What is the difference between char[] and char? - powershell

Question

If I use the [char] datatype, it requires the value to be one character, e.x.
[char]"a"
a
Since if I use it with more than one character it will give me an error:
[char]"ab"
Cannot convert value "ab" to type "System.Char". Error: "String must be exactly one
character long."
At line:1 char:1
+ [char]"ab"
+ ~~~~~~~~~~
+ CategoryInfo : InvalidArgument: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvalidCastParseTargetInvocation
However if I use
[char[]]"ab"
I get the output
a
b
If I compare Get-Member on both, I get no result:
PS C:\Users\nijoh> Compare-Object -ReferenceObject $([char] | gm) -DifferenceObject $([char[]] | gm) -PassThru
PS C:\Users\nijoh>
But I can see that they are two distinct types since they show up differently:
PS C:\Users\nijoh> ([char[]]"a").GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Char[] System.Array
PS C:\Users\nijoh> ([char]"a").GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Char System.ValueType
So what is the difference between the [char] and [char[]] datatypes in Powershell?

Answer 1

In PowerShell, a type name enclosed in [...] is a type literal, i.e. an object that represents the .NET type named.
Independently of that, inside [...], [] immediately after a type name represents an array containing elements of that type; for an overview of the notation used inside [...], see this answer.
Therefore:
[char] refers to type System.Char (the System. prefix is generally optional in PowerShell; additionally - as is the case here - PowerShell has a fixed set of type accelerators that allow you to use simple names for types located in any namespace - see this answer).
To get a given type literal's full (namespace-qualified) .NET type name, use [...].FullName; e.g., [char].FullName yields System.Char
[char[]] refers to an array type whose elements are of type char ( System.Char); [char[]].FullName yields System.Char[].
Since if I use it with more than one character it will give me an error ([char]"ab")
PowerShell has no [char] literals, only string ([string]) literals. When you cast a string to [char] - which represents a single character - only a single-character string is accepted; e.g. [char] 'a' works, but [char] 'ab' or [char] "ab" doesn't.
However if I use [char[]]"ab" ...
Casting a string to an array of characters returns a character array comprised of the string's individual characters.
In other words: [char[]] "ab" is equivalent to "ab".ToCharArray().
If I compare Get-Member on both, I get no result:
The reason is that Get-Member operates on the types of the input objects, and if the input objects are type literals - such as [char] and [char[]] - their type is examined, which is System.RuntimeType, a non-public PowerShell type that derives from System.Reflection.TypeInfo, which describes a .NET type.
In other words: all type literals piped to Get-Member will result in the same output, irrespective of what specific type they refer to, because it is the one and only type-describing type whose members are being reported.
Therefore, using Compare-Object on Get-Member calls with different type literals predictably produces no output, because the Get-Member calls result in the same output[1]. (Not producing output is Compare-Object's way of indicating that no differences were detected.)
[1] Get-Member outputs an array of Microsoft.PowerShell.Commands.MemberDefinition instances, one for each member of the input object's type. In the absence of a -Property argument passed to Compare-Object, these instances are compared as a whole, by their .ToString() value, which yields a meaningful representation of each member.