Question

I have a function accepting an enum value as parameter. As an example, consider something like:

(PS) > function IsItFriday([System.DayOfWeek] $dayOfWeek) { 
    if($dayOfWeek -eq [System.DayOfWeek]::Friday) {
        "yes"
    } else {
        "no"
    } 
}

Now, if I invoke it like this, everything is fine:

(PS) > $m = [System.DayOfWeek]::Monday
(PS) > IsItFriday $m
no

But if I call the function passing directly the enum value, I get a rather cryptic error:

(PS) > IsItFriday [System.DayOfWeek]::Monday
IsItFriday : Cannot convert value "[System.DayOfWeek]::Monday" to type "System.DayOfWeek" 
due to invalid enumeration values. Specify one of the following enumeration values and 
try again. The possible enumeration values are "Sunday, Monday, Tuesday, Wednesday, 
Thursday, Friday, Saturday".
At line:1 char:11
+ IsItFriday  <<<< [System.DayOfWeek]::Monday

What’s the difference between initializing a variable with the enum value and passing the enum value directly?

Answer 1

Yes, that is a rather confusing error message. I think you would understand better with an example:

Get-ChildItem -Path C:\

Notice there are no quotes around C:\ because, one, it implcitly gets converted to a string, and two, it is not necessary to enclose a path which does not contain spaces when you pass the path as a parameter to some callee.

So lets go back to your function, and change it slightly:

function IsItFriday($dayOfWeek) 
{
    $dayOfWeek.GetType()

    if ($dayOfWeek -eq [System.DayOfWeek]::Friday) 
    {
        "yes"
    } 
    else 
    {
        "no"
    }
}

IsItFriday [System.DayOkWeek]::Monday

…and the output:

IsPublic IsSerial Name                                     BaseType                                                                                        
-------- -------- ----                                     --------                                                                                        
True     True     String                                   System.Object                                                                                   
no

See what happened there? PowerShell thinks you are passing in a string instead of an enumeration value, so that’s why you get Cannot convert value "[System.DayOfWeek]::Monday" because that is the literal string that gets passed in.