c# linq graph database based path search

Situation example (oriented cyclic graph with some edge restrictions):

4 different entity Types (A, B, C, D), 7 Relations (directed graph edges: A <-> B, A -> D, B -> D, B <-> C, C -> D). Graph dynamically changes. The only graph function is to list all objects of one type, where path exists from selected vertices.

I'm looking for design pattern, which can be used in realtime queries on (MS) SQL (best using c# linq). I know some base algorithms for path search in graphs, but I can't imagine performance on database. Basically it's used for security purposes. I read about some interesting security access patterns (lattice-based access control), but I'm not sure about solution and mainly about performance (1kk object of each of 15 entity types per user).

Any literature, thesis, ideas, blogs are welcomed.