The difference between T-SQL ISNULL and = NULL

Method 1、

string sql=”insert into………………”;
string sql2 = “select @@identity as ‘lastid’”;
            try
            {
                OpenConnection();
                SqlCommand=objSqlCommand = new SqlCommand(sql, objSqlConnection);
                objSqlCommand.ExecuteNonQuery();
                SqlDataAdapter=objSqlDataAdapter = new SqlDataAdapter(sql2, objSqlConnection);
                DataSet=objDataSet = new DataSet();
                objSqlDataAdapter.Fill(objDataSet, “identity”);
                String num = objDataSet.Tables["identity"].Rows[0]["lastid"].ToString();
                CloseConnection();
                return int.Parse(num);
            }
            catch (Exception ex)
            {
                throw new ApplicationException(“查询ID出错n” + ex.Message);
            }

Methods 2、（From CSDN）

if you are using ms sql server
…
sqlcommand.CommandText = ”INSERT INTO sometable (filed1) VALUES (@filed1);”
+” SELECT @ID = @@identity ”;
sqlcommand.Parameter.Add(“@field1″, …);
…
sqlcommand.Parameter.Add(“@ID”, SqlDbType.Int, 4).Direction = ParameterDirection.Output;
sqlcommand.ExecuteNonQuery();
int id = cmd.Parameters["@ID"].Value;