EDIT: As of PowerShell 7 Preview 2, -not [System.DBNull]::Value
evaluates to $true
, thanks to Joel Sallow via pull request 9794
Spending more time pulling SQL data in PowerShell. Running into issues with [System.DBNull]::Value and how PowerShell behaves with this during comparisons.
Here's an example of the behavior I see, along with workarounds
#DBNull values don't evaluate like Null...
if([System.DBNull]::Value){"I would not expect this to display"}
# The text displays.
if([string][System.DBNull]::Value){"This won't display, but is not intuitive"}
# The text does not display.
#DBNull does not let you use certain comparison operators
10 -gt [System.DBNull]::Value
# Could not compare "10" to "". Error: "Cannot convert value "" to type "System.Int32". Error: "Object cannot be cast from DBNull to other types.""
[System.DBNull]::Value -gt 10
# Cannot compare "" because it is not IComparable.
#No real workaround. Must use test for null workaround in conjunction to avoid comparison altogether:
[string][System.DBNull]::Value -and [System.DBNull]::Value -gt 10
#Example scenario with a function that uses Invoke-Sqlcmd2 to pull data
Get-XXXXServer | Where-Object{$_.VCNumCPUs -gt 8}
#Error for every line where VCNumCPU has DBNull value
#workaround
Get-XXXXServer | Where-Object{[string]$_.VCNumCPUs -and $_.VCNumCPUs -gt 8}
Am I missing anything, or is there no 'simple' workaround for this that would let folks with little experience use PowerShell comparisons as expected?
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?
Any tips, or have I exhausted my options for now?
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