LINQ: When to use SingleOrDefault vs. FirstOrDefault() with filtering criteria

Question

Consider the IEnumerable extension methods SingleOrDefault() and FirstOrDefault()

MSDN documents that SingleOrDefault:

Answer 1

I don't understand why you're using FirstOrDefault(x=> x.ID == key) when this could retrieve results much faster if you use Find(key). If you are querying with the Primary key of the table, the rule of thumb is to always use Find(key). FirstOrDefault should be used for predicate stuff like (x=> x.Username == username) etc.

this did not deserve a downvote as the heading of the question was not specific to linq on DB or Linq to List/IEnumerable etc.