自动递增编号 [英] Auto increment number

问题描述

我想知道如何在asp.net/C#中制作自动递增ID，我需要在文本框中显示此处，我的代码是

i want to know that how to make auto-increment id in asp.net/C# i need display in textbox here my code is

con.Open();
int a=0;
SqlCommand com = new SqlCommand("select max(makeid) from make",con);
SqlDataReader dr = com.ExecuteReader();
if(dr.Read())
{
   a = Convert .ToInt32 (dr[0].ToString ())+1;(here show the error)
}
txtmakeid.Text = Convert.ToString(a);
con.Close();

但是我得到的错误是"输入字符串的格式不正确."如何解决这个问题

But i got error is "Input string was not in a correct format." how to solve this one

推荐答案

a = dr[0] == DBNull.Value ? 0 : Convert.ToInt32(dr[0]) +1;

即使您解决了它，也是这样做的危险方法.
如果不同的用户同时调用相同的代码会怎样?还是在执行此操作之后，但在将记录添加到数据库之前呢?答案:你们两个都以相同的号码结尾.
首先，放弃转换，并通过标量将其返回:

Even if you do solve it, that is a dangerous way to do it.
What happens if a different user calls that same code at the same time? Or even after you do this, but before you add your record to the database? Answer: you both end up with the same number.
First off: ditch the conversions, and return it via a scalar:

con.Open();
int a=0;
SqlCommand com = new SqlCommand("select max(makeid) from make",con);
int a = com.ExecuteScalar();
txtmakeid.Text = Convert.ToString(a);
con.Close();

哪个可以解决您的当前问题.

但这不会解决重复的值！

Which would cure your immediate problem.

But it won''t solve the duplicate values!

亲爱的朋友，

试试这个:-

Dear Friend,

Try this :-

a = dr[0] == DBNUll.Value ? 0 : dr.GetInt32(0) +1;

我希望这可以.

I hope this will work.

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