Question

Here is the basic gist of my problem:

1. My main Window class instantiates Class A.
2. Class A instantiates Class B in a secondary AppDomain.
3. Class B raises an event and Class A handles the event successfully.
4. Class A raises an event of its own.

Problem: In step 4, when Class A raises its own event from the event handler method that caught Class B’s event, the event is raised; however, the subscribing handler in the Window class is never called.

There are no exceptions being thrown. If I remove the secondary AppDomain, the event gets handled without a problem.

Does anyone know why this doesn’t work? Is there another way to make this work without using a callback?

I would think, if anything, the problem would occur in step 3 instead of step 4.

Here’s a real code sample to illustrate the problem:

Class Window1

    Private WithEvents _prog As DangerousProgram    

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles Button1.Click    
        _prog = New DangerousProgram()
        _prog.Name = "Bad Program"  
    End Sub

    Private Sub MyEventHandler(ByVal sender As Object, ByVal e As NameChangedEventArgs) Handles _prog.NameChanged
        TextBox1.Text = "Program's name is now: " & e.Name
    End Sub

End Class


<Serializable()> _    
Public Class DangerousProgram

    Private _appDomain As AppDomain
    Private WithEvents _dangerousProgram As Program
    Public Event NameChanged(ByVal sender As Object, ByVal e As NameChangedEventArgs)


    Public Sub New()

        // DangerousPrograms are created inside their own AppDomain for security.

        _appDomain = AppDomain.CreateDomain("AppDomain")    
        Dim assembly As String = System.Reflection.Assembly.GetEntryAssembly().FullName 
        _dangerousProgram = CType( _   
                    _appDomain.CreateInstanceAndUnwrap(assembly, _    
                        GetType(Program).FullName), Program)

    End Sub


    Public Property Name() As String
        Get
            Return _dangerousProgram.Name
        End Get
        Set(ByVal value As String)
            _dangerousProgram.Name = value
        End Set
    End Property


    Public Sub NameChangedHandler(ByVal sender As Object, ByVal e As NameChangedEventArgs) Handles _dangerousProgram.NameChanged    
        Debug.WriteLine(String.Format("Caught event in DangerousProgram. Program name is {0}.", e.Name))
        Debug.WriteLine("Re-raising event...")

        RaiseEvent NameChanged(Me, New NameChangedEventArgs(e.Name))   
    End Sub

End Class


<Serializable()> _    
Public Class Program
    Inherits MarshalByRefObject

    Private _name As String
    Public Event NameChanged(ByVal sender As Object, ByVal e As NameChangedEventArgs)

    Public Property Name() As String
        Get
            Return _name
        End Get
        Set(ByVal value As String)
            _name = value
            RaiseEvent NameChanged(Me, New NameChangedEventArgs(_name))
        End Set
    End Property   

End Class


<Serializable()> _   
Public Class NameChangedEventArgs
    Inherits EventArgs

    Public Name As String

    Public Sub New(ByVal newName As String)
        Name = newName
    End Sub

End Class

Answer 1

In my first attempt at solving this issue, I removed Class B‘s inheritance of MarshalByRefObject and flagged it as serializable instead. The result was the the object was marshaled by value and I just got a copy of Class C that executes in the host AppDomain. This is not what I wanted.

The real solution, I found, was that Class B (DangerousProgram in the example) should also inherit from MarshalByRefObject so that the call back also uses a proxy to transition the thread back to the default AppDomain.

By the way, here’s a great article I found by Eric Lippert that explains marshal by ref vs. marshal by value in a very clever way.