Mike,
Something along this one should at least give You an relevant error-message (at least something to start with):
Try
'Here You write the TryStatement
'Catch expression' As Exception
Console.WriteLine(" Error:" & f.Message)
Exit Sub
End Try
Does the error occur when You use VSTools or is it via the standard COM-connection?
Which version of XL are You working with?
Which version of office.dll in Net ?
Have faith although You working with Net

Dennis