pass in parameter from c#

[C# Programming]

Dear All,
I have sql query which is

create proc sp_detail
(
@p varchar(max)
)
as
select * from tblName where code in (@p)

I am passing @p from c# application, but it doesent reterieve any data while there is data

cmd=new SqlCommand("sp_detail",p.con);
cmd.CommandType= CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@p",codes);
DataTable dtResult=new DataTable();
if (cmd.Connection.State!=ConnectionState.Open)
cmd.Connection.Open();
dtResult.Load((cmd.ExecuteReader(CommandBehavior.CloseConnection)));
codes is a string which contains codes seperated by comma, as 28-2801,28-2802...
what is wrong here

Abdul Rahaman Hamidy
Database Developer
Kabul, Afghanistan