CREATE PROCEDURE dbo.AVTORIZATION
@login varchar( 20 ),
@name varchar( 50 ) OUTPUT,
@id tinyint OUTPUT
AS
BEGIN
 SELECT @id = U.ID, @name = U.Name FROM Users U WHERE U.Login = @login;
END

            SqlCommand SQL = new SqlCommand("dbo.AVTORIZATION", Cn);
            SqlParameter prm = new SqlParameter();

            SQL.CommandType = CommandType.StoredProcedure;
            prm.ParameterName = "@login";
            prm.SqlDbType = SqlDbType.VarChar;
            prm.Direction = ParameterDirection.Input;
            prm.Value = Login;
            SQL.Parameters.Add(prm);

            SqlParameter prmName = new SqlParameter();
            prmName.ParameterName = "@name";
            prmName.SqlDbType = SqlDbType.VarChar;
            prmName.Size = 50;
            prmName.Direction = ParameterDirection.Output;
            SQL.Parameters.Add(prmName);

            SqlParameter prmID = new SqlParameter();
            prmID.ParameterName = "@id";
            prmID.SqlDbType = SqlDbType.TinyInt;
            prmID.Direction = ParameterDirection.Output;
            SQL.Parameters.Add(prmID);

            SQL.ExecuteNonQuery();
                        
            Program.User = (string)prmName.Value;
            Program.ID = (int)prmID.Value;// Здесь затык типа Specified cast is not valid. Если ID определять как object, то всё пучком.

if (prmName.Value != System.DBNull.Value)
            {
                Program.User = (string)prmName.Value;
            }
            else return false;