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ManzZup 21Apr2011 11:07

Events + Multi-Threading to Overcome Cross Thread Operation Error in VB.NET
 
Though VB is an object oriented language with multithreading I was always dissapointed with its less flexibility in using threads with the GUI. Of course many of you may have seen this InvalidOperationBlahException saying you cannot put up a progress bar/ timer./ count down iun a seperate thread while it interacts with the GUI. VB's solution was given as the background worker but I personally found it not-so-coordinate as well. So i thought of bringing up my own solutions. [Sure someother may have found it already but fot the sake of Googlers i'll continue with the tute :D]

The solution i came up is using the high support given to the Events by VB.NET.

Introduction



Logic:
  1. Make a variable with encapsulation using the get/set methods.
  2. Raise an event if the variable is changed.
  3. Make the thread change the variable value as raise the event.
  4. Catch the event using the GUI.
  5. Function in the GUI will change the values [ex: progress] within the form so no cross thread calls.

The Code



Make a new project in VB.NEt and add a progressBar onto it and name it as " pbMain ".

In Form1 add this code.

Code: vb

Public Class Form1
    Public Event VariableChange(ByVal val As Integer)
    Private Delegate Sub setProgressCallback(ByVal e As Integer)
    Public Property progress() As Integer
        Get
            Return progress
        End Get
        Set(ByVal value As Integer)
            RaiseEvent VariableChange(value)
        End Set
    End Property
    Public Sub progressChange(ByVal value As Integer)
        If pbMain.InvokeRequired Then
            Dim d As New setProgressCallback(AddressOf progressChange)
            Me.Invoke(d, New Object() {value})
        Else
            pbMain.Value = value
        End If
    End Sub
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        AddHandler VariableChange, AddressOf progressChange

        Dim t1 As New Threading.Thread(AddressOf threadAction)
        t1.Start()
End Sub

    Sub threadAction()
        For i As Integer = 0 To 100 Step 1
            progress = i
            Threading.Thread.Sleep(200)
        Next
    End Sub
End Class


Lets go code by code,

Code: vb

Public Event VariableChange(ByVal val As Integer)

Public Property progress() As Integer
        Get
            Return progress
        End Get
        Set(ByVal value As Integer)
            RaiseEvent VariableChange(value)
        End Set
    End Property


This makes a new event, as it names to fire up if a variable is changed.And the next block use Encapsulation like code for updating the the variable progress and note that in the Set method we raise the VariableChanged event.

Code: vb

AddHandler VariableChange, AddressOf progressChange

Private Delegate Sub setProgressCallback(ByVal e As Integer)
Public Sub progressChange(ByVal value As Integer)
        If pbMain.InvokeRequired Then
            Dim d As New setProgressCallback(AddressOf progressChange)
            Me.Invoke(d, New Object() {value})
        Else
            pbMain.Value = value
        End If
End Sub


Then in the form load we add a handler to handle the VaiableChanged Event if someone fired it. We direct the catching part to the progressChanged function and here's where the tricky part comes. In here we cannot just update the progressBar because they exists in separate threads, so we first check wheather it needed to be Invoked, if yes, we make a callBackRequest with the integer value we have back to the function, and this callBack calls the function as it is called by the Form1 thread or the main thread. So NO errors are given


Then we just make this threadAction mwthod to test the thing, it will update the progress variable value with an interval of 200 miliseconds. As the sleep() is given to the thread handling the progress, the main thread will work well without any hanging or stuff while the progress is updated.

THAT'S IT :D, by this mean you can add MANY THREADS so your customers will think your program is SUPER-FAST. This is juz a basic implementation but you can develop it into anything.

Hope ya'll enjoy threads :) cyah all

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