Question

使用Powershell脚本我有几个地方我想要A除非它是null，否则B.本质上是?? C＃中的运算符。我最后编写了下面显示的函数，但我不禁想到有一种内置的方法可以做到这一点。

是否有更好的内置方式？

function Get-ValueOrDefault()
{
    foreach ($value in $args)
    {
        if ($value -ne $null) { return $value }
    }
}

我认为这样做效果更好：

function Get-ValueOrDefault() { $args | select -first 1 }

Answer 1

这是我们在PowerShell Community Extensions中提供的内容：

<#
.SYNOPSIS
    Similar to the C# ?? operator e.g. name = value ?? String.Empty
.DESCRIPTION
    Similar to the C# ?? operator e.g. name = value ?? String.Empty;
    where value would be a Nullable&lt;T&gt; in C#.  Even though PowerShell
    doesn't support nullables yet we can approximate this behavior.
    In the example below, $LogDir will be assigned the value of $env:LogDir
    if it exists and it's not null, otherwise it get's assigned the
    result of the second script block (C:\Windows\System32\LogFiles).
    This behavior is also analogous to Korn shell assignments of this form:
    LogDir = ${$LogDir:-$WinDir/System32/LogFiles}
.PARAMETER PrimaryExpr
    The condition that determines whether the TrueBlock scriptblock is used or the FalseBlock
    is used.
.PARAMETER AlternateExpr
    This block gets evaluated and its contents are returned from the function if the Conditon
    scriptblock evaluates to $true.
.EXAMPLE
    C:\PS> $LogDir = ?? {$env:LogDir} {"$env:windir\System32\LogFiles"}
    $LogDir is set to the value of $env:LogDir unless it doesn't exist, in which case it 
    will then default to "$env:windir\System32\LogFiles".
#>
filter Invoke-NullCoalescing {
    param([scriptblock]$PrimaryExpr   = $(throw "Parameter '-primaryExpr' (position 1) required"), 
          [scriptblock]$AlternateExpr = $(throw "Parameter '-alternateExpr' (position 2) required"))

    if ($primaryExpr -ne $null) {
        $result = &$primaryExpr
        if ($result -ne $null -and "$result" -ne '') {
            $result
        }
        else {
            &$alternateExpr
        }
    }
    else {
        &$alternateExpr
    }
}

New-Alias ?? Invoke-NullCoalescing

PS> ?? {$xyzzy} {"empty"}
empty

PS> ?? {$psversiontable} {"empty"}

Name                           Value
----                           -----
CLRVersion                     2.0.50727.4927
BuildVersion                   6.1.7600.16385
PSVersion                      2.0
WSManStackVersion              2.0
PSCompatibleVersions           {1.0, 2.0}
SerializationVersion           1.1.0.1
PSRemotingProtocolVersion      2.1

空值的默认值

1 个答案: