无效的尝试访问字段的数据读取器问题 [英] Data Reader Problem for Invalid attempt to access a field

问题描述

嘿evry1 ..

我已经开发了一个项目，其中编写了以下代码，但在第3或第4次迭代后，在while循环中出现了错误.

所以请让我知道查询是错误的还是任何其他问题.



字符串qr2 =从sampleOB中选择quan，其中日期<(从sampleob中选择max(date)，日期<""+ CDate +")和(Special =(从产品代码中选择RMtype，其中RMCode ="" + RMCode + "限制1)或subparticular =''"+ RMCode +"'')限制1;


while()
{
con.Open();
cmd = new MySqlCommand(qr2，con);
dr = cmd.ExecuteReader();
dr.Read();
currBalOB = Convert.ToDouble(dr [0]);//在调用Read()之前尝试访问字段无效尝试
con.Close();
}


谢谢
swapnil

Hey evry1..

I had developed a project, in which following code is written But i got the ERROR in t while loop after 3rd or 4th iteration.

So plz let me kn whether the query is wrong or any other problem.



string qr2 = "select Quan from sampleOB where date<(select max(date) from sampleob where date<''"+ CDate +"'') and (Particular=(select RMtype from productcode where RMCode=''"+ RMCode +"'' limit 1) or subparticular=''" + RMCode + "'') limit 1";


while()
{
con.Open();
cmd = new MySqlCommand(qr2, con);
dr = cmd.ExecuteReader();
dr.Read();
currBalOB = Convert.ToDouble(dr[0]);//Invalid attempt to access a field before calling Read()
con.Close();
}


thank you
swapnil

推荐答案

您为什么这样做?
充其量，您将坐在一个循环中，从DB中读取相同的数据，并加载相同的记录，并将相同的字段转换为双精度，然后丢弃结果.

为什么?
为什么不查看读者返回的所有记录呢?
为什么不退出循环?

顺便说一句:您也应该处理MySqlCommand对象，而不是每次都创建一个新对象.

Why are you doing that?
At best, you will sit in a loop reading the same data from teh DB and loading the same record and converting the same field to a double, then throwing away the result.

Why?
Why not look at all the records the reader returns instead?
Why not have an exit from the loop?

BTW: you should Dispose your MySqlCommand objects as well, rather than just creating a new one each time.

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