I have a stored procedure:
ALTER PROCEDURE [dbo].[pr_Tbl_Test_Insert] @guidid uniqueidentifier output, @sname nvarchar(50) AS -- INSERT a new row in
You need to construct a SqlParameter using one of the constructors that lets you specify a ParameterDirection, such as this one. Alternatively, construct your parameter and then set the direction using the Direction property.
Check this link on MSDN for more information.