Powershell 如何从另一个变量中减去一个变量，两者都包含 Exchange 邮箱 Object 身份

Question

I have two variables both which contain the Get-Mailbox object "Identity" of user accounts. I need to subtract the contents of one from the other IE:

$termednofwd = (domain.local/OUname/SubOU/Users/first1 last1, domain.local/OUname/SubOU/first2 last2)

$termedfmr = (domain.local/OUname/SubOU/Users/first1 last1)

I want something that would subtract the contents of $termedfmr from $termednofwd giving something like the below. Compare-Object only lists the contents that are in both, I basically need to subtract what is in both from the first variable. essentially: $termednofwdnofmr = $termednofwd - $termedfmr resulting in this:

$termednofwdnofmr = (domain.local/OUname/SubOU/first2 last2)

Answer 1

In set theory terms, you're looking for the relative complement between two collections, which Compare-Object can provide, although it requires additional effort:

By default, Compare-Object provides the symmetric difference between two sets, ie it lists the union of relative complements; that is, given two sets A and B, it lists both those elements of B not present in A and those elements of A not present in B, and it uses the .SideIndicator property to indicate which is which:

• '<=' indicates objects unique to set A ( -ReferenceObject argument, or first positional argument), whereas
• '=>' indicates elements unique to set B ( -DifferenceObject argument, or second positional argument).

Therefore, you need to filter the output objects by their .SideIndicator values.
The -PassThru switch additionally ensures that the input objects are passed through (as opposed to wrapping them in a [pscustomobject] instance whose .InputObject contains them):

$termednofwd = 'domain.local/OUname/SubOU/Users/first1 last1',
               'domain.local/OUname/SubOU/first2 last2'

$termedfmr = 'domain.local/OUname/SubOU/Users/first1 last1'

# Return the elements in $termednofwd that aren't also present in $termedfmr
Compare-Object $termednofwd $termedfmr -PassThru |
  Where-Object SideIndicator -eq '<='

The above yields 'domain.local/OUname/SubOU/first2 last2' , ie those element(s) in $termednofwd that aren't also present in $termedfmr .

Note: The above uses strings as input objects for brevity; in your case, since you're working with the objects returned by the Get-Mailbox cmdlet and want to compare based on their .Identity property values, you need to use:

# If you only need the identity values as results.
Compare-Object $termednofwd.Identity $termedfmr.Identity -PassThru |
  Where-Object SideIndicator -eq '<='

# Alternatively, if you need the whole mailbox objects.
Compare-Object -Property Identity $termednofwd $termedfmr -PassThru |
  Where-Object SideIndicator -eq '<='

See also: GitHub issue #4316 , which proposes enhancing Compare-Object with set operations .

Answer 2

So this is essentially a practical example of Set theory ie I'd like all the items from Set 1 that isn't in Set 2. PowerShell doesn't have direct commandlets to do that but through the power of.Net you can lervage the Microsoft Linq libraries to do this work for you.

The only trick is that you need to cast your PowerShell variables to arrays of Objects to make the function call work right:

$results = [System.Linq.Enumerable]::Except([object[]]$mainArray, [object[]]$subArray)

Lastly, the items you are comparing in the two arrays have to be comparable. For simple things like strings this is always works. If the things you are comparing are more complex objects, they may not be comparable directly.