How to Build a Dynamic WHERE Clause with OR in LINQ to Entities?

Dynamic WHERE clause (OR) in LINQ to Entities

Creating dynamic queries using the OR operator in LINQ requires PredicateBuilder, a LINQKit utility. Here’s how to do it:

<code class="language-csharp">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();</code>

However, using AsExpandable() is another option:

<code class="language-csharp">return query.AsExpandable().Where(pred).FirstOrDefault();</code>

The

Expand method allows handling of calling expressions that EF cannot handle. Without it, an exception will be thrown.

Another Predicate Builder option that achieves the same functionality without Expand can be found at http://petemontgomery.wordpress.com/2011/02/10/a-universal-predicatebuilder/.

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