Situation:

1. Get-ChildItem $Path -Filter *.dll works for me
2. This works:
   
   Default

   $Path = "$env:windir\system32\*" $GuyArray = @("*.dll", "*.exe") Get-ChildItem $Path -Include $GuyArray

   1 2 3 4 5

   $Path = "$env:windir\system32\*" $GuyArray = @("*.dll", "*.exe")   Get-ChildItem $Path -Include $GuyArray

3. But I cannot get this working:
   
   Default

   $Path = "$env:windir\system32\*" $GuyArray = @("*.dll", "*.exe") Get-ChildItem $Path -Filter $GuyArray

   1 2 3 4 5

   $Path = "$env:windir\system32\*" $GuyArray = @("*.dll", "*.exe")   Get-ChildItem $Path -Filter $GuyArray

Error message:

Cannot convert ‘System.Object[]’ to the type ‘System.String’ required by parameter ‘Filter’. Specified method is not supported.

Questions:

1. Does this mean that -Include supports multiple values, but -Filter only allows one value?
2. If the above explaination is correct, is there a way I could have discovered this from Get-Help gci?

Does this mean that -Include supports multiple values, but -Filter only allows one value?

Yes.

If the above explaination is correct, is there a way I could have discovered this from Get-Help gci?

Yes, but you do not get much information by Get-Help gci -Parameter Filter. But you still can see it is a string, not an array. As for the details, Filter is a provider-specific filter. Get-Help gci cannot tell you anything about implementation in a particular provider. In theory, Get-Help FileSystem (help about this provider) should have explained this.

P.S. Also note that this filter rather uses CMD wildcard rules than PowerShell wilcard rules.