`[enum]` Without `Flags` Inconsistent Behavior And Implicit Casting

[mattcargile]

I found this trap while passing an @() to a [enum] parameter without the Flags attribute. This occurs on 5.1.22621.4391 and 7.4.6.

The simplest demo is below just with the [type].

[DayOfWeek]'Monday,Tuesday'
# Returns Wednesday
[DayOfWeek]@('Monday','Tuesday')
# Returns Wednesday

In a [scriptblock] or function{}, it is a bit more problematic as I would expect it to throw and the implicit cast returns inconsistent results.

& {
    param( [DayOfWeek]$d )
    $d -eq 'Wednesday'
} Monday,Tuesday
# Returns Wednesday

[nightroman]

Interesting... FWIW it's how .NET methods Enum.Parse work, too.

The value parameter contains the string representation of an enumeration member's underlying value or named constant, or a list of named constants delimited by commas (,). One or more blank spaces can precede or follow each value, name, or comma in value. If value is a list, the return value is the value of the specified names combined with a bitwise OR operation.

The result of bitwise OR may be valid value on not:

> [DayOfWeek]'Monday,Tuesday'
Wednesday

> [DayOfWeek]'Friday,Saturday'
InvalidArgument: Cannot convert value "Friday, Saturday" to type "System.DayOfWeek" due to enumeration values that are not valid.

[nightroman]

Why are results inconsistent, what do you mean? All examples return Wednesday. This might be not what you expect but it is consistent.

[mattcargile]

I would expect it to fail in every case unless there is a Flags attribute that allows multiple values to be set. The counter point would be behavior of [IO.FileAttributes]'Hidden,Directory'. PowerShell doesn't really expose which type of [enum] the user is working with.

[nightroman]

Yes, agree. I did not know about this feature and could make a mistake, too. I'll create a topic. This may take some time due to other stuff.

[nightroman]

I have created the topic https://github.com/nightroman/PowerShellTraps/tree/main/Basic/Enums-without-FlagsAttribute