InfoPath Dev

I Have a text box and I write this code in the OnBeforeChange Event:

If eventObj.IsUndoRedo Then
Exit Sub
end if

If eventObj.Operation="Delete" Then
Exit Sub
end if

dim str1

str1=eventObj.NewValue

if len(str1)<4 then
eventObj.ReturnStatus=false
eventObj.ReturnMessage="should be more than 4"
end if

That code work. But when I write the same code in the OnAfterChange Event I dont get the ReturnMessage. I get it only if I change it to:

XDocument.UI.Alert("should be more than 4")

Why it's happen???

Becuase the return message is in OnBeforeChange to send a user a message if you are not going to allow the change to continue. In the case of OnAfterChange, the change has been committed to the DOM.