boolean weirdness

[allencrawford]

Whew! Now that I actually found these forums again thanks to VMware's constant need to make navigation of the VMware communities more difficult, allow me to ask my question.

Example: You have a VM with a single NIC that is connected.

If I run this command:

Get-VM <vm name> | Get-NetworkAdapter | Where-Object {$_.ConnectionState.Connected -eq "False"}

It still returns my one and only connected NIC (more details in this next example):

Get-VM <vm name> | Get-NetworkAdapter | Where-Object {$_.ConnectionState.Connected -eq "False"} | ft Name,Type,@{N="Connected";E={$_.ConnectionState.Connected}} -autosize

However, if I use:

Get-VM <vm name> | Get-NetworkAdapter | Where-Object {$_.ConnectionState.Connected -eq $false}

then it works just fine. Why does it do this when

"false" -eq $false

?

[allencrawford]

Oh, running PowerShell 2.0 and PowerCLI 4.1 if it matters...

[LucD · Accepted solution]

The reason for what your seeing is called silent conversion or type casting.

The Connected property has a Boolean value ($true or $false).

When you compare this with "true" (a string), the -eq operator will convert one of the operands to have the same type as the other operand.

That's why

"true" -eq $true

will return True.

Interesting to know is that PowerShell evaluates the -eq operator left-to-right.

The left operand is the example above is a string, so PS will convert the right operand ($true) to a string.

And ultimately compare the 2 strings.

Try this

"true" -ceq $true

It will return $false because the conversion of $true to a string gives "True" and the -ceq operator is a case-sensitive comparison.

Hence the $false return.

____________

Blog: LucD notes

Twitter: lucd22

[allencrawford]

Ah, understood now. Thanks for the intel.

Still somewhat odd that $false -ceq "False" isn't true (that converting the string "False" doesn't result in boolean $false), but apparently any non-boolean value that is not integer 0 would be typecast to boolean $true, except for $null which is neither $true nor $false according to some testing me and a buddy just did.

[LucD]

That is correct. You can play with explicit casting and see some, at first sight, strange results

[Boolean]"false"               # Will return $true
[Boolean]""                       # Will return $false

Are you sure about the $null ?

[Boolean]$null                  # Returns $false

____________

Blog: LucD notes

Twitter: lucd22

[allencrawford]

Well, I guess I'm not sure. But it seems to be that way when using comparison operators...

C:\> $false -ceq $null
False
C:\> $false -eq $null
False
C:\> $true -ceq $null
False
C:\> $true -eq $null
False

It seems to not be typecasting $null when comparing but if I force it to do so as you did...

C:\> $false -ceq [Boolean]$null
True

[LucD]

$null is a special value of course but I didn't know the -eq behaved like that when it sees a $null.

Good to know

____________

Blog: LucD notes

Twitter: lucd22