powershell - Get-Member へのパイプ変数で「オブジェクトを指定する必要があります」

Question

コマンドレットの結果を Get-Member に正常にパイプできるのに、変数を使用できないのはなぜですか?

PM> Get-ProjectFolder "Services" -Project "Foobar" | Get-Member


   TypeName: System.__ComObject#{8e2f1269-185e-43c7-8899-950ad2769ccf}

Name              MemberType Definition                                  
----              ---------- ----------                                  
AddFolder         Method     ProjectItem AddFolder (string, string)      
AddFromDirectory  Method     ProjectItem AddFromDirectory (string)       
AddFromFile       Method     ProjectItem AddFromFile (string)            
AddFromFileCopy   Method     ProjectItem AddFromFileCopy (string)        
AddFromTemplate   Method     ProjectItem AddFromTemplate (string, string)
Item              Method     ProjectItem Item (Variant)                  
ContainingProject Property   Project ContainingProject () {get}          
Count             Property   int Count () {get}                          
DTE               Property   DTE DTE () {get}                            
Kind              Property   string Kind () {get}                        
Parent            Property   IDispatch Parent () {get}                   

.

PM> $f = Get-ProjectFolder "Services" -Project "Foobar"
PM> $f | Get-Member
Get-Member : You must specify an object for the Get-Member cmdlet.
At line:1 char:6
+ $f | Get-Member
+      ~~~~~~~~~~
    + CategoryInfo          : CloseError: (:) [Get-Member], InvalidOperationException
    + FullyQualifiedErrorId : NoObjectInGetMember,Microsoft.PowerShell.Commands.GetMemberCommand

編集: https://i.imgur.com/AeUW6Ge.png