Rename columns in Select-Object - powershell

Question

I have an object with spaces in their properties names. I want to Select-Object #{n='NewName';e={$_.'Old Name'}} for every NoteProperty. Since there is a lot of them, I created this function. Running this code will return an array of hash tables but I can't get the old name $col to be replaced with the actual old name. I think it's because it's bound to a new execution context but I can't make it to work.
function Rename-Columns {
# Array of HashTable splat to rename columns in Select-Object (TSQL SELECT [col with space] as colwithspace)
param (
[string[]]$Columns,
[hashtable]$RenamePattern = #{Replace='\s|\:|\.|\+|\|';With=''} # remove unwanted cars
)
$return = #()
foreach ($col in $Columns) {
$newName = $col -replace $RenamePattern['Replace'], $RenamePattern['With']
$return += #{n="$newName";e={$_.'"$col"'}} # <- can't replace $col
}
$return
}

Answer 1

Your current approach can't work because by the time you pass the {$_."$col"} block to Select-Object, $col no longer resolves to the value it did when you created the scriptblock inside the loop.
In order to bind the current iterator value of $col to the expression block, you need a closure:
function Rename-Columns {
# Array of HashTable splat to rename columns in Select-Object (TSQL SELECT [col with space] as colwithspace)
param (
[string[]]$Columns,
[hashtable]$RenamePattern = #{Replace='\s|\:|\.|\+|\|';With=''} # remove unwanted cars
)
foreach ($col in $Columns) {
# calculate new name
$newName = $col -replace $RenamePattern['Replace'], $RenamePattern['With']
# close over `{$_.$col}` to bind the current value to `$col`
#{n=$newName;e={$_.$col}.GetNewClosure()}
}
}
GetNewClosure() will see that the $col variable exists in the scope where it was called, and so it copies its value and stores it along-side the scriptblock.
As a result, when Select-Object executes the property expression (at a later time), the scriptblock "remembers" the original value and $col resolves correctly/as expected.
With sample data:
$data = [pscustomobject]#{
'Old Name' = 123
'Schema Name' = 456
}
$originalColumnNames = $data |Get-Member -MemberType Properties |ForEach-Object -MemberName Name
$data |Select -Property #(Rename-Columns $originalColumnNames)
It yields the expected result (spaces replaced in all names):
OldName SchemaName
------- ----------
123 456