Define the change event for variable

' A custom Event Handler for your Event.
' Contains the old and the new value of the Property.
Public Class MmmSixChangedEventArgs
    Inherits System.EventArgs
    
    Private _oldValue As String
    Private _newValue As String
    
    Public Sub New(ByVal oldValue As String, ByVal newValue As String)
        MyBase.New()
        _oldValue = oldValue
        _newValue = newValue
    End Sub
    
    Public ReadOnly Property OldValue As String
        Get
            Return _oldValue
        End Get
    End Property
    
    Public ReadOnly Property NewValue As String
        Get
            Return _newValue
        End Get
    End Property
    
End Class

' The delegate that represents your Event Handler.
Public Delegate Sub MmmSixChangedEventHandler(ByVal sender As Object, ByVal e As MmmSixChangedEventArgs)

Public Class TestClass

    ' The Event to raise.
    Public Event MmmSixChanged As MmmSixChangedEventHandler
    
    Private _MMM6 As String
    Public Property MMM6 As String
        Get
            Return _MMM6
        End Get
        Set(ByVal value As String)
            Dim oldValue As String = _MMM6
            _MMM6 = value
            RaiseEvent MmmSixChanged(Me, New MmmSixChangedEventHandler(value, oldValue)
        End Set
    End Property

End Class