How do I create and use a custom function attribute in PowerShell?

Question

I want to be able to create and assign a custom attribute to my powershell functions. I looked everywhere and it seems to be possible but I have not seen an example. I have created a custom attribute in C# and am referencing the assembly in my powershell script. However, I receive an error stating Unexpected attribute 'MyDll.MyCustom'.

Here is what I have:

MyCustomAttribute in MyDll.dll:

namespace MyDll
{
    [AttributeUsage(AttributeTargets.All, Inherited = true, AllowMultiple = false)]
    public sealed class MyCustomAttribute : Attribute
    {
        public MyCustomAttribute(String Name)
        {
            this.Name= Name;
        }

        public string Name { get; private set; }
    }
}

PowerShell Script:

Add-Type -Path "./MyDll.dll";
function foo {
    [MyDll.MyCustom(Name = "This is a good function")]

    # Do stuff 
}

Of note, however, is that if I do this:

$x = New-Object -TypeName "MyDll.MyCustomAttribute" -ArgumentList "Hello"

It works fine. So the type is clearly being loaded correctly. What am I missing here?

Mathias R. Jessen · Accepted Answer · 2017-01-07 20:12:15Z

2

Seemingly two things you need to change:

Command attributes need to syntactically precede the param() block.
Using the Name = specifier seems to cause the PowerShell parser to treat the attribute argument as an initializer, at which point the constructor won't get resolved.

function foo {
    [MyDll.MyCustom("This is a good function")]
    param()
    # Do stuff 
}

answered Jan 7, 2017 at 20:12

Mathias R. Jessen

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