I need some guidance with getting archetype values via the API (compiling a dll);

I can run the following code in Razor and it works just fine.

var inventRoot = Umbraco.Content(1100);
foreach (var inventory in inventRoot.Descendants("course")){

var entries = inventory.GetPropertyValue<Archetype.Models.ArchetypeModel>("propertyX");
..
}

Then I tried to implement it in the .CS (api usage to compile as dll) and realized
.Descendants("course") was not working even though i have included using Umbraco.Web.

It was giving error "IContent does not contain definition for 'Descendants' "

So I have tried another approach (IContent vs IPublishedContent is not very clear to me yet) to iterate all the inventory items via

ContentService.GetContentOfContentType(1081) where 1082 is the Id of the document type "course"

    using Umbraco.Core;
    using Umbraco.Web;
    using Archetype;
    using Archetype.Models;
    using Archetype.Extensions;

    var inventories = global::Umbraco.Core.ApplicationContext.Current.Services.ContentService.GetContentOfContentType(1081);

                foreach (var inventory in inventories)
                {

                    var entries = inventory.GetValue<Archetype.Models.ArchetypeModel>("propertyX");
                                  ^--here throws exception "The given key was not present in the dictionary."
               }

which seems to iterate all the "course" nodes but throws exception while reading the Archetype with GetValue

Can someone shed light on how to read archetype property/values for all descendants of a node via the api properly?