How do I use Event Handlers

[Brian Henry]

Hi all I'm am trying to do sum custom events in a Class. My class listens for a UDP Broadcast, I want to Raise and Event when a message is received to display the message from a windows form. I am having no luck. Can sum one help.

Here is my code.

Public Class UDP_Broadcast
    Public Event NewMessage(ByVal Message As String)
    Public Port As Integer = 2456

    Public Sub Receiver_Load()
        Dim t As New Threading.Thread(AddressOf listen)
        t.IsBackground = True
        t.Start()
    End Sub

    Public Sub Receiver_Closing()
        If udp IsNot Nothing Then udp.Close()
    End Sub

    Private udp As Net.Sockets.UdpClient
    Private Sub listen()
        Try
            udp = New Net.Sockets.UdpClient(Port)
            udp.EnableBroadcast = True
            Dim ep As New Net.IPEndPoint(Net.IPAddress.Broadcast, Port)
            Do
                Dim b() As Byte = udp.Receive(ep)
                'Me.Invoke(safeAddText, System.Text.Encoding.UTF32.GetString(b))
                safeAddText(System.Text.Encoding.UTF32.GetString(b))
            Loop
        Catch ex As Exception
            If udp IsNot Nothing Then udp.Close()
        End Try
    End Sub

    Private Delegate Sub delAddText(ByVal text As String)
    Private safeAddText As New delAddText(AddressOf AddText)
    Private Sub AddText(ByVal text As String)
        RaiseEvent NewMessage(text)
        'MsgBox(text)
    End Sub

End Class

Form Code

 Private WithEvents Mess As UDP_Broadcast
    Private Sub UDP_Broadcast1(New_UDP_Mess As String) Handles Mess.NewMessage
        MsgBox(New_UDP_Mess)
    End Sub

[ident]

do you ever get past Dim b() As Byte = udp.Receive(ep)????

[DataMiser]

You have to define an event in your class

Public Event EventName(ArgumentList)

Then you have to raise the event somewhere

RaiseEvent EventName(ArgumentList)

In your main form or wherever it is to be used you would dim it with events as you have and the have a sub routine that handles the event.

looks like you are only missing the event definition.

[DataMiser]

Never mind, when I looked at the code the line where the event was defined had scrolled off the top and I did not see it.

[Shaggy Hiker]

If you raise that event from a background thread, you'll be raising it on the wrong thread.

Over in the .NET CodeBank I have a UDP class that is derived from a class I use in a robotics project (which can be found in the Networking forum in a thread I started). It deals with broadcasts, too, so it might be of iterest for a variety of reasons.

[Brian Henry]

Thanks I will check it out.

[Evil_Giraffe]

I am having no luck.

In what way is it not working? What do you expect to happen, and what actually happens?

If you raise that event from a background thread, you'll be raising it on the wrong thread.

True dat. You don't want to be messing around with Invokes in the form code if you can avoid it. You can make sure events are raised on the thread that created the UDP_Broadcast instance (presumably it is created on the UI thread?) by taking a reference to the SynchronizationContext.Current property in the constructor (add a Public Sub New() method). This will give you a SynchronixationContext instance that you can call Post or Send from the secondary thread to execute code on the first thread. These two methods are analogous to BeginInvoke and Invoke, respectively.

[Shaggy Hiker]

That's the exact technique I used for posting events to the UI thread in the class I referenced. I believe JMC has also suggested a different method that might be easier to use, but it can't be MUCH easier, because SynchronizationContext is about as simple as anything can be.

[Brian Henry]

Thanks for the help, I got it to work.

[jmcilhinney]

That's the exact technique I used for posting events to the UI thread in the class I referenced. I believe JMC has also suggested a different method that might be easier to use, but it can't be MUCH easier, because SynchronizationContext is about as simple as anything can be.

There's basically only three ways to do it:

1. In Windows Forms, use the Invoke or BeginInvoke method of a Control.
2. In WPF, use the Invoke or BeginInvoke method of a Dispatcher.
3. Use the Send or Post method of a SynchronizationContext.

The SynchronizationContext class is actually inherited by the WindowsFormsSynchronizationContext and DispatcherSynchronizationContext classes and it is an instance of one of those that SynchronizationContext.Current returns. Those derived classes actually use options 1 and 2 under the hood anyway, which is why SynchronizationContext is only of use in a Windows Forms or WPF app. I believe that the Current property is always Nothing in any other type of app.

[Evil_Giraffe]

In contexts other than a Windows Forms or WPF app, you would be expected to supply your own (or the framework to supply) a derivation of SynchronizationContext. You can set the SyncronizationContext of a thread manually (WinForms and WPF just do this for you in their start-up code).

For instance, you could unit test the class discussed in this thread by supplying a test-double SynchronizationContext. (Actually, aside from the tests for the thread invoking behaviour, you'd probably just use the SynchronizationContext implementation directly)