VB.NET：如何与数据库列值和可为空的类型结合使用？还是有更好的解决方案？ [英] VB.NET: How do I use coalesce with db column values and nullable types? Or is there a better solution?

问题描述

我正在尝试执行与此处所述类似的操作，但是具有可为空的类型。

I'm trying to do something similar to what's described here, but with nullable types.

http://www.csharp-station.com/Tutorials/Lesson23.aspx

int availableUnits = unitsInStock ?? 0;

在VB中，应该是这样：

In VB, it would be this:

Dim availableUnits As Int32 = If(unitsInStock, 0)

但是我正在使用db列（可以是DbNull）和可空类型（可以是Nothing）（与DbNull不同）。如果列是DbNull，我想返回Nothing，否则返回值。例如：

However I'm working with db columns, which could be DbNull, and nullable types, which can be Nothing (which is different to DbNull). If a column is DbNull, I want to return Nothing, otherwise return the value. For eg:

Dim availableUnits As Int32? = If(myDataReader("UnitsInStock").Value, Nothing)

我得到的错误是指定的演员表无效，但我不确定为什么。我也尝试过这样做：

The error I'm getting is "Specified cast is not valid" but I'm not sure why. I've tried this as well:

Dim availableUnits As Int32? = If(isDbNull(myDataReader("UnitsInStock").Value), myDataReader("UnitsInStock").Value, Nothing)

这是混乱的，只会导致相同的错误。唯一有效的方法是：

Which is messy and just results in the same error. The only thing that works is this:

Dim availableUnits As Int32?
If isDbNull(myDataReader("UnitsInStock").Value) Then
  availableUnits = myDataReader("UnitsInStock").Value
Else
  availableUnits = Nothing
End If

这只是愚蠢。有没有更好的方法可以将可空的db值转换成我不知道的可空变量？

Which is just silly. Is there a better way of getting nullable db values into nullable variables that I'm not aware of?

推荐答案

这很愚蠢，但是在设置时，您必须投射 Nothing ：

It is silly, but you have to cast the Nothing when you set it:

Dim availableUnits As Int32? = If(myDataReader.IsDBNull("UnitsInStock"), _
                                  CType(Nothing, Int32?), _
                                  myDataReader.GetInt32("UnitsInStock"))

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