Linq.First 等价于 PowerShell 是什么？

Question

The snippet below detects from a list of files which of them is a Directory on Ftp

as C# it will be like below

var files = new List<string>(){"App_Data", "bin", "Content"};
var line = "drwxr-xr-x 1 ftp ftp              0 Mar 18 22:41 App_Data"
var dir = files.First(x => line.EndsWith(x));

How I can transalte the last line in PowerShell?

Answer 1

Something like this...

$files = @("App_Data", "bin", "Content")
$line = "drwxr-xr-x 1 ftp ftp              0 Mar 18 22:41 App_Data"
$dir = $files | Where { $line.EndsWith($_) } | Select -First 1

These versions of the last line would all accomplish the same:

$dir = @($files | Where { $line.EndsWith($_) })[0]

$dir = $files | Where { $line.EndsWith($_) } | Select -index 0

$dir = $files | Where { $line.EndsWith($_) } | Select -First 1

It was pointed out that the above is not exactly equivalent in behavior to Linq.First because Linq.First throws exceptions in two cases:

• Throws ArgumentNullException when source or predicate is null.
• Throws InvalidOperationException when source sequence is empty or no element satisfies the condition in predicate.

If you wanted that behavior exactly, you'd need some extra guard code.

Answer 2

as Robert Groves said, Select-Object -First Occurence do the tricks, you can also use -Last Occurence.

by the way, like any other static .Net method you can use linq in powershell.

[Linq.Enumerable]::First($list)

[Linq.Enumerable]::Distinct($list)

[Linq.Enumerable]::Where($list, [Func[int,bool]]{ param($item) $item -gt 1 })

Answer 3

Doug Finke produced a great video ( only 7 mins ) about converting C# to Powershell http://dougfinke.com/video/CSharpToPowerShell.html

Roberts example is very good indeed, though comma delimiting will implicitly be treated as an array

the shortest way of doing it would be to put it all into a single pipeline :

$dir = "App_Data", "bin", "Content" | % { if("drwxr-xr-x 1 ftp ftp              0 Mar 18 22:41 App_Data".EndsWith($_)) { $_ } } | select -first 1

Answer 4

There is a native way to do this using the Powershell Array's Where Function by passing in a WhereOperatorSelectionMode like this:

(1..9).Where({ $_ -gt 3}, "First") # 4

---

You can also use the mode straight from the enum as well:

$mode = [System.Management.Automation.WhereOperatorSelectionMode]::First
(1..9).Where({ $_ -gt 3}, $mode) # 4

Using any of the values from the WhereOperatorSelectionMode enum

Name	Val	Description
Default	0	Return all items
First	1	Return the first item
Last	2	Return the last item
SkipUntil	3	Skip items until condition is true
Until	4	Return all items until condition is true
Split	5	Return an array of two elements

See Also : Checking Out The Where and ForEach Operators in PowerShell V4

Answer 5

This is a really simple implementation for First :

function First($collection)
{
    foreach ($item in $collection)
    {
        return $item
    }
    return $null
}

Instead of returning $null , you could throw an InvalidOperationException exception.