Mongo C# driver – Building filter dynamically with nesting

Builders is really flexible class, it also has overridden operators “& = AND” and “| = OR”

Your example will be

var filter = Builders<User>.Filter.Eq(x => x.A, "1");
filter &= (Builders<User>.Filter.Eq(x => x.B, "4") | Builders<User>.Filter.Eq(x => x.B, "5"));