How to exit from ForEach-Object in PowerShell

问题:

I have the following code:

$project.PropertyGroup | Foreach {
    if($_.GetAttribute('Condition').Trim() -eq $propertyGroupConditionName.Trim()) {
        $a = $project.RemoveChild($_);
        Write-Host $_.GetAttribute('Condition')"has been removed.";
    }
};

Question #1: How do I exit from ForEach? I tried using "break" and "continue", but it doesn't work.

Question #2: I found that I can alter the list within a foreach loop... We can't do it like that in C#... Why does PowerShell allow us to do that?

回答1:

Item #1. Putting a break within the foreach loop does exit the loop, but it does not stop the pipeline. It sounds like you want something like this:

$todo=$project.PropertyGroup 
foreach ($thing in $todo){
    if ($thing -eq 'some_condition'){
        break
    }
}

Item #2. PowerShell lets you modify an array within a foreach loop over that array, but those changes do not take effect until you exit the loop. Try running the code below for an example.

$a=1,2,3
foreach ($value in $a){
  Write-Host $value
}
Write-Host $a

I can't comment on why the authors of PowerShell allowed this, but most other scripting languages (Perl, Python and shell) allow similar constructs.

回答2:

To stop the pipeline of which ForEach-Object is part just use the statement continue inside the script block under ForEach-Object. continue behaves differently when you use it in foreach(...) {...} and in ForEach-Object {...} and this is why it's possible. If you want to carry on producing objects in the pipeline discarding some of the original objects, then the best way to do it is to filter out using Where-Object.