Because Signer ID was not exposed in current .Net SDK implementation, this code share was using REST method querying,parsing and filtering package JSON by signer's Email and return you the corresponding signer ID.
Signer ID are required in multiple API/SDK functions. Even though most of the cases, signer ID can be replaced by signer Email, though still some time you are required to use signer ID.
(If you have set .CustomID() for your signer, signer ID are align with custom ID/role ID/role Name, so you probably don't need to worry about this problem)