PowerShell function will not return an object

Question

I have a simple function that creates a generic list:

function test() { $genericType = [Type] "System.Collections.Generic.List``1" [type[]] $typedParameters = ,"System.String" $closedType = $genericType.MakeGenericType($typedParameters) [Activator]::CreateInstance($closedType) } $a = test 

The problem is that $a always null, regardless of what I'm trying to do. If I execute the same code outside the function, it works correctly.

Thoughts?

Answer 1

IMHO, that the trap number 1. If you return an object from a function that is somehow enumerable (I don’t know for sure whether the IEnumerable implementation is the only case), PowerShell expands the object and returns the elements in it.

Your new list was empty, so nothing was returned. To make it work, just use this:

 ,[Activator]::CreateInstance($closedType) 

This will create one array of elements to be expanded, and the element (general list) is assigned $a .

Additional Information

Here is a list of related questions to help you understand what is happening:

• Powershell Traps
• Preventing smoothing of Agnostic flexural arrays in Powershell
• Strange PowerShell behavior returning a DataSet / DataTable
• What determines if a Powershell pipeline will expand a collection?

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Note: you do not need to declare the title of the function as a bracket. If you need to add parameters, the function will look like this:

 function test { param($myParameter, $myParameter2) } 

or

 function { param( [Parameter(Mandatory=true, Position=0)]$myParameter, ... again $myParameter2) ... 

Answer 2

An easier way to work with generics. This does not directly address the [Activator] approach, although

 Function test { New-Object "system.collections.generic.list[string]" } (test).gettype()