如果Sql Server返回错误，则应抛出SqlException。但事实并非如此

If I bind GridView to SqlDataSource and also set AutoGenerateEditButton to true, and if I then try to update a field ( this field being a primary key in database ), then database should return an error and thus SqlException should be thrown?! So why doesn’t page report an exception? Instead, all Gridview does is setting all fields in that row back to their original values. If I bind GridView to SqlDataSource and also