Update label on form from other class

Hello,

I have this problem quite a time. I made an add in for Solidworks with a template. This add in loads a taskpane in solidworks. This is a form. On this form in the class:

public partial class KoppelingTaskpane : UserControl

are a label label1 and a button button1. I update label1 when I pres the button. That works fine. But now I want to update label1 when I change a custom property of the loaded part. For this there is an eventhandler: 

doc.ChangeCustomPropertyNotify += new DPartDocEvents_ChangeCustomPropertyNotify(PropertyChange);

in the class: 

public class PartEventHandler : DocumentEventHandler

Now my question, how can I get this to work? I tried it with backgroundworker and invoke ect. I don't have much experience with programming. I use VS2010. 

Can anyone help me? I can post the complete code if you like.


Thanks!