Dimitri's Wanderings

Creating a WMI Class from registry entries

 
  • Your Name
  • Oct 28, 2016

As you may know, Windows Management Instrumentation (WMI) is the way Microsoft has made available many API available since Windows XP over the time. Creating a WMI class can be a tedious class but creating a WMI class from registry entries is in contrast fairly easy.

Creating one’s own WMI Class Steps

As you perhaps know, creating WMI objects is a matter of

  • describing the objects and its instances, usually in a MOF format
  • Compiling the description into something useable by the WMI engine

Registry use in WMI

Registry has its own provider in the WMI world in order to easily create or update which allows a registry key and its values to be mapped to a WMI class.

In order for you to use the registry in a WMI, you must then

  1. register the WMI provider in the MOF File
  2. create a new class
  3. for each property of this class, indicate which registry key stores the underlying data.

The only problem is that every WMI class instance must have a unique identifier. Fortunately the plumbing inside the registry provider allows us to define a dummy property called InstanceKey that will do the trick once it has been prefixed by the [key] keyword.

WMI Class template

The following template uses the eps engine which brings Razor equivalent templating capabilities to Powershell:

<pre class="lang:c# decode:true" title="WMI Class creation from MOF File ">// create WMI Class that maps a registry key
// DJ - 1.0 - 18.10.2016
//==================================================================
// 1. Register Registry property provider (shipped with WMI)
//==================================================================

#pragma namespace("\\\\.\\root\\cimv2")

// Registry instance provider
instance of __Win32Provider as $InstProv
{
	Name    ="RegProv" ;
	ClsID   = "{fe9af5c0-d3b6-11ce-a5b6-00aa00680c3f}" ;
	ImpersonationLevel = 1;
	PerUserInitialization = "False";
};

instance of __InstanceProviderRegistration
{
	Provider    = $InstProv;
	SupportsPut = True;
	SupportsGet = True;
	SupportsDelete = False;
	SupportsEnumeration = True;
};


// Registry property provider
instance of __Win32Provider as $PropProv
{
	Name    ="RegPropProv" ;
	ClsID   = "{72967901-68EC-11d0-B729-00AA0062CBB7}";
	ImpersonationLevel = 1;
	PerUserInitialization = "False";
};

instance of __PropertyProviderRegistration
{
	Provider     = $PropProv;
	SupportsPut  = True;
	SupportsGet  = True;
};


//==================================================================
// 2. <%= $Classname %>
//==================================================================

#pragma namespace ("\\\\.\\root\\cimv2")

// Class definition

#pragma deleteclass("<%=$Classname%>",nofail)
[DYNPROPS]
class <%= $classname %>
{
// unique identifier for the class
	[key]
	string InstanceKey;
// 3.
<% $properties | % { %>
   string <%= $_ %>;
<% } %>
};


// Instance definition

[DYNPROPS]
instance of <%= $Classname%>
{
	InstanceKey = "@";

<% $properties | % { %>
   [PropertyContext("local|<%=$keyexpanded%>|<%=$_%>"), Dynamic, Provider("RegPropProv")]
   <%= $_ %>;
<% } %>

};

You can then use the above file saved as ‘WMItemplate.txt’ in a command like:

<pre class="lang:ps decode:true">$moftext = Expand-Template -file 'WMITemplate.txt' -safe -binding @{Key='HKLM:\Software\MySoftware',@keyexpanded='HKEY_LOCAL_MACHINE\\Software\\MySoftware,@classname='MyCompany_MySoftware'}
$moftext | Set-Content 'mycompany_mysoftware.mof' -force
mofcomp -autorecover -f mycompany_mysoftware.mof

KeyExpanded is just the not abbreviated version of describing a registry key, where you do not use HKLM but HKEY_LOCAL_MACHINE and replace simple backslashes by double backslashes

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