The easiest way to create a MongoX509Credential involves passing a null parameter to the username, to avoid having to extract the username from the X509 certificate, a somewhat error prone process.
 A sample username is: `CN=myName,OU=myOrgUnit,O=myOrg,L=myLocality,ST=myState,C=myCountry" (See the username format here: https://docs.mongodb.com/manual/tutorial/configure-x509-client-authentication/#add-x-509-certificate-subject-as-a-user).

See the suggested method here, which involves passing in a null: https://mongodb.github.io/mongo-csharp-driver/2.11/reference/driver/authentication/

It'd be nice to have a method that simply doesn't require a username parameter.