How to handle this error?

I ahve written a winforms app which has a method that is looping through a datareader and printing stuff from it...

The problem is that some values are dbnull and I want to display a message (from an exception object) to the user "column "name" in the current record is NULL, aborting!"

Hoe can I throw such an exception? And how do I check the datareader before I access it if it's null or not?

kind regards
Henrik

VB Code:

If Not objDR("FIELDNAME") Is System.DBNull.Value Then
            TextBox1.Value = objDR("FIELDNAME")
        Else
            'Here is  where you would throw an exception
        End If