Sql Statement Case In Where Works Management Studio Not In C#

I have a bit of an odd problem in an sql statement where it works in management studio but not in c# when setting up a table adapter the sql is:


DECLARE @week char(2)

SET @Week = 7

SELECT student_id, acad_period, register_id, register_group, week_no, absence_code, attendance_type

FROM dbo.sttdstud

WHERE (student_id LIKE '%') AND (register_id LIKE '%') AND (register_group LIKE '%') AND

week_no = Case @Week when null Then '#' Else @Week End


the idea is if they don't enter a number it would bring up all records. This works in mangaement studio but c# brings up the error sql server doesn't support udt on excecution of the sql. This is sql server 2000. Any ideas why this doesn't work and how to fix it.