Using reflection to get round PSv2 lack of PSSerializer class

I had some code that used PSSerializer to serialize objects into XML. This blew up when run on PSv2, because PSv2 doesn’t expose the System.Management.Automation.PSSerializer class - leaving me looking at an annoying refactor to use Export-Clixml everywhere, which only ever writes to the filesystem.

I thought I’d have a look at what PSSerializer does under the hood, so I opened the source code for Powershell - which you can clone from Github - and searched for it.

I found it in serialization.cs. I’m really only interested in the Serialize and Deserialize methods and, fortunately, they turn out to be quite simple. Here’s the method declaration for Serialize:

public static string Serialize(Object source, int depth)
{
    // Create an xml writer
    StringBuilder sb = new StringBuilder();
    XmlWriterSettings xmlSettings = new XmlWriterSettings();
    xmlSettings.CloseOutput = true;
    xmlSettings.Encoding = System.Text.Encoding.Unicode;
    xmlSettings.Indent = true;
    xmlSettings.OmitXmlDeclaration = true;
    XmlWriter xw = XmlWriter.Create(sb, xmlSettings);

    // Serialize the objects
    Serializer serializer = new Serializer(xw, depth, true);
    serializer.Serialize(source);
    serializer.Done();
    serializer = null;

    // Return the output
    return sb.ToString();
}

Pretty simple, right… if I can also use those other classes. Well, StringBuilder and XmlWriterSettings are public, and I can find them even in PSv2, but Serializer is declared as an internal class, so I just get an error:

[System.Management.Automation.Serializer]
Unable to find type [System.Management.Automation.Serializer].
At line:1 char:1
+ [System.Management.Automation.Serializer]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Management.Automation.Serializer:TypeName) [], RuntimeException
    + FullyQualifiedErrorId : TypeNotFound

If I can access this class, then I can brew up an sort-of monkeypatch of PSSerializer’s Serialize() and Done() methods. This is where reflection comes in.

First, we use the assembly containing the class to get the type:

# System.Management.Automation
# Quickest way to get the right assembly
# is from a type in the same assembly
$SmaAssembly = [powershell].Assembly
$Type = $SmaAssembly.GetType('System.Management.Automation.Serializer')

$Type

IsPublic IsSerial Name           BaseType
-------- -------- ----           --------
False    False    Serializer     System.Object

This is a RuntimeType, just like the output from calling GetType() with no arguments on any object, except that it would be otherwise inaccessible (because it was declared internal rather than public).

Next we get a constructor (note that ‘.ctor’ is a common abbreviation for ‘constructor’):

$Ctor = $Type.GetConstructors('Instance, NonPublic') |
    Where-Object {$_.GetParameters().Count -eq 3}

I have Where-Object {$_.GetParameters().Count -eq 3} because Serializer has three constructors, and I want the one that matches the signature of the one used in the PSv3+ declaration of the PSSerializer class, which is new Serializer(xw, depth, true) in the C# source code.

The GetConstructors method takes an argument of type System.Reflection.BindingFlags. That is an enum. These are the values of the enum:
[Enum]::GetValues([System.Reflection.BindingFlags])
Default
IgnoreCase
DeclaredOnly
Instance
Static
Public
NonPublic
  ... etc ...
As the name suggests, this is a flag-type enum, which means that you can combine the values. This is usually the case wherever you see options that have power-of-two values, like 0x400, 0x800 etc etc. You bitwise-combine these to send the combination of options that you want - so 0x400 and 0x800 would be 0xC00. We want the Instance and the NonPublic options. In Powershell, the long way to write this out would be:
[System.Reflection.BindingFlags]::Instance -bor [System.Reflection.BindingFlags]::NonPublic
Fortunately, a single string containing comma-separated enum names will be combined, so we can just pass in 'Instance, NonPublic' to get the same effect.

To get back from our digression, we now have the constructor that we want and can invoke it:

# Constructor params
$Depth = 10    # like -Depth in ConvertTo-Json
$OutputBuilder = [Text.StringBuilder]::new()
$XmlWriter = [System.Xml.XmlWriter]::Create($OutputBuilder)

$Serializer = $Ctor.Invoke(@($XmlWriter, $Depth, $true))

We’re not done with reflection, unfortunately. To use this object, we need to call Serialize followed by Done. And those methods are also nonpublic. So we neeed to grab those:

$Methods = $Type.GetMethods('Instance, NonPublic')
$SerializeMethod = $Methods | Where-Object {$_.Name -eq 'Serialize' -and $_.GetParameters().Count -eq 1}
$DoneMethod = $Methods | Where-Object {$_.Name -eq 'Done'}

Now we can Do The Thing:

$DataToSerialize = "Foo"

$SerializeMethod.Invoke($Serializer, @($DataToSerialize))   # single param for .Serialize(data)
$DoneMethod.Invoke($Serializer, @())                        # empty list of params for .Done()

return $OutputBuilder.ToString()
# <?xml version="1.0" encoding="utf-16"?>
# <Objs Version="1.1.0.1" xmlns="http://schemas.microsoft.com/powershell/2004/04">
#     <S>Foo</S>
# </Objs>