LINQ extension methods - Any() vs. Where() vs. Exists()

Question

Unfortunately the names of these methods make terrible search terms, and I've been unable to find a good resource that explains the difference between these methods--as in when to use each.

Thanks.

Edit:

The sort of query that I'm trying to fully understand is something like this:

context.Authors.Where(a => a.Books.Any(b => b.BookID == bookID)).ToList(); 

And thanks to all who've answered.

Answer 1

Where returns a new sequence of items matching the predicate.

Any returns a Boolean value; there's a version with a predicate (in which case it returns whether or not any items match) and a version without (in which case it returns whether the query-so-far contains any items).

I'm not sure about Exists - it's not a LINQ standard query operator. If there's a version for the Entity Framework, perhaps it checks for existence based on a key - a sort of specialized form of Any? (There's an Exists method in List<T> which is similar to Any(predicate) but that predates LINQ.)

Answer 2

context.Authors.Where(a => a.Books.Any(b => b.BookID == bookID)).ToList();

a.Books is the list of books by that author. The property is automatically created by Linq-to-Sql, provided you have a foreign-key relationship set up.

So, a.Books.Any(b => b.BookID == bookID) translates to "Do any of the books by this author have an ID of bookID", which makes the complete expression "Who are the authors of the book with id bookID?"

That could also be written something like

  from a in context.Authors   join b in context.Books on a.AuthorId equal b.AuthorID   where b.BookID == bookID   select a; 

UPDATE: Any() as far as I know, only returns a bool. Its effective implementation is:

 public Any(this IEnumerable<T> coll, Func<T, bool> predicate)  {      foreach(T t in coll)      {          if (predicte(t))             return true;      }      return false;  }