Dynamic where clause (OR) in Linq to Entities

With LINQKit's PredicateBuilder you can build predicates dynamically.

var query = from u in context.Users select u;
var pred = Predicate.False<User>();

if (type.HasFlag(IdentifierType.Username))
    pred = pred.Or(u => u.Username == identifier);

if (type.HasFlag(IdentifierType.Windows))
    pred = pred.Or((u => u.WindowsUsername == identifier);

return query.Where(pred.Expand()).FirstOrDefault();
// or return query.AsExpandable().Where(pred).FirstOrDefault();

This is what the Expand is for:

Entity Framework's query processing pipeline cannot handle invocation expressions, which is why you need to call AsExpandable on the first object in the query. By calling AsExpandable, you activate LINQKit's expression visitor class which substitutes invocation expressions with simpler constructs that Entity Framework can understand.

Or: without it an expression is Invoked, which causes an exception in EF:

The LINQ expression node type 'Invoke' is not supported in LINQ to Entities.

Later addition:

There is an alternative predicate builder that does the same but without Expand: http://petemontgomery.wordpress.com/2011/02/10/a-universal-predicatebuilder/