是否可以创建我自己的“自动变量"? [英] Is it possible to create my own 'automatic variable'?

问题描述

我本质上想创建一个每次都会执行的变量.举个最简单的例子:

I essentially want to create a variable that would be executed every time. For the simplest example:

$myvar = `write-host foo`;

然后每次我引用 $myvar 时，它都会输出 foo:

Then everytime I referenced $myvar, it would output foo:

dir $myvar

Directory of foo:

重点是每次引用 $myvar 时都会重新执行 write-host foo 部分

The point being that the write-host foo portion would be re-executed everytime I reference $myvar

推荐答案

通过创建您自己的 PSVariable 派生类，在托管代码 (C#/VB) 中是可行的，但不能直接在纯脚本中使用，抱歉.我说纯脚本"是因为在 powershell v2 中，您可以使用 add-type 内联 C#.也就是说，您可以通过依赖隐式 ToString 调用在脚本中破解它，但这在每种情况下都不可靠.示例:

It's doable in managed code (C#/VB) by creating your own PSVariable derived class, but not directly in pure script, sorry. I say "pure script" because in powershell v2 you could inline the C# with add-type. That said, you could hack it in script by relying on implicit ToString calls but this would not be reliable in every situation. Example:

# empty custom object
$o = new-object psobject

# override ToString with a PSScriptMethod member
$o.psobject.members.add((new-object `
     System.Management.Automation.PSScriptMethod "ToString", {
         "ticks: $([datetime]::now.ticks)" }))

ps> $o
ticks: 634256043148813794

ps> $o
ticks: 634256043165574752

请注意，每次对变量求值时，滴答计数都不同.当然，如果你只是使用常规函数而不是变量，这会容易得多.

Note the tick count is different on each evaluation of the variable. If of course if you just use a regular function instead of a variable, this is much easier.

function Ticks { [datetime]::now.ticks }

# use as a parameter - note the use of ( and )
ps> write-host (ticks)
634256043148813794

# use in a string - note the use of $( and )
ps> write-host "ticks $(ticks)"
ticks 634256043165574752

希望能帮到你

-Oisin

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