VB Code:
'add the common interface
Public Interface ICommon
Sub DoSave()
End Interface
'implement it in the mdichildren
Public Class Form1
Inherits System.Windows.Forms.Form
Implements ICommon
Public Sub DoSave() Implements ICommon.DoSave
MsgBox(Me.Name)
End Sub
'then use it in the MdiParent
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
'check that active form has interface
If TypeOf Me.ActiveMdiChild Is ICommon Then
'cast to interface and use it
DirectCast(Me.ActiveMdiChild, ICommon).DoSave()
End If
End Sub