Avoid duplicated event handlers?

How do I avoid an event from being handled twice (if is the same handler?)

Module Module1
  Sub Main()
    Dim item As New Item
    AddHandler item.TitleChanged, AddressOf Item_TitleChanged
    AddHandler item.TitleChanged, AddressOf Item_TitleChanged

    item.Title = "asdf"

    Stop
  End Sub
  Priva开发者_Go百科te Sub Item_TitleChanged(sender As Object, e As EventArgs)
    Console.WriteLine("Title changed!")
  End Sub
End Module

Public Class Item
  Private m_Title As String
  Public Property Title() As String
    Get
      Return m_Title
    End Get
    Set(ByVal value As String)
      m_Title = value
      RaiseEvent TitleChanged(Me, EventArgs.Empty)
    End Set
  End Property

  Public Event TitleChanged As EventHandler
End Class

Output:

Title changed!
Title changed!

Desired output:

Title changed!

I want the event manager to detect that this event is already handled by this handler and so it shouldn't rehandle (or readd) it.

You can also just always call RemoveHandler before AddHandler. I have found this practical in specific scenarios.

Wrapping the event handler list in a HashSet will make sure the handlers are not duplicate references, the following snippet replacement for the question's Item class will work under the above sample (it won't re-add the handler if it's already in the HashSet):

Public Class Item
  Private m_Title As String
  Public Property Title() As String
    Get
      Return m_Title
    End Get
    Set(ByVal value As String)
      m_Title = value
      RaiseEvent TitleChanged(Me, EventArgs.Empty)
    End Set
  End Property

  Private handlers As New HashSet(Of EventHandler)

  Public Custom Event TitleChanged As EventHandler
    AddHandler(value As EventHandler)
      handlers.Add(value)
    End AddHandler

    RemoveHandler(value As EventHandler)
      handlers.Remove(value)
    End RemoveHandler

    RaiseEvent(sender As Object, e As System.EventArgs)
      For Each handler In handlers.Where(Function(h) h IsNot Nothing)
        handler(sender, e)
      Next
    End RaiseEvent
  End Event
End Class