[RESOLVED] System Timer and DataBinding

Hi All,
If I have a class that implements INotifyPropertyChanged, and I have a property in that class that is bound to a label on a form, how do I avoid a Cross-threaded exception if I set the property value from a System.Timers.Timer.Elapsed event handler?

The following code demonstrates the exception.

Thanks for looking
Kevin

VB.Net Code:

Imports System.ComponentModel
Imports System.Timers
 
Public Class Form1
 
    Private thisClass As New aClass
 
    Private lbl As System.Windows.Forms.Label
    Public Sub New()
 
        InitializeComponent()
 
        'create the label and add it to the form
        lbl = New Label
        lbl.Text = "some text"
        Me.Controls.Add(lbl)
 
        'set the data binding and start a timer
        lbl.DataBindings.Add("Text", thisClass, "X")
 
 
    End Sub
 
 
End Class
 
Public Class aClass
    Implements INotifyPropertyChanged
 
    Private WithEvents tmr As New System.Timers.Timer()
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
    Public Sub New()
 
        AddHandler tmr.Elapsed, AddressOf tmr_Elapsed
        tmr.Interval = 1000
 
        tmr.Start()
    End Sub
 
    Private Sub tmr_Elapsed(sender As Object, e As ElapsedEventArgs)
        'change the property value when the timer elapses
        X = Guid.NewGuid.ToString
    End Sub
 
 
    Private _x As String = ""
    Public Property X As String
        Get
            Return _x
        End Get
        Set(value As String)
            _x = value
            RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs("X"))
        End Set
    End Property
End Class

I'm mainly posting this because I want to subscribe to this thread because I'm not entirely sure. This looks like something JMcIlhinney, .paul., or TG would have a good idea on.

The way to handle it in the original code where the timer is a member of the form is to simply Invoke in the timer elapsed handler...

VB.net Code:

Me.Invoke(New Action(Sub() thisClass.X = Guid.NewGuid.ToString()))

That said, my real problem is when the timer is not in the form and is a member of the class. I have updated the code in the original post to reflect this.

I suppose we'll have to wait for one of the smart kids.

The way I would do this would be to Post an event to the SynchronizationContext. If you want to see an example of that, look for the UDP class I posted in the CodeBank. That used a background thread as a listener, and when a message was received, it added it to a Queue, then posted an event. By posting to the UI SynchronizationContext, the event gets raised on the UI thread. You'd then set the label in there.

I believe that JMC showed a simpler way to do that than the one that I used in that UDP class, but the one in the UDP class is pretty doggone easy.

Quote:

---

Originally Posted by dday9

I'm mainly posting this because I want to subscribe to this thread because I'm not entirely sure. This looks like something JMcIlhinney, .paul., or TG would have a good idea on.

---

Hello Dday9,

You can subscribe to a thread without posting: At the top of the thread, there is "Thread Tools" list and in it there is the command "subscribe to this thread..."

Quote:

---

Originally Posted by Shaggy Hiker

The way I would do this would be to Post an event to the SynchronizationContext. If you want to see an example of that, look for the UDP class I posted in the CodeBank. That used a background thread as a listener, and when a message was received, it added it to a Queue, then posted an event. By posting to the UI SynchronizationContext, the event gets raised on the UI thread. You'd then set the label in there.

I believe that JMC showed a simpler way to do that than the one that I used in that UDP class, but the one in the UDP class is pretty doggone easy.

---

If you have code that is not UI-related itself but may affect an application's UI and you want to make it thread-safe then the SynchronizationContext is definitely the way to go. The Send method is akin to Invoke and Post is akin to BeginInvoke but you don't need access to a control created on the UI thread in order to call them. You just need to make sure that you create your SynchronizationContext instance on the UI thread in the first place. That's not usually a problem because you'll generally be creating the parent object on the UI thread and you just create the SynchronizationConext at the same time.

Thanks Shaggy.
I have this resolved using the code below. I've added a SynchronizationContextStringArgs class that is used to pass a property name and value to the SynchronizationContext.Post method. In the method, I then use reflection to set the property value (via casting with CtypeDynamic) which in turn raises the PropertyChange event and updates the label.

