Example for defining custom type conversion in PowerShell using [System.Management.Automation.PSTypeConverter]

DictionaryPSTypeConverter.ps1

#!/usr/bin/env pwsh
using namespace System
using namespace System.Collections
using namespace System.Collections.Generic
using namespace System.Collections.ObjectModel
using namespace System.Collections.Specialized
using namespace System.ComponentModel
using namespace System.Management.Automation
using namespace System.Reflection

class DictionaryPSTypeConverter : PSTypeConverter {
	# A parameterless constructor is required.
	DictionaryPSTypeConverter() {}

	# Gets invoked when [DictionaryPSTypeConverter] was added to the destination
	# type's type data. Not used here because you need to add it to the *exact*
	# output type (e.g. [Dictionary[string, int]]) - adding it to a generic type
	# definition (e.g. [Dictionary`2]) or an interface type (e.g.
	# [IDictionary]) does **not** work!
	[bool]CanConvertFrom([object]$SourceValue, [type]$DestinationType) {
		return $false
	}

	# Gets invoked when [DictionaryPSTypeConverter] was added to the source
	# values's type data, in this case [hashtable] and [ordered].
	[bool]CanConvertTo([object]$SourceValue, [type]$DestinationType) {
		return (
			(-not [object]::ReferenceEquals($null, $SourceValue)) -and
			(-not [object]::ReferenceEquals($null, $DestinationType)) -and
			($SourceValue -is [IDictionary]) -and
			($DestinationType -is [TypeInfo]) -and
			($DestinationType.ImplementedInterfaces -contains [IDictionary])
		)
	}

	# This won't ever be called as [DictionaryPSTypeConverter]::CanConvertFrom
	# always returns false.
	[object]ConvertFrom([object]$SourceValue, [type]$DestinationType, [IFormatProvider]$FormatProvider, [bool]$IgnoreCase) {
		throw [NotSupportedException]::new(
			"[$($this.GetType())] does not support converting into arbitrary destination types."
		)
	}

	[object]ConvertTo([object]$SourceValue, [type]$DestinationType, [IFormatProvider]$FormatProvider, [bool]$IgnoreCase) {
		[IDictionary]$Output = try {
			$DestinationType::new($SourceValue.Count, [StringComparer]::"Ordinal$(if ($IgnoreCase) {
				'IgnoreCase'
			})")
		} catch {
			try {
				$DestinationType::new($SourceValue.Count)
			} catch {
				$DestinationType::new()
			}
		}

		$SourceValue.GetEnumerator() | ForEach-Object {
			[void]$Output.Add($_.Key, $_.Value)
		}

		return $Output
	}
}

# Register [DictionaryPSTypeConverter] to be invoked whenever PowerShell tries
# to convert either a [hashtable] or an [ordered] into any other type, assuming
# that no other conversion mechanism with a higher priority could be found (like
# casting to [object], which is a base type of both, or to [psobject], which is
# hard-coded to call [psobject]::AsPSObject()).
[hashtable], [ordered] | Update-TypeData -TypeConverter ([DictionaryPSTypeConverter])

################################################################################
#                                   EXAMPLE                                    #
################################################################################

# This would normally fail, because PowerShell would try to set each entry in
# the source hashtable as a property instead of as an entry on the dictionary.
# Beware that this will still happen if you use keys like `Count` that exist
# on the given destination type, which can lead to some very odd behaviour.
[Dictionary[string, int]]@{
	Foo = 69
	Bar = 420
}