How to pass reference parameter to PowerShell script from C#

Question

I can't seem to be able to pass in a reference parameter to PowerShell from C#. I keep getting the following error:

"System.Management.Automation.ParentContainsErrorRecordException: Cannot process argument transformation on parameter 'Test'. Reference type is expected in argument."

Example:

For the simple script:

Param (
[ref]
$Test
)

$Test.Value = "Hello"
Write-Output $Test

And here's the C# code:

string script = {script code from above};
PowerShell ps = PowerShell.Create();
ps = ps.AddScript($"New-Variable -Name \"Test\" -Value \"Foo\""); // creates variable first
ps = ps.AddScript(script)
        .AddParameter("Test", "([ref]$Test)"); // trying to pass reference variable to script code

ps.Invoke(); // when invoked, generates error "Reference type is expected in argument

I've tried AddParameter as well as AddArgument.

What I have gotten to work is to create my script first as a script block:

ps.AddScript("$sb = { ... script code ...}"); // creates script block in PowerShell
ps.AddScript("& $sb -Test ([ref]$Test)"); // executes script block and passes reference parameter
ps.AddScript("$Test"); // creates output that shows reference variable has been changed

Any help?

What help are you looking for? You already said/shown you can get your result using your script block workaround. Why stress out for something different? Why do you believe using a script blick is a bad thing? — postanote
– postanote, Commented Mar 12, 2020 at 4:12
Fair point - I should have said that I wanted to understand why my first approach wasn't working, in an effort to better understand the C# interface to PowerShell as the documentation is very limited. — Mike Oliver
– Mike Oliver, Commented Mar 12, 2020 at 11:37
To note, though, the solution from @mklement0 is better than my script block approach because I don't have to explicitly output the [ref] variable in PS in order to know the output variable. Instead, I can just check the C# PSReference object I pass to AddParameter. — Mike Oliver
– Mike Oliver, Commented Mar 12, 2020 at 12:08
Understood. So, please update your post to show what you did as well for the clarity of others that may dine the need for the same approach. — postanote
– postanote, Commented Mar 12, 2020 at 15:41

mklement0 · Accepted Answer · 2020-03-12 10:51:49Z

3

I can't seem to be able to pass in a reference parameter to PowerShell from C#

The only way to make your original approach work is to create your [ref] instance in C# and pass that, which means creating an instance of System.Management.Automation.PSReference and passing that to your .AddParameter() call:

// Create a [ref] instance in C# (System.Management.Automation.PSReference)
var psRef = new PSReference(null);

// Add the script and pass the C# variable containing the
// [ref] instance to the script's -Test parameter.
ps.AddScript(script).AddParameter("Test", psRef);

ps.Invoke();

// Verify that the C# [ref] variable was updated.
Console.WriteLine($"Updated psRef: [{psRef.Value}]");

The above yields Updated psRefVar: [Hello]

Full code:

using System;
using System.Management.Automation;

namespace demo
{
  class Program
  {
    static void Main(string[] args)
    {
      var script = @"
        Param (
        [ref]
        $Test
        )

        $Test.Value = 'Hello'
        Write-Output $Test
        ";

      using (PowerShell ps = PowerShell.Create())
      {
        var psRef = new PSReference(null);
        ps.AddScript(script).AddParameter("Test", psRef);
        ps.Invoke();
        Console.WriteLine($"Updated psRef: [{psRef.Value}]");
      }

    }
  }
}

edited Mar 12, 2020 at 10:51

answered Mar 12, 2020 at 10:46

mklement0

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2 Comments

Mike Oliver Over a year ago

Awesome - thanks! Makes sense. Can you explain the real difference between AddParameter vs. AddArgument? Is it just that AddParameter is named whereas AddArgument relies on PS script referring to $args[] array? Thanks again.

mklement0 Over a year ago

Glad to hear it was helpful, @MikeOliver; an argument is an unnamed value (a mere value, not preceded by a parameter name), passed positionally. For robustness, you should use .AddParameter(). Does that explain it?

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