Dynamic where clause (OR) in Linq to Entities

Question

In the post here I learned how to build a dynamic query using the deferred execution of Linq. But the query is actually using an AND concatenation of the WHERE condition.

How can I achieve the same query but with an OR logic?

Due to the Flags enum, the query should search for Username, WindowsUsername or both:

public User GetUser(IdentifierType type, string identifier)
{
    using (var context = contextFactory.Invoke())
    {
        var query = from u in context.Users select u;

        if (type.HasFlag(IdentifierType.Username))
            query = query.Where(u => u.Username == identifier);

        if (type.HasFlag(IdentifierType.Windows))
            query = query.Where(u => u.WindowsUsername == identifier);

        return query.FirstOrDefault();
    }
}

Answer 1

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/