String interpolation in PowerShell for hashmaps

Question

I have a list of hashmaps. I want to create comma separated string for each item. If I use the following, it works fine.

$users | ForEach-Object {$_.firstname + "," + $_.surname}

But when I try to do it with string interpolation like this,

$users | ForEach-Object {"$_.firstname,$_.surname"}

It shows

System.Collections.Hashtable.firstname,System.Collections.Hashtable.surname

What mistake am I doing?