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Dynamic Events in VB.NET

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26 Mar 2013CPOL2 min read 38.9K   728   20   3
How to handle dynamic, late binding, events for COM objects in VB.NET.


This article shows a way to use events with late bound COM objects. To solve the problem some C# script was used...


There are two common ways in VB.NET to create a late bound object.

  • Using the CreateObject method.

    Dim ie As Object = CreateObject("InternetExplorer.Application")
  • Using the System.Activator.CreateInstance method.

    Dim iet As Type = Type.GetTypeFromProgID("InternetExplorer.Application")
    Dim ie As Object = Activator.CreateInstance(iet)

And there are three common ways in VB.NET to call methods and properties on a late bound object.

  • With Option Strict turned off

    ' call a method
    ' read a property
    Dim readyState As Integer = ie.ReadyState
    ' write a property
    ie.Visible = True
  • Using the Microsoft.VisualBasic.Interaction.CallByName method.

    ' Call a method 
    CallByName(ie, "Navigate", CallType.Method, "about:blank")
    ' Read a property
    readyState = CInt(CallByName(ie, "ReadyState", CallType.Get))
    ' Write a property
    CallByName(ie, "Visible", CallType.Let, True)
  • Using the System.Type.InvokeMember method.

    ' Call a method 
    ie.GetType().InvokeMember("Navigate", BindingFlags.InvokeMethod, Type.DefaultBinder, ie, {"about:blank"})
    ' Read a property
    readyState = CInt(ie.GetType().InvokeMember("ReadyState", BindingFlags.GetProperty, Type.DefaultBinder, ie, Nothing))
    ' Write a property
    ie.GetType().InvokeMember("Visible", BindingFlags.SetProperty, Type.DefaultBinder, ie, {True})

There are some ways discussed on how to use events with late bound objects, see Google search ' Eventhandler for late bound objects). But there isn't any way to add event handlers on late bound COM objects like with the dynamic Type in C#.

  • Working in C#

    public delegate void OnQuitDelegate();
    static void Main(string[] args)
        var iet = Type.GetTypeFromProgID("InternetExplorer.Application");
        dynamic ie = Activator.CreateInstance(iet);
        ie.OnQuit += new OnQuitDelegate(ie_OnQuit);
    public static void ie_OnQuit()
  • Not working in VB.NET

    Dim iet As Type = Type.GetTypeFromProgID("InternetExplorer.Application")
    ' No dynamic type...
    Dim ie = Activator.CreateInstance(iet)
    ' Not working...
    ' AddHandler ie.OnQuit, AddressOf ie_OnQuit
    ' Not working also...
    'ie.OnQuit += New OnQuitDelegate(AddressOf ie_OnQuit)

A first idea was to write a library in C# but the name of the event is hard coded. So that wasn't possible too, except ...It is when compiling C# script at runtime...

  • So first a interface was made to make calls into the compiled C# script.

    Public Interface IEventBinder
        Sub [AddHandler](ByVal source As Object, ByVal [delegate] As [Delegate])
        Sub [RemoveHandler](ByVal source As Object, ByVal [delegate] As [Delegate])
    End Interface
  • Then a class was written that implemented the interface with Factory methods that compiled and returned instances of the class/interface. From then on events could be attached to and detached from an object.

