This trend metric is written in CQLinq, C# LINQ over the NDepend code model.
It ships with every new NDepend project: you can read it, adapt it, or use it as a starting point for your own custom rules.
// <TrendMetric Name="# Third-Party Types Used" Unit="Types" />
from t in ThirdParty.Types
select new { t, t.TypesUsingMe }