How to add a class method to a built-in .net class in PowerShell?

Question

I know I can add an instance method to a built-in .net class in PowerShell like this.

PS C:\Users\a> [string] | Add-Member -MemberType ScriptMethod -Name test -Value {
>> param(
>>     $name
>> )
>> Write-Host $name
>> }
>>
PS C:\Users\a> [string].test("Joe")
Joe
PS C:\Users\a>

I want to know if it's possible to add a class method that I can call it like [string]::test("blah").

Thanks.

Edit:

The reason I want to do this is because I have written a PowerShell script against .net 4.5 and I used the method [string]::isnullorwhitespace which is a .net 4 and .net 4.5 method. When I run the on a machine that only have .net 3.0 installed, I got errors. I don't want to modify all the place that used that method, instead I want to just add that method if it does not exist. Thanks.

Joey · Accepted Answer · 2013-08-01 13:01:53Z

4

Documentation says

The Add-Member cmdlet lets you add members (properties and methods) to an instance of a Windows PowerShell object.

So I fear it's not possible to add static members that way.

answered Aug 1, 2013 at 13:01

Joey

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1 Comment

x0n Over a year ago

Correct - it's not possible. You could name a function like that with a hack: new-item function:"[string]::isnullorwhitespace" -value { ... } but powershell will never be able to see it.

Aaron Jensen · Accepted Answer · 2013-08-01 18:26:19Z

I don't think you add members to a class, only to instances of a class. I would create your own function, Test-StringIsNullOrWhiteSpace, that uses the built-in IsNullOrWhiteSpace method when running under .NET 4.0/4.5, but your own implementation under previous versions of .NET:

filter Test-StringIsNullOrWhiteSpace
{
    param(
        [Parameter(Mandatory=$true,ValueFromPipeline=$true)]
        [string]
        # The string to test.
        $InputObject
    )

    if( [string].GetMethods() | Where-Object { $_.Name -eq 'IsNullOrWhiteSpace' } )
    {
        return [string]::IsNullOrWhitespace( $InputObject )
    }

    return ($InputObject -eq $null -or $InputObject -match '^\s*$')
}

You should be able to use it like so:

if( Test-StringIsNullOrWhiteSpace $var )
{
    Write-Host ('$var is null or only whitespace.')
}

$emptyVars = $vars | Test-StringIsNullOrWhiteSpace

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