How Does Member Enumeration Work in PowerShell 3?

问题

In PoweShell 2 we did:

Get-ChildItem | ForEach-Object {$_.LastWriteTime} | Sort-Object  

In Powershell 3 we do:

(Get-ChildItem).LastWriteTime | Sort-Object

But how does it work, i read this blog post on MSDN and they say that its faster because the foreach loop isnt running? So how does it enumerate the properties then ?

回答1:

PowerShell is doing the hard work for us and it loops over the collection internally. I like to call this "implicit foreach". Assuming the member you specified is present on each object, if the member you specified is a property, you get back its value. If it's a method, it invokes the method on the each object.

In v2, to get all process names you had to take care of looping yourself:

Get-Process | Foreach-Object {$_.Name}

In v3, the equivalent would be:

(Get-Process).Name

Same applies to methods. To kill all processes with name starting with note*:

(Get-Process note*).Kill()

回答2:

The blog says foreach-object cmdlet is not running. Now it is taken care of by the language engine and not a cmdlet, making it faster. How it EXACTLY works is internal implementation detail and I think that is not what you really want to know.

来源：https://stackoverflow.com/questions/12131416/how-does-member-enumeration-work-in-powershell-3