如何在VB中正确检查DBNull？

Why does the following code:

为什么以下代码：

  A = not IsDBNull(CurRow("BuyBook")) AndAlso CType(CurRow("BuyBook"), string) = "Yes"

results in the following error:

导致以下错误：

 Conversion from type 'DBNull' to type 'String' is not valid.

When AndAlso is supposed to short-circuit according to this article:

当AndAlso根据这篇文章被认为是短路的时候：

http://support.microsoft.com/kb/817250

5 个解决方案

#1

You are correct. AndAlso is short circuiting.

你是对的。而且也是短路的。

However, the error comes by calling CurRow("GuyBook") (verify this in a debugger to make sure I'm not a liar or making some crazy assumptions or just misremembering* ;-). Before you ask for a value, you need to ask the DataRow if it has a value. That is, use:

然而，错误来自调用CurRow（“GuyBook”）（在调试器中验证这一点，以确保我不是骗子或做出一些疯狂的假设或只是误报* ;-)。在询问值之前，您需要询问DataRow是否有值。也就是说，使用：

CurRow.IsNull("BuyBook")

Happy coding.

快乐的编码。

*One should just be able to compare with DBNull.Value or use IsDBNull. However, I am fairly certain that I ran into a row before that threw these exceptions instead of returning a DBNull object. Start by finding out -- in the Immediate Window of the Debugger -- exactly which expression throws the exception.

*应该只能与DBNull.Value进行比较或使用IsDBNull。但是，我很确定在抛出这些异常而不是返回DBNull对象之前遇到了一行。首先找出 - 在调试器的立即窗口中 - 确切地说哪个表达式抛出了异常。

#2

Have you tried comparing like this:

您是否尝试过这样比较：

If CurRow("BuyBook") Is DBNull.Value Then
     '...
End If

#3

Instead of not IsDBNull(CurRow("BuyBook")), use NOT (CurRow("BuyBook")) is System.Dbnull.Value). Try this:

而不是IsDBNull（CurRow（“BuyBook”）），使用NOT（CurRow（“BuyBook”））是System.Dbnull.Value）。尝试这个：

A = (NOT (CurRow("BuyBook")) is System.Dbnull.Value) AndAlso CType(CurRow("BuyBook"), string) = "Yes"

要么

A = not string.IsNullOrEmpty(CurRow("BuyBook")) AndAlso CType(CurRow("BuyBook"), string) = "Yes"

#4

If dt.Rows(0)("BuyBook") = DBNull.Value Then
End If

Hope it helps.

希望能帮助到你。

#5

-1

In vb.net I usually do something like this:

在vb.net中，我通常会这样做：

A = (CurRow("BuyBook") & "" = "Yes")

Old trick from vb6 era. If CurRow("BuyBook") is null, vb.net will consider it as empty string when concatenating it with another string.

vb6时代的老把戏。如果CurRow（“BuyBook”）为null，则vb.net在将其与另一个字符串连接时会将其视为空字符串。

#1