Is it possible to attach an event to a PSObject?

Question

Say I've a psobject like this :

$o=New-Object PSObject -Property @{"value"=0}
Add-Member -MemberType ScriptMethod -Name "Sqrt" -Value {
    echo "the square root of $($this.value) is $([Math]::Round([Math]::Sqrt($this.value),2))"
} -inputObject $o

Is it possible to attach an event so that the method Sqrt() is executed when the value attribute change ? ie :

PS>$o.value=9

will produce

the square root of 9 is 3

update

As per @Richard answer this is the working recipe :

$o=New-Object PSObject -property @{"_val"=1}
Add-Member -MemberType ScriptMethod -Name "Sqrt" -Value {
    write-host "the square root of $($this._val) is $([Math]::Round([Math]::Sqrt($this._val),2))"
} -inputObject $o


Add-Member -MemberType ScriptProperty -Name 'val' -Value{ $this._val }  -SecondValue { $this._val=$args[0];$this.Sqrt() } -inputObject $o

Answer 1

Rather than making value a NoteProperty make it a ScriptProperty, this includes defining separate get and set methods that are called rather than directly modifying a field.

$theObject | Add-Member -MemberType ScriptProperty -Name 'Value' 
                        -Value{ $this._value }
                        -SecondValue { $this._value = $args[0]; $this.Sqrt }

(Value defines the get method, SecondValue the set.)

Note as PowerShell doesn't provide any ability to encapsulate data, the underlying field is still accessible to callers. Coding a custom type in C# (or other .NET language) and use of Add-Type can avoid this, but is unlikely to be worth it unless you really have callers who will not follow the rules.

Second Issue

In a ScriptProperty there is no output pipe (any output is thrown away), so echo (being an alias for Write-Output) won't do anything useful. Replacing it with Write-Host works. In general side effects in a property get or set (including output) are poor practice (there is an expectation of low overhead when using them).