在PowerShell中处理System.DBNull [英] Dealing with System.DBNull in PowerShell

问题描述

编辑：从PowerShell 7 Preview 2开始，-不是[System.DBNull] :: Value 的评估结果为 $ true ，感谢 Joel Sallow 通过拉出请求9794

As of PowerShell 7 Preview 2, -not [System.DBNull]::Value evaluates to $true, thanks to Joel Sallow via pull request 9794

花更多时间在PowerShell中提取SQL数据。遇到[System.DBNull] :: Value问题以及在比较期间PowerShell的行为方式。

Spending more time pulling SQL data in PowerShell. Running into issues with [System.DBNull]::Value and how PowerShell behaves with this during comparisons.

这是我看到的行为示例，以及解决方法

Here's an example of the behavior I see, along with workarounds

#DBNull values don't evaluate like Null...
    if([System.DBNull]::Value){"I would not expect this to display"}
    # The text displays.
    if([string][System.DBNull]::Value){"This won't display, but is not intuitive"}
    # The text does not display.

#DBNull does not let you use certain comparison operators
    10 -gt [System.DBNull]::Value 
    # Could not compare "10" to "". Error: "Cannot convert value "" to type "System.Int32". Error: "Object cannot be cast from DBNull to other types.""

    [System.DBNull]::Value -gt 10
    # Cannot compare "" because it is not IComparable.

    #No real workaround.  Must use test for null workaround in conjunction to avoid comparison altogether:
    [string][System.DBNull]::Value -and [System.DBNull]::Value -gt 10

#Example scenario with a function that uses Invoke-Sqlcmd2 to pull data
    Get-XXXXServer | Where-Object{$_.VCNumCPUs -gt 8}
    #Error for every line where VCNumCPU has DBNull value

    #workaround
    Get-XXXXServer | Where-Object{[string]$_.VCNumCPUs -and $_.VCNumCPUs -gt 8}

我是否遗漏了任何东西，或者没有简单的解决方法可以让缺乏经验的人按预期使用PowerShell比较？

Am I missing anything, or is there no 'simple' workaround for this that would let folks with little experience use PowerShell comparisons as expected?

我提交了对Connect的建议，并有一个临时< Dave Wyatt提供的href = http://powershell.org/wp/forums/topic/dealing-with-dbnull/ rel = nofollow noreferrer>解决方法，该方法将数据行转换为psobject，而dbnull转换为null ，但是会增加一些开销。

I submitted a suggestion on Connect and have a temporary workaround from Dave Wyatt that converts datarows to psobjects with dbnulls converted to nulls, but this adds a bit of overhead. Seems like something that should be handled under the covers, given the existing 'loose' behavior of PowerShell?

任何提示，还是我现在已经精疲力尽了？

Any tips, or have I exhausted my options for now?

p>

谢谢！

Thanks!

推荐答案

我认为您在这里采用了错误的方法。如文档所述， DBNull 类表示一个不存在的值，因此类似 -gt 或 -lt 的比较没有任何意义不存在的值既不大于也不小于任何给定值。 值 字段具有 Equals（）方法，该方法可让您检查值是否为 DBNull ：

I think you're taking a wrong approach here. As documented, the DBNull class represents a non-existing value, so comparisons like -gt or -lt don't make any sense. A value that doesn't exist is neither greater nor less than any given value. The Value field has an Equals() method, though, which allows you to check if a value is or isn't DBNull:

PS C:> ([DBNull]::Value).Equals(23)
False
PS C:> ([DBNull]::Value).Equals([DBNull]::Value)
True

这篇关于在PowerShell中处理System.DBNull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！