c# - dates being ignored when type is passed -

any idea when pass type stored proecedure dates being ignored

 alter procedure [dbo].[hms_getbenefitsbydatetype] ( @id int, @startdate datetime, @enddate datetime, @btype int  )  set nocount on;      select        benefit.emp_no, benefit.record_id, benefit.contract_id,    benefit.career_id, benefit.date_from, benefit.date_to, benefit.benefit_type, benefit.monthly_value, benefit.benefit_provider, benefit.level_of_cover,   benefit.current_benefit, benefit.previous_monthly_cost, benefit.benefit_change_details, benefit.notes, benefit.level_description, benefit.monthly_annual, benefit.cover_level,     benefit.qualifying_reason, benefit_type.desc_ benefit inner join                      benefit_type on benefit.benefit_type = benefit_type.code (benefit.benefit_type = @btype) or (benefit.emp_no = @id)  , (benefit.date_from >= @startdate) , (benefit.date_to <= @enddate)  

where date 2006-01-01 00:00:00.000 , date 1900-01-01 00:00:00.000 result being produced following 11,4,21,123,2006-01-01 00:00:00.000,1900-01-01 00:00:00.000,3,15.00,1,2.00,1,0.00,none ,notes ,children ,1,1,1,medical cover - children
12,6,14,27,2013-06-21 00:00:00.000,2013-06-21 00:00:00.000,3,500.00,0,0.00,1,0.00,

i go one

where (benefit.benefit_type = @btype) or (benefit.emp_no = @id)  , ( (benefit.date_from >= @startdate) , (benefit.date_to <= (@enddate + 1)) ) 

#update 1

   (benefit.benefit_type = @btype) or (benefit.emp_no = @id)  , ( (benefit.date_from null or benefit.date_from >= @startdate) , ( benefit.date_to null or benefit.date_to <= (@enddate + 1)) )