在Powershell中调用函数时，使用与参数和存储相同的变量

Question

Recently I had this problem. After investigating a lot of hours, finally I discovered that the problem was in using the same variable for:

• Storing the value returned by the function
• Pass it as a parameter to the function

So taken into account below function:

Function Create-Filter($filter)
{    
    $filter.Split(',') | ForEach-Object {"*.$($_.Trim())"}
    return
}

(Above function gets a string variable such as "csproj, vbproj" and converts it into *.csproj *.vbproj)

...below code is not working, variable $filter used for -Include parameter does not like to Get-ChildItem and it is returning nothing:

$filter = "csproj, vbproj"
$filter = Create-Filter ($filter)
Get-ChildItem "D:\Path\To\My\Root\Folder" -Include $filter -Recurse

Instead below one is working by using a different variable for storing and pass it as a parameter:

$filter = "csproj, vbproj"  
$formattedfilter = Create-Filter ($filter)
Get-ChildItem "D:\Path\To\My\Root\Folder" -Include $formattedfilter -Recurse

... now Get-ChildItem works.

In other languages one can use the same variable for passing it as a parameter and storing the value returned by the function. So could you explain me why in powershell this does not work if one uses the same variable?

Answer 1

Not sure why your code won't work, as the return in your case is redundant and the result is put on the output stream.

So the same code but refactored:

function New-Filter
{    
    param (
        [Parameter(Mandatory=$true)]
        [String[]] $filter
    )

    return $filter.Split(',') | ForEach-Object {"*.$($_.Trim())"}
}

$filter = @("csproj, vbproj")
$filter = New-Filter $filter
Get-ChildItem -Path "D:\Path\To\My\Root\Folder" -Include $filter -Recurse