About Enums

This is the first of three posts that will each build on the previous post. Come back next week for the next one!

Edit - 05-May-2021 - The whole series is now live.

• Part 2 - Classes
• Part 3 - Bitwise

So what’s an enum?

A distinct type that has a set of named constants. An enumerator is a data type that has preset values (constants). When we set a variable to an enum value it will always be one of our predefined values

How do we create an Enum then?

we define an enum a bit like a hash table of values we want to work with, except there is no Name part, its just a set of values.

PS> enum Animals {
    Cat
    Dog
    Fish
    Tiger
}

How do we reference an enum once we have created one?

You can reference an enumerator with [ and ]

PS> [Animals]

# does that look familiar?
PS> (get-date).dayofweek
PS> (get-date).dayofweek.gettype()

We can get a specific item from the enumerator with an index value

PS> [Animals].GetEnumName(2)

Using enums as values

Setting a variable to a value and setting a variable to an enum are similar in syntax but subtley different in what implications there are. The first results in a string variable type whereas the other results in a type that is our enum.

$var1 = 'fish'
[animals]$Var2 = [Animals]::Fish

$Var1.gettype().fullname
$Var2.gettype().fullname

How do these two variables compare?

PS> ($var1 -eq $var2)

We can very simply convert our Animals type variable into a string with its built-in ToString() method

PS> $var2.tostring()

Strange things happen when we add a second value to our [Animals] type variable though

PS> $var2 += [Animals]::tiger
PS> $var2

The value of our variable has become the sum of the enum object values (Fish is 2 and Tige is 3 in our original declaration of the enum. However, we cant get those values back out very simply. These lines simply return False.

PS> $var2 -eq 'fish'
PS> $var2 -eq 'tiger'

Trying other comparisons is similarly doomed

PS> $var2 -like 'tiger'
PS> $var2 -eq 'fish, tiger'
PS> $var2

The value, 5, is also useless to us as both 2+3 and 1+4 could be combinations of values that make up our current value of 5

If we want to work with more than one value in an enum then we need to declare it with the [Flags()] designation. Looking at the different attributes that a file can have, we could define an enum as follows

[Flags()] enum FileAttributes {
    Archive = 1
    Compressed = 2
    Device = 4
    Directory = 8
    Encrypted = 16
    Hidden = 32
}

Once that is done we could define $file1 as [FileAttribute].

[FileAttributes]$file1 = [FileAttributes]::Archive
[FileAttributes]$file1 +=[FileAttributes]::Compressed
[FileAttributes]$file1 +=  [FileAttributes]::Device

Having done this, we can see the attributes of our imaginary file with this line

"file1 attributes are: $file1"

Equally, we can use the sum of our selected attribute values to set our variable. Encrypted, Directory, and Device are 16 + 8 + 4 respectively, referencing our declaration above. This totals 28 so we can set our $file2 variable as follows

[FileAttributes]$file2 = [FileAttributes]28  
"file2 attributes are: $file2"

Further reading

about enum