cross thread access

by: jagguyPosted on 2009-11-06 at 02:54:39ID: 25758058

this works but I am not sure if there can be problems with this method
Private Sub bw_DoWork(ByVal sender As Object, ByVal e As DoWorkEventArgs)

For i = 1 To 10
System.Threading.Thread.Sleep(500)
bw.ReportProgress(i * 10)
a.X = i
a.Y = i
e.Result = "l"
Next

End Sub

Private Sub bw_ProgressChanged(ByVal sender As Object, ByVal e As ProgressChangedEventArgs)
img = New Image
_x = 100
_y = 300
url = New Uri("images/circle.png", UriKind.Relative)
uu = New BitmapImage(url)
uu.UriSource = url
img.Source = uu
Canvas.SetLeft(img, _x)
Canvas.SetTop(img, _y)
Canvas.SetZIndex(img, 10)
canvas1.Children.Add(img)
AddHandler uu.ImageOpened, AddressOf uu_downloadComplete

Me.tb1.Text = e.ProgressPercentage.ToString() & "%"

End Sub

by: vo1dPosted on 2009-11-07 at 02:29:06ID: 25765724

a cross thread exception occurs when a thread other than the creating thread of a control tries to access one of that control's methods or properties. to avoid that problem, you have to invoke a delegate for your ui update.

by: jagguyPosted on 2009-11-07 at 04:03:11ID: 25765944

>>you have to invoke a delegate for your ui update.

In vb.net how do i do this as I only have c# examples

worker.DoWork +=
delegate(object sender, DoWorkEventArgs e)

by: vo1dPosted on 2009-11-07 at 05:34:06ID: 25766262

i'm not a vb developer.
but the reason, why your code example1 crashes but example2 not, is the fact, that the bw_DoWork eventhandler runs in the new thread of the backgroundworker, whereas your bw_ProgressChanged runs in the ui thread, which got the controls. you can prove that by extending both eventhandler with a line (in c#) :
Console.WriteLine(Thread.CurrentThread.ManagedThreadId);

by: adgPosted on 2009-11-07 at 13:15:46ID: 25768088

The solution is to use the control's Invoke method.

There is a property on a control (a Form is a control), that is true when you read it from a thread that is not the GUI thread, called InvokeRequired.

If InvokeRequired is true, then you have to use the Invoke method and pass in a delegate that does the work. My example uses an anonymous delegate and checks InvokeRequired to tell whether to use Invoke or just execute the anonymous delegate.

I use ThreadPool.QueueUserWorkItem to get ThreadProcedure to execute in a thread that is not the GUI thread (in a system thread pool thread).

Action is a standard delegate type that declares a function that takes no arguments, and doesn't return a value. An anonymous delegate declared the way I have declared it magically has access to all of the local variables that are in scope at the point that the delegate is declared.

Invoke is a method on a control (in this case, the Form) that executes the passed delegate in the GUI thread. If you are familliar with Win32 programming, the GUI thread message pump eventually gets around to executing the method.

If you are familliar with the async pattern, there are BeginInvoke and EndInvoke methods that allow you to asynchronously execute code in the GUI thread without waiting for it to complete.

using System;
using System.Threading;
using System.Windows.Forms;
 
namespace EECrossThreadAccess
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            ThreadPool.QueueUserWorkItem(ThreadProcedure);
 
        }
 
        private void ThreadProcedure(object p)
        {
            Action doSomething = new Action(delegate
                {
                    label1.Text = "This was changed from another thread";
                });
 
            if (InvokeRequired)
            {
                // Running in some thread other than the GUI thread
                Invoke(doSomething);
            }
            else
            {
                // I'm running in the GUI thread, it's ok to just do it
                doSomething();
            }
        }
    }
}

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by: adgPosted on 2009-11-08 at 14:26:16ID: 25772270

Oh, you wanted it in VB. Since this is the C# area, I just blindly assumed you wanted it in C#.

I am not a VB expert, but I figured it out :)

Basically, I have added a little block of code at the top of the function that tests InvokeRequired, and if it is true, invokes itself using Invoke.

I tried to get it to work using an anonymous method, but nothing happened. Perhaps I am getting something slightly wrong with the lambda syntax in VB. Instead, I have put it in a separate sub and used AddressOf. It works that way.

Imports System.Threading
 
Public Class Form1
    Private Sub SetLabelTextExample(ByVal text As String)
        If InvokeRequired Then
            Dim invokeMyself As Action(Of String) = AddressOf SetLabelTextExample
            Invoke(invokeMyself, text)
            Return
        End If
 
        Label1.Text = text
    End Sub
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        REM Queue a user work item to artificially get execution into another thread
        ThreadPool.QueueUserWorkItem(AddressOf ThreadProcedure)
    End Sub
 
    Private Sub ThreadProcedure()
        REM Executing in non-UI thread...
        SetLabelTextExample("Called from non-UI thread")
    End Sub
 
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        REM Executing in UI thread...
        SetLabelTextExample("Called from UI thread")
    End Sub
End Class

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screenshot.png
- 6 KB
Screenshot of sample implementation (EECrossThreadAccess)

by: chinu1310Posted on 2009-11-11 at 11:25:55ID: 25798111

What you need to do is use Dispatcher.BeginIvoke in order to update the UI from backend thread.

http://forums.silverlight.net/forums/t/19637.aspx

by: cafejavaPosted on 2009-11-16 at 08:09:24ID: 25831297

You can use this code in C# to make them access each other.

Control.CheckForIllegalCrossThreadCalls = false;

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by: vo1dPosted on 2009-11-16 at 08:14:31ID: 25831341

please do not use Control.CheckForIllegalCrossThreadCalls = false .

this is what MSDN sais about that property:
---
When a thread other than the creating thread of a control tries to access one of that control's methods or properties, it often leads to unpredictable results. A common invalid thread activity is a call on the wrong thread that accesses the control's Handle property. Set CheckForIllegalCrossThreadCalls to true to find and diagnose this thread activity more easily while debugging. Note that illegal cross-thread calls will always raise an exception when an application is started outside the debugger.
---

as explained before use invocation to be threadsafe.

Question

cross thread access

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