How do I pass a class object in a argument list to a another computer and call a function on it?

Question

I am attempting to create a class object and use Invoke-Command to call a function on the class on a remote machine. When I use Invoke-Command with no computer name this works fine but when I attempt to do this on a remote computer I get an error saying the that the type does not contain my method. Here is the script I am using for testing this.

$ComputerName = "<computer name>"

[TestClass]$obj = [TestClass]::new("1", "2")

Get-Member -InputObject $obj

$credentials = Get-Credential

Invoke-Command -ComputerName $ComputerName -Credential $credentials -Authentication Credssp -ArgumentList ([TestClass]$obj) -ScriptBlock {
    $obj = $args[0]

    Get-Member -InputObject $obj

    $obj.DoWork()
    $obj.String3
}

class TestClass {
    [string]$String1
    [string]$String2
    [string]$String3

    [void]DoWork(){
        $this.String3 = $this.String1 + $this.String2
    }

    TestClass([string]$string1, [string]$string2) {
        $this.String1 = $string1
        $this.String2 = $string2
    }
}

Here is the output I get.

PS > .\Test-Command.ps1

cmdlet Get-Credential at command pipeline position 1
Supply values for the following parameters:
User: <my user>
Password for user <my user>: *

   TypeName: TestClass

Name        MemberType Definition
----        ---------- ----------
DoWork      Method     void DoWork()
Equals      Method     bool Equals(System.Object obj)
GetHashCode Method     int GetHashCode()
GetType     Method     type GetType()
ToString    Method     string ToString()
String1     Property   string String1 {get;set;}
String2     Property   string String2 {get;set;}
String3     Property   string String3 {get;set;}


   TypeName: Deserialized.TestClass

Name     MemberType Definition
----     ---------- ----------
GetType  Method     type GetType()
ToString Method     string ToString(), string ToString(string format, System.IFormatProvider formatProvider), string IFormattable.ToString(string format, System.IFormatProvider formatProvider)
String1  Property   System.String {get;set;}
String2  Property   System.String {get;set;}
String3  Property    {get;set;}
Method invocation failed because [Deserialized.TestClass] does not contain a method named 'DoWork'.
    + CategoryInfo          : InvalidOperation: (DoWork:String) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound
    + PSComputerName        : <computer name>

I can see that the type changes from TestClass to Deserialized.TestClass and I am wondering if there is a way around this? My goal is to be able to ship the functions I need to each of the machines I am running a script on so that I don't have to rewrite the functions in the context of the Invoke-Command script block.

I think you need to serialize the object. See S.O. [Is there a way to pass serializable objects to a PowerShell script with start-process?](https://stackoverflow.com/questions/34076478/is-there-a-way-to-pass-serializable-objects-to-a-powershell-script-with-start-pr) — VA systems engineer, Jan 24 '20 at 19:35
just include class definition in the script block and pass constructor arguments as parameter. Since 2 computers do not share memory they can only exchange with serialized (key/value pair) objects — Mike Twc, Jan 24 '20 at 19:59

score 1 · Accepted Answer · answered Jan 26 '20 at 22:31

In short: The XML-based serialization / deserialization that PowerShell employs behind the scenes during remoting and in background jobs only handles a handful of known types with type fidelity.

Instances of custom classes such as yours are emulated with method-less "property bags" in the form of [pscustomobject] instances, which is why the emulated instances of your class instances have no methods on the remote machine.

For a more detailed overview of PowerShell's serialization/deserialization, see the bottom section of this answer.

As Mike Twc suggests, you can work around the problem by passing your class definition along to your remote command as well, allowing you to redefine the class there and then recreate instances of your custom class in the remote session.

However, since you cannot dynamically obtain a custom class' definition, you'd have to either duplicate the class definition code or define it as a string to begin with, requiring evaluation via Invoke-Expression, which should generally be avoided.

A simplified example, which uses the $using: scope rather than parameters (via -ArgumentList) to include values from the caller's scope.

# Define your custom class as a *string* first.
$classDef = @'
class TestClass {
    [string]$String1
    [string]$String2
    [string]$String3

    [void]DoWork(){
        $this.String3 = $this.String1 + $this.String2
    }

    TestClass([string]$string1, [string]$string2) {
        $this.String1 = $string1
        $this.String2 = $string2
    }

    # IMPORTANT:
    # Also define a parameter-less constructor, for convenient
    # construction by a hashtable of properties.
    TestClass() {}

}
'@

# Define the class in the caller's session.
# CAVEAT: This particular use of Invoke-Expression is safe, but
#         it should generally be avoided.
#         See https://blogs.msdn.microsoft.com/powershell/2011/06/03/invoke-expression-considered-harmful/
Invoke-Expression $classDef

# Construct an instance
$obj = [TestClass]::new("1", "2")

# Invoke a command remotely, passing both the class definition and the input object. 
Invoke-Command -ComputerName . -ScriptBlock {
    # Define the class in the remote session too.
    Invoke-Expression $using:classDef
    # Now you can cast the emulated original object to the recreated class.
    $recreatedObject = [TestClass] $using:obj
    # Now you can call the method...
    $recreatedObject.DoWork()
    # ... and output the modified property
    $recreatedObject.String3
}

I'm glad to hear it was helpful, @MaxYoung; my pleasure. – mklement0 Jan 27 '20 at 13:29 — mklement0, Jan 27 '20 at 13:29

How do I pass a class object in a argument list to a another computer and call a function on it?

1 Answers1

Linked