    Imports Microsoft.CSharp
    Imports System.CodeDom
    Imports System.CodeDom.Compiler
    Imports System.Reflection
    Imports System.Text
    Public NotInheritable Class EventBinder
        Implements IEventBinder
        Private _name As String
        Private _event As IEventBinder
        Private Sub New(ByVal name As String, ByVal [event] As IEventBinder)
            Me._name = name
            Me._event = [event]
        End Sub
        Public ReadOnly Property Name As String
                Return _name
            End Get
        End Property
        Public Sub [AddHandler](source As Object, [delegate] As System.Delegate) Implements IEventBinder.AddHandler
            Me._event.AddHandler(source, [delegate])
        End Sub
        Public Sub [RemoveHandler](source As Object, [delegate] As System.Delegate) Implements IEventBinder.RemoveHandler
            Me._event.RemoveHandler(source, [delegate])
        End Sub
        Public Shared Function CreateEventBinders(ByVal names() As String) As Dictionary(Of String, EventBinder)
            Using provider As CodeDomProvider = New CSharpCodeProvider()
                Dim parameters As New CompilerParameters() With {
                    .GenerateInMemory = True
                Dim uid As String = Guid.NewGuid().ToString("N")
                Dim code As New StringBuilder()
                    "using System;" & vbCrLf &
                    "" & vbCrLf &
                    "namespace DynamicEvent {" & vbCrLf
                For Each name As String In names
                            "public class {0}Event{2} : {1}.IEventBinder" & vbCrLf &
                            "{{" & vbCrLf &
                            "   public void AddHandler(dynamic source, Delegate del) {{ source.{0} += del; }}" & vbCrLf &
                            "   public void RemoveHandler(dynamic source, Delegate del) {{ source.{0} -= del; }}" & vbCrLf &
                            "}}" & vbCrLf,
                Dim codeResult As CompilerResults = provider.CompileAssemblyFromSource(parameters, code.ToString())
                If codeResult.Errors.Count = 0 Then
                    Dim result As New Dictionary(Of String, EventBinder)
                    For Each name As String In names
                        Dim codeType As Type = codeResult.CompiledAssembly.GetType(String.Format("DynamicEvent.{0}Event{1}", name, uid))
                        Dim codeObject As Object = Activator.CreateInstance(codeType)
                        result.Add(name, New EventBinder(name, DirectCast(codeObject, IEventBinder)))
                    Return result
                    ' todo: Compiler errors...
                    Return Nothing
                End If
            End Using
        End Function
        Public Shared Function CreateEventBinder(ByVal name As String) As EventBinder
            Return CreateEventBinders({name})(name)
        End Function
    End Class

Using the code

To attach an event now:

  • A delegate declaration is required.

    Delegate Sub OnQuitDelegate()
  • A event handler is required.

    Sub ie_OnQuit()
        ' ...
    End SUb	
  • A instance of the delegate is required.

    Dim [delegate] As [Delegate] = New OnQuitDelegate(AddressOf ie_OnQuit)
  • And a instance of EventBinder for the specific event is required.

    Dim binder As EventBinder = EventBinder.CreateEventBinder("OnQuit")
  • From then on it's possible to attach, detach events...

    Dim ie As Object = CreateObject("InternetExplorer.Application")
    binder.AddHandler(ie, [delegate])
    binder.RemoveHandler(ie, [delegate])

But it's better for performance to create the interfaces for all events at once. (Not that speed matters, this is VB.NET and has the speed of light...)

  • Bind to more then one event:
    ' Create event binders
    Dim binders As Dictionary(Of String, EventBinder)
    binders = EventBinder.CreateEventBinders({
    ' Create event handlers
    Dim delegates() As [Delegate] = {
        New BeforeNavigate2Delegate(AddressOf ie_BeforeNavigate2),
        New DocumentCompleteDelegate(AddressOf ie_DocumentComplete),
        New NavigateComplete2Delegate(AddressOf ie_NavigateComplete2),
        New OnQuitDelegate(AddressOf ie_OnQuit),
        New StatusTextChangeDelegate(AddressOf ie_StatusTextChange)
    ' Bondage...
    For i As Integer = 0 To binders.Count - 1
        binders(binders.Keys(i)).AddHandler(ie, delegates(i))


  • 18/03/2013: First draft.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Software Developer
Belgium Belgium
Currently i work in a small company making software for windows mobile. I have to be very "all-round" working in C++, VB.Net, VBScript and WinDev.

This i do for Windows, Web and Mobile development.

Learning new things is what i like to do most, improving my style and attitude TID!

Comments and Discussions

GeneralMy vote of 5 Pin
Ștefan-Mihai MOGA12-Apr-13 19:14
professionalȘtefan-Mihai MOGA12-Apr-13 19:14 
GeneralMy vote of 5 Pin
Member 373058726-Mar-13 2:02
Member 373058726-Mar-13 2:02 
GeneralRe: My vote of 5 Pin
Vozzie226-Mar-13 8:30
Vozzie226-Mar-13 8:30 
Thank you. Tried to keep it simple and to the point, not losing the subject in details...

There are other ways to bind to events, but thought this idea was unique and worth sharing here.
It feels good to learn and achieve

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