VB.net Code:

Imports System.ComponentModel
Imports System.Reflection
Imports System.Timers
 
Public Class Form1
 
    Private thisClass As New aClass
    Private lbl As System.Windows.Forms.Label
 
    Public Sub New()
 
        InitializeComponent()
 
        'create the label and add it to the form
        lbl = New Label
        lbl.Text = "some text"
        Me.Controls.Add(lbl)
 
        'set the data binding and start a timer
        lbl.DataBindings.Add("Text", thisClass, "X")
 
    End Sub
End Class
 
Public Class aClass
    Implements INotifyPropertyChanged
 
    Private WithEvents tmr As New System.Timers.Timer()
    Private Shared myContext As System.Threading.SynchronizationContext
 
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
 
    Public Sub New()
 
        myContext = System.Threading.SynchronizationContext.Current
 
        AddHandler tmr.Elapsed, AddressOf tmr_Elapsed
        tmr.Interval = 1000
        tmr.Start()
 
    End Sub
 
    Private Sub tmr_Elapsed(sender As Object, e As ElapsedEventArgs)
 
        Dim newValue As String = Guid.NewGuid.ToString
        Dim args As New SynchronizationContextStringArgs
        args.PropertyName = "X"
        args.NewValue = newValue
        [B]myContext.Post(AddressOf setProperty, args)[/B]
 
    End Sub
 
    Private _x As String = ""
    Public Property X As String
        Get
            Return _x
        End Get
        Set(value As String)
            _x = value
            RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs("X"))
        End Set
    End Property
 
    Private Sub setProperty(e As SynchronizationContextStringArgs)
 
        Dim prop As PropertyInfo = Me.GetType().GetProperty(e.PropertyName, BindingFlags.Public Or BindingFlags.Instance)
        If (prop IsNot Nothing) Then
            prop.SetValue(Me, CTypeDynamic(e.NewValue, prop.PropertyType), Nothing)
        End If
 
    End Sub
End Class
 
Public Class SynchronizationContextStringArgs
    Inherits EventArgs
    Public Property PropertyName As String
    Public Property NewValue As Object
End Class

As I was moving this code into my project, I realized it could be simplified greatly by posting to a ThreadSafe_PropertyChanged method directly from the property setter. That method has a parameter for the property name and it raises the Property_Change method. The following code illustrates this.

VB.net Code:

Imports System.ComponentModel
Imports System.Reflection
Imports System.Timers
 
Public Class Form1
 
    Private thisClass As New aClass
    Private lblX As System.Windows.Forms.Label
    Private lblY As System.Windows.Forms.Label
 
 
    Public Sub New()
 
        InitializeComponent()
 
        'create the label and add it to the form
        lblX = New Label
        lblX.Text = "X label"
        Me.Controls.Add(lblX)
 
        lblY = New Label
        lblY.Location = New Point(0, 30)
        lblY.Text = "Y label"
        Me.Controls.Add(lblY)
 
        'set the data binding and start a timer
        lblX.DataBindings.Add("Text", thisClass, "X")
        lblY.DataBindings.Add("Text", thisClass, "Y")
    End Sub
End Class
 
Public Class aClass
    Implements INotifyPropertyChanged
 
    Private WithEvents tmr As New System.Timers.Timer()
    Private myContext As System.Threading.SynchronizationContext
    Private rnd As New Random
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
 
    Public Sub New()
 
        myContext = System.Threading.SynchronizationContext.Current
 
        AddHandler tmr.Elapsed, AddressOf tmr_Elapsed
        tmr.Interval = 1000
        tmr.Start()
 
    End Sub
 
    Private Sub tmr_Elapsed(sender As Object, e As ElapsedEventArgs)
 
        X = Guid.NewGuid.ToString()
        Y = rnd.NextDouble()
 
    End Sub
 
    Private _x As String = ""
    Public Property X As String
        Get
            Return _x
        End Get
        Set(value As String)
            _x = value
            myContext.Post(AddressOf ThreadSafe_PropertyChanged, "X")
        End Set
    End Property
 
    Private _y As Double = 0
    Public Property Y As String
        Get
            Return _y
        End Get
        Set(value As String)
            _y = value
            myContext.Post(AddressOf ThreadSafe_PropertyChanged, "Y")
        End Set
    End Property
 
    Private Sub ThreadSafe_PropertyChanged(propertyName As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
 
    End Sub
End Class