How to generate invoice number using ASP.NET C#

declare @PrevReferenceNo varchar(50)
 declare @PrevMonth int
 declare @PrevNo int
 declare @ReferenceNo varchar(10)

 set @PrevReferenceNo = (select ReferenceNo from tbl_FuelConsumption where FuelConsumptionID = (select MAX(FuelConsumptionID) from tbl_FuelConsumption))
 set @PrevMonth = SUBSTRING(@PrevReferenceNo,5,2)
 set @PrevNo = SUBSTRING(@PrevReferenceNo,7,4)

 if(@PrevMonth = MONTH(GETDATE()))
  begin
   set @ReferenceNo  =  (CAST(YEAR(GETDATE())as varchar(10))+ RIGHT('00' + CAST(MONTH(GETDATE()) as varchar(10)),2) + RIGHT('0000' + CAST((@PrevNo + 1)as varchar(4)) ,4))
  end
 else
  begin
   set @ReferenceNo  =  (CAST(YEAR(GETDATE())as varchar(10))+ RIGHT('00' + CAST(MONTH(GETDATE()) as varchar(10)),2) + RIGHT('0000' + CAST((1)as varchar(4)) ,4))
  end