DataSet、ExecuteScalar、ExecuteReader

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Data;
using System.Data.SqlClient;


namespace test
{
    public class Class1
    {
        public DataSet RunQuery(String QueryString)
        {

            String ConnectionString = "server=localhost;database=NorthWind;Integrated Security=SSPI";
            try
            {
                using (SqlConnection DBConnection = new SqlConnection(ConnectionString))
                {
            DBConnection.Open();
                    DataSet ResultsDataSet = new DataSet();
                    SqlDataAdapter DBAdapter = new SqlDataAdapter(QueryString, DBConnection);
                    DBAdapter.Fill(ResultsDataSet);

                }

            }
            catch (Exception ex)
            {
              ex.Message;               
            }

            return ResultsDataSet;

        }

    }
}




        string sql = "select  *  from users ";
        string stringconcetion = System.Configuration.ConfigurationManager.ConnectionStrings["Conn"].ConnectionString;
        using (SqlConnection conn = new SqlConnection(stringconcetion))
        {
            conn.Open();
            SqlCommand cmd = new SqlCommand(sql,conn);
            string LastTime = cmd.ExecuteScalar().Tostring();
        }




String ConnectionString = "server=localhost;database=NorthWind;Integrated Security=SSPI";
string sql="select * from users";
using (SqlConnection cn = new SqlConnection(ConnectionString ))
{
SqlCommand command=new SqlCommand(sql,cn);
cn.Open()
//SqlDataReader 对象中保存查询的结果，不能new。相当于游标，先打开 再读取 最后关闭。
SqlDataReader reader=command.ExecuteReader(CommandBehavior.CloseConnection);  //CloseConnection 指定在关闭DataReader的时候，自动关闭数据库连接。
while (reader.Read())
{
//int id=(int)reader[0];  第二种写法
//int id=reader.GetInt(0); 第三种写法
int id=(int)reader["userid"];
}
reader.Close();
}