Why does my mandatory hashtable parameter not take my value - powershell

Question

I have a function with parameters like this:
function foo {
Param(
[Parameter(Mandatory)]
[string]$foo,
[Parameter(Mandatory)]
[hashtable]$bar
)
}
if i call it like this everything works fine:
foo -foo "abc" -bar #{"a"="b"}
but if I only call foo and PowerShell asks me to specify values for my mandatory parameters, it won't accept my hashtable.
PS C:\Users\abc> foo
cmdlet foo at command pipeline position 1
Supply values for the following parameters:
foo: "abc"
bar: #{"a"="b"}
Cannot convert the "#{"a"="b"}" value of type "System.String" to type "System.Collections.Hashtable".
why does PowerShell not take my specified value?
I also tried to write it different ways like #{a=b}, "a"="b", #{a=b;}, #{"a"="b";} etc.

Answer 1

When you mark a parameter as Mandatory and don't specify a value (argument) for it on invocation, PowerShell automatically prompts you for a value.
Unfortunately, this automatic prompting mechanism for missing mandatory parameter values is broken (it has never worked, and still doesn't as of PowerShell v7):
You currently cannot interactively supply a value for any of the following types:
[hashtable],[scriptblock], [bool], [switch] (the latter two being uncommon as mandatory parameters) - their literal forms #{ ... }, { ... } and $true / $false are simply not recognized.
There's also no way to interactively specify $null.
Additionally, the automatic prompts, when they do work, are inconvenient (no tab completion, no re-prompting on invalid input), and therefore currently of little use.
These problems are being discussed in this GitHub issue.
Given the above, it's better to bypass the automatic prompts altogether and instead throw an error when no value was provided:
function foo {
Param(
# Use a default value that throws an error.
[string] $foo = $(Throw "Please pass a value for -foo."),
# Use a default value that throws an error.
[hashtable] $bar = $(Throw "Please pass a value for -bar.")
)
}
Note that [Parameter(Mandatory)] then mustn't be used, as that would still trigger an automatic prompt, despite the presence of a default value.

Answer 2

Read-Host (which is invoked automatically when this happens) always returns strings. If you want to take hashtable input like that, you will need to process the input. I would do something like:
if ($foo -is [system.Collections.hashtable]) {
//proceed
} else if ($foo -is [system.array]) {
Foreach ($fooItem in $foo) {
$myhash.add($key, $fooItem)
}
} else if ($foo -is [system.string]) {
$fooItem = $foo.split(,)
...
} else {
//error
}