如何抢占“指定的转换无效”例外？ [英] How can I preempt a "Specified cast is not valid" exception?

问题描述

在Web API应用程序中查询MS Access数据库时，如果我尝试为变量分配一个空字符串，我会得到一个指定的转换无效异常。 IOW，当这个代码：

In querying an MS Access database in a Web API app, I get a "Specified cast is not valid" exception if I'm trying to assign an empty string to a variable. IOW, when this code:

var accountID = oleDbD8aReader.GetInt16(0); 
var platypusName = oleDbD8aReader.GetString(1); 

...达到一个记录，其中结果集中的第二列包含空/

...reaches a record where the second column in the result set contains an empty/null string, it bombs.

所以，我想我可以在传球这样的头像：

So, I thought I could head that off at the pass like this:

string platypusName;
var accountID = oleDbD8aReader.GetInt16(0); 
if (!string.IsNullOrEmpty(oleDbD8aReader.GetString(1)))
{
    platypusName = oleDbD8aReader.GetString(1); 
}
else
{
    platypusName = string.Empty;
}

...但它不工作 - 我还是得到一个

...but it doesn't work - I still get a "Specified cast is not valid" exception.

如何安全地检查空字符串/空值，并忽略通过结果集的通过以便它将获得后续记录？

How can I safely check for an empty string/null value there and ignore that pass through the result set so that it will get subsequent records?

或者可以通过更改SQL语句以排除结果集中的空/空字符串来排除这种情况？如果是，如何？查询采用以下格式：

Or can I preclude this by changing the SQL statement to exclude empty/null strings from the result set? If so, how? The query is in this format:

SELECT td_duckbill_accounts.platypus_no, t_accounts.name 
FROM t_accounts 
INNER JOIN td_duckbill_accounts ON t_accounts.account_no = td_duckbill_accounts.account_no      
ORDER BY td_duckbill_accounts.platypus_no

推荐答案

我认为查询返回空字符串很容易解决：

I think having the query return empty string is easy solution:

SELECT td_duckbill_accounts.platypus_no, 
       IIF(ISNULL(t_accounts.name),'',t_accounts.name) AS name
FROM t_accounts 
INNER JOIN td_duckbill_accounts ON t_accounts.account_no = td_duckbill_accounts.account_no      
ORDER BY td_duckbill_accounts.platypus_no

这应该也可以，但我现在不能测试：

This should also work but I can't test it right now:

SELECT td_duckbill_accounts.platypus_no, 
       Nz(t_accounts.name,'') AS name
FROM t_accounts 
INNER JOIN td_duckbill_accounts ON t_accounts.account_no = td_duckbill_accounts.account_no      
ORDER BY td_duckbill_accounts.platypus_no

这篇关于如何抢占“指定的转换无效”例外？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！