How does the Count property work in Powershell?

Question

I've made a most unfortunate typo costing me quite some precious time:

$errors.Count

This returns "0", even if there are errors, because the variable name should be singular. This does work:

$error.clear()                # To ensure a correct repro
Copy-Item asdf fdsa           # Will show an error
$error.Count                  # Will output "1"

However, I now want to know why $errors.Count gave me anything at all, and why it gave me "0". So I went on to do some testing, and got the following results:

$asdf.Count                   # Will output "0"
$nrOfEinsteinsInSpace.Count   # Will output "0"
$a = 0; $a.Count;             # Will output "1"
$b = 2; $a.Count;             # Will output "1"
$x = 1,2,3; $x.Count;         # Will output "3"

And gathering even more data to be able to ask a sensible question here I did:

$true.Count                   # Will output "1"
$false.Count                  # Will output "1"

So we have the following different cases:

1. Array(like) variables, where .Count will output the number of items.
2. Non-existent variables, where .Count will output "0".
3. Declared variables, where .Count will output "1".
4. Built-in variables, where .Count will output "1".

Case 2, 3, and 4 don't make any sense to me (yet). What is going on here? Where is this documented? How does the .Count property work?

Answer 1

To complement Paul's answer, this might be related to extended type data. To quote the relevant part of the documentation:

Extended type data defines additional properties and methods ("members") of object types in Windows PowerShell. You can extend any type that is supported by Windows PowerShell and use the added properties and methods in the same way that you use the properties that are defined on the object types.

And:

There are three sources of extended type data in Windows PowerShell sessions. The Types.ps1xml files in the Windows PowerShell installation directory are loaded automatically into every Windows PowerShell session.

If you open that Types.ps1xml file (in $pshome), you'll see this at the beginning of the file:

   <Type>
        <Name>System.Array</Name>
        <Members>
            <AliasProperty>
                <Name>Count</Name>
                <ReferencedMemberName>Length</ReferencedMemberName>
            </AliasProperty>
        </Members>
    </Type>

So my guess is that by providing the ".Count" property, PowerShell assumes this is an array.

Answer 2

Here is how i think it works:

Case 1: In Arrays the .Count Property actually links to the .Length property which shows the number of Items in the Array

Case 2: Non-exitent variables get automatically created by powershell and initialized with value $null

Case 3 / 4: On this one i am not exactly sure why it happens but since neither String nor Int or boolean Objects have a .Count property i could imagine that the Property is inherited by a parent-object.

The behaviour suggests that the variable is treated as array so with 1 Value assigned the output will be 1, without a value the result will be 0.

Edit: For the sake of completeness here is the Link to the Documentation: Technet, thanks @David Brabant