How do I call a parameterless generic method from Powershell v3?

Question

For example I have a .NET object $m with the following method overloads:

PS C:\Users\Me> $m.GetBody

OverloadDefinitions
-------------------    
T GetBody[T]() 
T GetBody[T](System.Runtime.Serialization.XmlObjectSerializer serializer)  

If I try to invoke the parameterless method I get:

PS C:\Users\Me> $m.GetBody()
Cannot find an overload for "GetBody" and the argument count: "0".
At line:1 char:1
+ $m.GetBody()
+ ~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodException
    + FullyQualifiedErrorId : MethodCountCouldNotFindBest

I understand PowerShell v3.0 is supposed to work more easily with generics. Obviously I need to tell it somehow what type I want returned but I cannot figure out the syntax.

Answer 1

It looks like you are trying to invoke a generic method.

In powershell this can be done by:

$nonGenericClass = New-Object NonGenericClass
$method = [NonGenericClass].GetMethod("SimpleGenericMethod")
$gMethod = $method.MakeGenericMethod([string]) 
# replace [string] with the type you want to use for T. 
$gMethod.Invoke($nonGenericClass, "Welcome!")

See this wonderful blog post for more info and additional examples.

For your example you could try:

$Source = @" 
public class TestClass
{
    public T Test<T>()
    {
        return default(T);
    }
    public int X;
}
"@ 

Add-Type -TypeDefinition $Source -Language CSharp 
$obj = New-Object TestClass

$Type  = $obj.GetType();
$m =  $Type.GetMethod("Test")
$g = new-object system.Guid
$gType = $g.GetType()
$gm = $m.MakeGenericMethod($gType)
$out = $gm.Invoke( $obj, $null)
#$out will be the default GUID (all zeros)

This can be simplified by doing:

$Type.GetMethod("Test").MakeGenericMethod($gType).Invoke( $obj, $null)

This has been testing in powershell 2 and powershell 3.

If you had a more detailed example of how you came across this generic method I would be able to give more details. I have yet to see any microsoft cmdlets return anything that give you generic methods. The only time this comes up is when custom objects or methods from c# or vb.net are used.

To use this without any parameters you can use Invoke with just the first parameter. $gMethod.Invoke($nonGenericClass)

Answer 2

Calling a generic method on an object instance:

$instance.GetType().GetMethod('MethodName').MakeGenericMethod([TargetType]).Invoke($instance, $parameters)

Calling a static generic method (see also Calling generic static method in PowerShell):

[ClassType].GetMethod('MethodName').MakeGenericMethod([TargetType]).Invoke($null, $parameters)

Note that you will encounter an AmbiguousMatchException when there is also a non-generic version of the method (see How do I distinguish between generic and non generic signatures using GetMethod in .NET?). Use GetMethods() then:

([ClassType].GetMethods() | where {$_.Name -eq "MethodName" -and $_.IsGenericMethod})[0].MakeGenericMethod([TargetType]).Invoke($null, $parameters)

(Mind that there could be more than one method that match the above filter, so make sure to adjust it to find the one you need.)

Hint: You can write complex generic type literals like this (see Generic type of Generic Type in Powershell):

[System.Collections.Generic.Dictionary[int,string[]]]

Answer 3

To call a (parameterless) generic method with overloads from Powershell v3, as shown in the OP example, use the script Invoke-GenericMethod.ps1 from the reference provided by @Chad Carisch, Invoking Generic Methods on Non-Generic Classes in PowerShell.

It should look something like

Invoke-GenericMethod $m GetBody T @()

This is a verified working code sample that I am using:

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Practices.ServiceLocation") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Practices.SharePoint.Common") | Out-Null

$serviceLocator = [Microsoft.Practices.SharePoint.Common.ServiceLocation.SharePointServiceLocator]::GetCurrent()

# Want the PowerShell equivalent of the following C#
# config = serviceLocator.GetInstance<IConfigManager>();

# Cannot find an overload for "GetInstance" and the argument count: "0".
#$config = $serviceLocator.GetInstance()

# Exception calling "GetMethod" with "1" argument(s): "Ambiguous match found."
#$config = $serviceLocator.GetType().GetMethod("GetInstance").MakeGenericMethod([IConfigManager]).Invoke($serviceLocator)

# Correct - using Invoke-GenericMethod
$config = C:\Projects\SPG2013\Main\Scripts\Invoke-GenericMethod $serviceLocator GetInstance Microsoft.Practices.SharePoint.Common.Configuration.IConfigManager @()

$config.CanAccessFarmConfig

Here is an alternate script that I haven't tried but is more recent and being actively maintained, Invoke Generic Methods from PowerShell.

Answer 4

marsze's helpful answer contains great general information about calling generic methods, but let me address the aspect of calling a parameter-less one specifically, as asked:

As hinted at in the question:

• in PSv3+ PowerShell can infer the type from the parameter values (arguments) passed to a generic method,
• which by definition cannot work with a parameter-less generic method, because there is nothing to infer the type from.

As of Windows PowerShell v5.1 / PowerShell Core v6.1.0, PowerShell has no syntax that would allow you to specify the type explicitly in this scenario.

However, there is a suggestion on GitHub to enhance the syntax in PowerShell Core that has been green-lighted in principle, but is awaiting implementation by the community.

For now, reflection must be used:

# Invoke $m.GetBody[T]() with [T] instantiated with type [decimal]
$m.GetType().GetMethod('GetBody', [type[]] @()).
    MakeGenericMethod([decimal]).
      Invoke($m, @())

• .GetMethod('GetBody', [type[]] @()) unambiguously finds the parameter-less overload of.GetBody(), due to passing in an empty array of parameter types.

• .MakeGenericMethod([decimal]) instantiates the method with example type [decimal].

• .Invoke($m, @()) then invokes the type-instantiated method on input object ($m) with no arguments (@(), the empty array).