阅读器一次读取一条记录，因此，如果要处理阅读器返回的所有记录，则必须在环形.

Reader reads a single record at a time so if you want to handle all the records returned by the reader, you must use the Read method in a loop. Something like

while (reader.Read()) { ... }

之类的东西您正在使用if语句而不是循环.更改代码，使其看起来像这样，以便覆盖所有结果并正确清理连接，读取器和命令对象.

You are using an if statement rather than a loop. Change your code to look something like this so that you cover all of the results and clean up the connection, reader and command objects properly.

DateTime datenow = DateTime.Now;
using(SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["racon"].ToString()))
{
   con.Open();
   using(SqlCommand cmd = new SqlCommand("SELECT id,MAX(date) as enddate from table1 group by id", con))
   {
      using(SqlDataReader reader = cmd.ExecuteReader())
      {
         while(reader.Read())
         {
            DateTime enddate = Convert.ToDateTime(reader["enddate"]);

            if(DateTime.Compare(datenow,enddate)>0)
            {
               string id = Convert.ToString(reader["id"]);
               string sql = "update table2 set type=''expired'' where id=''" + id + "''";
               using(SqlCommand cmd1 = new SqlCommand(sql, con))
               {
                  cmd1.ExecuteNonQuery();
               }
            }
         }
      }
   }
}

这是Kythen回答的延续.从循环中移出该代码块.

Hi,

This is in continuation to Kythen''s answer. Move out this code block from the loop.

string sql = "update table2 set type='expired' where id='" + id + "'";
using(SqlCommand cmd1 = new SqlCommand(sql, con))
{
   cmd1.ExecuteNonQuery();
}

相反，只需在循环中收集ID，然后在循环外执行查询即可.

Instead just collect the ids in the loop and execute the query outside the loop.

//have a string builder instance before the loop
StringBuilder sb=new StringBuilder();

//Inside the loop just append the ids. Take care the last id no need to append a comma.
sb.append(id); sb.append(",");

//outside the loop use like

string sql=@"UPDATE table2 SET type='expired' WHERE id IN ("+sb.ToString()+")";

然后使用nonquery执行它.这样您就不会对sql服务器产生兴趣了.

Then execute it using the nonquery. So you won''t be chatty with the sql server.