There are two ways to write PowerShell Cmdlets - you can use either C# or PowerShell script. It is far, far easier to write Cmdlets using PowerShell script than using C#. Previously, the Cmdlets in PowerTools for Open XML were written in C#. With PowerTools for Open XML 3.0, we are moving to the approach of using scripts, which will make it much easier to create and enhance PowerTools for Open XML. This screen-cast tells you what you need to know in order to create new Cmdlets for PowerTools for Open XML.
