Powershell Error When Returning Collections.Generic.List[String]

Question

I have the following code:

[Collections.Generic.List[String]]$script:Database = New-Object Collections.Generic.List[String]
[string]$script:DatabaseFile = "C:\NewFolder\database.csv"

function RunProgram
{
    $script:Database = getCurrentDatabase #errors after this line
}
function getCurrentDatabase
{
    [string[]]$database = Get-Content -Path $script:DatabaseFile
    [Collections.Generic.List[String]]$databaseAsList = $database
    return $databaseAsList
}

I get this exception after getCurrentDatabase returns:

Cannot convert the "system.object[]" value of type "system.object[]" to type "system.collections.generic.list`1[system.string]"

To get the code to work, I need to do this:

[Collections.Generic.List[String]]$script:Database = New-Object Collections.Generic.List[String]
[string]$script:DatabaseFile = "C:\NewFolder\database.csv"

function RunProgram
{
    getCurrentDatabase #this works fine
}
function getCurrentDatabase
{
    [string[]]$database = Get-Content -Path $script:DatabaseFile
    $script:Database = $database
}

Why does the first way throw that exception, but the 2nd way doesn't?

EDIT: I am using PS version 2.0 and C:\NewFolder\database.csv contains this one line:

Release Group,Email Address,Template ID,Date Viewed

I tried running your first snippet and it runs without errors (including invoking RunProgram and writing $script:Database to host)... is that the exact code in your script? — SteveChapman
– SteveChapman, Commented May 26, 2014 at 2:05
note, there's a syntax error on line 11 (missing a bracket): [Collections.Generic.List[String]]$databaseAsList = $database — SteveChapman
– SteveChapman, Commented May 26, 2014 at 2:06
sorry that typo wasn't in my original code. and yes this is the entire code. Could it be something to do with the fact that I'm using Powershell v2.0? — David Klempfner
– David Klempfner, Commented May 26, 2014 at 2:17

BartekB · Accepted Answer · 2014-05-26 08:51:57Z

2

That's something people trip on a lot... When you output collections from function PowerShell will enumerate them, loosing original type as a part of the process. To prevent it from happening you can use unary comma:

function getCurrentDatabase
{
    [string[]]$database = Get-Content -Path $script:DatabaseFile
    [Collections.Generic.List[String]]$databaseAsList = $database
    , $databaseAsList
}

Unary comma should prevent PowerShell from enumerating your collection (it actually enumerates collection of one that we've just created using unary comma).

I wrote a blog post about it a while ago - should help understand it in more detail.

answered May 26, 2014 at 8:51

BartekB

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3 Comments

David Klempfner Over a year ago

wow, I'm sure you can understand how unintuitive that would be for programmers who started with C#!

David Klempfner Over a year ago

do you know why my code would have worked for Chappoo? He wrote in the comments below my question that it worked for him.

BartekB Over a year ago

I would guess it was PowerShell version: when I tried on v3 it worked just fine, had to test it with powershell -version 2 to get same errors you got.

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