Of String)
Private _count As Integer
Sub Main()
Console.WriteLine("concurrent threads")
_count = 0
Dim t5 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessageConcurrently(x))))
Dim t6 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessageConcurrently(x))))
Dim t7 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessageConcurrently(x))))
Dim t8 As New Thread(New ParameterizedThreadStart(Sub() Addmessages(Sub(x) AddMessageConcurrently(x))))
t5.Start()
t6.Start()
t7.Start()
t8.Start()
Do While _count < 4
Loop
_count = 1
For Each q In _concurrentQueue.Reverse.ToList
Console.WriteLine(_count & ". " & q)
_count += 1
Next
Console.ReadLine()
End Sub
Public Sub Addmessages(ByVal action As Action(Of String))
For x = 1 To 1000000
action.Invoke("test")
Next
_count += 1
End Sub
Public Sub AddMessageConcurrently(message As String)
If _concurrentQueue Is Nothing Then
_concurrentQueue = New ConcurrentQueue(Of String)()
End If
If Not String.IsNullOrEmpty(message) Then
_concurrentQueue.Enqueue(message)
End If
Dim result As String
If _concurrentQueue.Count > 10 Then
_concurrentQueue.TryDequeue(result)
End If
End Sub
End Module
And now the result is.
concurrent threads
test
test
test
test
test
test
test
test
test
test
And that is exactly what I expected.
Conclusion
There is a namespace in .Net since 4.0 that contains threadsafe collections. And it is there for you to use. I would however not overuse it since I expect the blocking to have an effect on performance.
Chris is awesome.
