通过StateChangeEvent事件(StateChangeEventHadler委托)计算数据庫访问时间次数

        通过委托计算数据庫访问时间，次数
每当客户打开，关闭連接时，为StateChange事件指定处理程序，
通过事件处理程序在列表框显示相关信息。
namespace @event
...{
    public partial class Form1 : Form
    ...{
        public Form1()
        ...{
            InitializeComponent();
        }

     
        private void cmdCnStateChange_Click(object sender, EventArgs e)
        ...{
            SqlConnection cn=
                new SqlConnection(@"Data Source=localhost;Integrated Security=SSPI;database=Northwind");

            SqlCommand cmd=new SqlCommand();
            cmd.CommandText="SELECT CustomerId,CompanyName FROM Customers";
            cmd.Connection=cn;
            cn.StateChange+=new StateChangeEventHandler(CnStateChange);

            try
            ...{
             
                cn.Open();
                SqlDataReader dr = cmd.ExecuteReader();
            
                while (dr.Read())
                ...{
                    lbResult.Items.Add(dr.GetString(0) + "-" + dr.GetString(1));
                   
                }
               
            }
            catch (SqlException se)
            ...{
                foreach (SqlError err in se.Errors)
                ...{
                    lbResult.Items.Add(err);
                }
            }
            finally
            ...{
                cn.Close();
            }
 

          }
        private void CnStateChange(object sender, StateChangeEventArgs ev)
        ...{
            lbResult.Items.Add("--------");
            lbResult.Items.Add("Entering StateChange EventHander");
            lbResult.Items.Add("Sender=" + sender.ToString());
            lbResult.Items.Add("Original State=" + System.DateTime.Now);//获取当前时间
            lbResult.Items.Add("Current State=" + ev.CurrentState.ToString());//获取当前状态
            lbResult.Items.Add("Exiting StateChange Event Hander");
            lbResult.Items.Add("--------");

        }
    
    }
}