Generic type of Generic Type in Powershell

Question

Well generics in Powershel are quite confusing. To instantiate a simple List you need to dance around with a tambourine:

$type = ("System.Collections.Generic.List"+'`'+"1") -as "Type"
$type= $type.MakeGenericType("System.string" -as "Type")
$o = [Activator]::CreateInstance($type)

But what if I need something a bit more complex like: <Dictionary<string,List<Foo>> for example

or for example here: Dictionary<string,List<string>>

$listType = ("System.Collections.Generic.List"+'`'+"1") -as "Type"
$listType = $listType.MakeGenericType("System.string" -as "Type")
$L = [Activator]::CreateInstance($listType)

$dicType = ("System.Collections.Generic.Dictionary"+'`'+"2") -as "Type"

#the next line is problematic
$dicType = $dicType.MakeGenericType( 
     @( ("system.string" -as "Type"), 
        ("System.Collections.Generic.List" as "Type)) # and that's of course wrong
      )

$D = [Activator]::CreateInstance($dicType )

Answer 1

While you can delve into CLR internal representations and make life hard for yourself, you don't have to:

$dict = new-object 'collections.generic.dictionary[string,int]'
$dict.add("answer", 42)

Want a type literal representation?

[collections.generic.dictonary[string,int]]

Done. How about generic type parameters?

$dictOfList = new-object 'collections.generic.dictionary[string,
    [collections.generic.list[int]]]'

Done.

However, there's an unfortunate catch. In PowerShell 2.0, there's a bug when you mix and match BCL and 3rd party types as type parameters. The latter need to be assembly qualified:

# broken over two lines for clarity with backtick escape
$o = new-object ('collections.generic.dictionary[[{0}],[{1}]]' -f `
        [type1].fullname, [type2].fullname)

Hope this helps. In PowerShell 3.0, this has been fixed.

Answer 2

Yeah, seems it's possible, but as almost everything else in PS, this is darn ugly too. Here's the real world example:

$requestItemsType is a Dictionary<string, List<Amazon.DynamoDB.Model.WriteRequest>>

$wrt = ("System.Collections.Generic.List``1" -as "Type") 
$wrt = $wrt.MakeGenericType( @( ("Amazon.DynamoDB.Model.WriteRequest" -as "Type")))

$requestItemsType = ("System.Collections.Generic.Dictionary"+'`'+"2") -as "Type"
$requestItemsType = $requestItemsType.MakeGenericType( @( ("System.string" -as "Type"), ($wrt)))
$ri = [Activator]::CreateInstance($requestItemsType)
$ri.Add("TaskLog",$writeRequests)

Answer 3

The example above doesn't have to be as complicated if you're creating a dictionary with a custom type defined:

$propertiesType = ("System.Collections.Generic.Dictionary"+'`'+"2") -as "Type"
$propertiesType = $propertiesType.MakeGenericType( @( ("System.string" -as "Type"), ("Namespace.CustomType" -as "Type")))
$properties = [Activator]::CreateInstance($propertiesType)

Answer 4

我知道这可能有点陈旧，但是此解决方法在PowerShell 2中似乎很好用，几乎可以用作直接替代方法。

$foo = [activator]::CreateInstance(([System.Collections.Generic.List[string]] -as 'Type'))

Answer 5

Weird. I've always just done like this:

$dict = [system.collections.generic.dictionary[string, int]]::new()

$dict.add("my string", 42)

I always thought it was odd seeing people use New-Object and other unnecessary stuff when the constructor overloads are all exposed in a much more convenient way.

Though I only started with PS 3.0, admittedly.