Question

How can I check with C# if the IIS6 compatibility role is enabled/disabled on IIS7 ?

Answer 1

you can check read the value in the registry

HKEY_LOCAL_MACHINE\Software\Microsoft\InetStp\Components\WMICompatibility

or, you can ouput the content of servermanagercmd to an xml file and parse that file looking for the iis6 compatibility component

ServerManagerCmd -query [SaveFile.xml]

If your doing this on R2, servermanagercmd is now deprecated so you might want to use powershell to achieve the same check.
Here are some powershell examples, in this case done remotely http://www.techmumbojumblog.com/?p=217

The WMI approach from the previous answer is probably good as well, espcialy if you have more configuration tasks to perform on the IIS, after validating that the compatibility tool is install.

btw, if you do find configuration settings that are not handled by the compatibility component, here is what I found doing it from C#, what I was configuring through wmi back in iis6 worked fine at the website level and under(website, virtual dir and pools), but to configure the webserver level, I had to use the the api that’s installed with iis7, Microsoft.Web.Administration.dll from System32\inetsrv.

using Microsoft.Web.Administration;