C＃中出现错误 [英] Error is coming in C#

问题描述

我试图总结一些项目的数量，所以我写了一个for循环。

因为我在sql quey和executioncalar（）的帮助下获取了itemid和item的最大值这两个值存储在numlow和numhigh变量中。

我在I中存储了numlow值，而我在for循环中使用了i。
当我运行时
代码一切正常如下

< pre> for（int i = lownum; i< = highnum; ++ i）
 {
 con.Close（）; 
 con.Open（）; 

 SqlCommand qty = new SqlCommand（从invDB中选择itmqty，其中itemid ='+ i + ''，con）; 
 double quant = Convert.ToDouble（qty.ExecuteScalar（））; 

当我将sum（）添加到itmqty时总和（itmqty）它显示错误

。其中数量也为零。

错误是

对象无法从DBNull转换为其他类型

我尝试过：



我用google搜索但没有合适的结果。

解决方案

你的查询没有返回值。

使用调试器确认<$的值c $ c>我，并手动检查您的数据库以确保数据符合您的预期。



如果您的查询不返回任何值，ExecuteScalar返回DbNull - 你不能将它转换为double。

如果它应该没有匹配的值，那么你需要在尝试转换返回值之前检查null值。

I am trying to sum the quantity of some items so I wrote one for loop.
for that I took max of itemid and min of itemid with the help of sql quey and executescalar() and that two values stored in numlow and numhigh variables.
I stored numlow value in "I" and that "i" i am using in for loop.
when I am running the code everything is ok like below

<pre> for (int i = lownum; i<= highnum; ++i)
            {
                con.Close();
con.Open();

SqlCommand qty = new SqlCommand("select itmqty from invDB where itemid='"+i+"'", con);
                double quant = Convert.ToDouble(qty.ExecuteScalar());

when I added sum() to that itmqty like sum(itmqty) it is showing error

.Some where quantity is zero also.
Error is "

Object cannot be cast from DBNull to other types"

What I have tried:

I googled but came with no suitable results.

解决方案

Your query returns no values.
Use the debugger to confirm the value of i, and manually check your DB to ensure that the data is what you expect it to be.

If your query returns no values, ExecuteScalar returns DbNull - and you can't convert that to a double.
If it's supposed to work with no matching values, then you need to check for null before attempting to cast the return value.

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