Type of Variable not as expected in Powershell

Question

This is a question of understanding.

I construct a Directory Structure using a string Variable $path. I append the name of the Directory I want to create, this works as expected. When the path is completed I need the complete Directory as System.IO.DirectoryInfo but assigning the Path in this way $Path = Get-Item $Path results in a string type.

$path = "c:\dir1"
If(-Not (Test-Path $path))New-Item -ItemType Directory -Path $path
$path = $path + "\dir2"
If(-Not (Test-Path $path))New-Item -ItemType Directory -Path $path

# Assigned to same variable
$path = Get-Item $path
echo $path.GetType() # = string

# assigned to different variable
$p_a_t_h = Get-Item $path
echo $p_a_t_h.GetType() # = System.IO.DirectoryInfo

# solution but not understood the behavior
[System.IO.DirectoryInfo]$path = Get-Item $path
echo $path.GetType() # = System.IO.DirectoryInfo

It took hours to find out this behavior and I couldn't find any documentation why this is - maybe because I don't know what to search for.

It is clear, that for appending something to a variable, the type of the variable is relevant, but a $path = ... is a "new" assignement and should have the type of the assigned value - at least in my eyes. In the languages I used so far a variable becomes the type of its value and is not converted to a type the variable had earlier or I define the type of a variable and get an error if assigned with wrong type.

Where is the error in my logic?

learn.microsoft.com/en-us/powershell/module/… The Path parameter accepts a string input not a System.IO.DirecetoryInfo object — trebleCode
– trebleCode, Commented Jan 5, 2018 at 16:16
Why do you need/want to reassign the same variable when using separate variables (of the type you require) would make your life much easier? — henrycarteruk
– henrycarteruk, Commented Jan 5, 2018 at 16:19
@James it is not too important to have the same variable reassigned - for me it is just the same information with a different tpye. It is just a little script and i didn't think too much about it until i came across this problem. The point is not to solve the problem - i did already in many ways, but to understand why this is the case. — HolgerZ
– HolgerZ, Commented Jan 5, 2018 at 16:31
@user2734259 i know this and that's the reason why i tried to reassign [System.IO.DirectoryInfo]$path after i have created the Directorystructure using [string]$path — HolgerZ
– HolgerZ, Commented Jan 5, 2018 at 16:36
@user2734259 I've changed my script according to the info from briantist and found out that the -Path parameter accepts a [System.IO.DirecetoryInfo] object - no need to use [System.IO.DirecetoryInfo]$path.FullName — HolgerZ
– HolgerZ, Commented Jan 5, 2018 at 17:37

briantist · Accepted Answer · 2018-01-05 16:20:24Z

I think that somewhere in your code you did a left-side cast (on the variable, not the value) to [String], just like you did later in your sample with [System.IO.DirectoryInfo]$path.

The most common way that this happens: Parameters.

Is this taken from a function? Like:

function Invoke-MyThing {
param([String]$Path)

}

Why that matters

When you put the type on the variable, all values assigned to that variable receive that cast.

[String]$Value = 'Hello'
$Value.GetType()

$Value = 3.141
$Value.GetType()

Casting the value only affects that one value:

$V2 = 5
$V2.GetType()

$V2 = [String]9
$V2.GetType()

$V2 = 45
$V2.GetType()

So, remove your previous variable-side cast, or if it's a parameter, just use a different local variable.

Better yet, if it's a parameter, you could make it of type [System.IO.DirectoryInfo] instead.. then it would accept that directly, or even accept a string. You just have to rework your code a little bit to deal with it.

Yes it was a parameter. Do i understand this correctly, if i don't cast the variable anywhere it will be of the type of the value i assigned and will change when reassigned? When i use [System.IO.DirectoryInfo] and assign a [string] to it, can i still append/reassign a new [string] or do i need the -Name or -FullName Attributes? $path.name = $path.name + "\somedit" or $path = $path.name + "\somedir"
@HolgerZ if the variable has no cast assigned, you can reassign new values of new types and it will accept those types. If the variable has a cast, that cast will apply to the value on assignment (following all the same rules of any cast).

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