Set up event handlers through reflection

Set up event handlers through reflection

Reflection makes it easy to invoke a method (or assign a field or aproperty) by its name. For example, suppose you have the following classes:

Class Person   Event PropertyChanged(ByVal propertyName As String, ByVal newValue As Object)   Dim m_Name As String   Property Name() As String      Get         Return m_Name      End Get      Set(ByVal Value As String)         If Value <> m_Name Then            m_Name = Value            RaiseEvent PropertyChanged("Name", m_Name)         End If      End Set   End PropertyEnd Class

Here’s how you can assign a Person’s Name and a Car’s Color throughreflection:

' this code assumes the following statement' Imports System.Reflection' create a Person objectDim pe As New Person()' note how you can create the array of arguments on the flype.GetType.InvokeMember("Name", BindingFlags.SetProperty, Nothing, pe, _    New Object() {"John"})

However, setting up an event handler for an event of which you know just thename isn’t as immediate. These are the steps you need to follow:

1. Create an event handler that reflects the event syntax
2. Get the EventInfo object corresponding to the event in question
3. Create a delegate to local event handler by means of the Delegate.CreateDelegate method
4. call the EventInfo.AddEventHandler method to establish the connection

Remember to remove the event handler when you don’t need to receive eventsany longer. Here’s a complete example that uses the Person class, but it canbe easily adapted to other classes:

Module MainModule   Sub Main      ' create a Person object      Dim pe As New Person()      ' get a reference to the EventInfo for this object      Dim peEv As EventInfo = pe.GetType.GetEvent("PropertyChanged")      ' create a delegate to the local procedure      Dim peDel As [Delegate] = [Delegate].CreateDelegate(peEv.EventHandlerType, _          Me, "PropertyChanged")      ' make the local procedure a handler for the event      peEv.AddEventHandler(pe, peDel)      ' prove that it works      pe.Name = "John"      ' (always remove event handlers)       peEv.RemoveEventHandler(pe, peDel)   End Sub   Private Sub PropertyChanged(ByVal propertyName As String, _       ByVal newValue As Object)      Console.WriteLine("Property {0} has changed. The new value is {1}", _          propertyName, newValue)   End SubEnd Module


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