How Can I Efficiently Implement OR Logic in a Dynamic LINQ WHERE Clause?

Use LINQKit to enhance the OR logic of dynamic WHERE clause

The previous article introduced a method to build dynamic queries using LINQ delayed execution. However, this query uses an AND condition to join the WHERE clause. This article explores how to adapt this method to use OR logic instead.

The specific goal is to create a query that searches for users based on their username, Windows username, or both (determined by the provided IdentifierType enumeration). The modified GetUser method is as follows:

<code>public User GetUser(IdentifierType type, string identifier)
{
    using (var context = contextFactory.Invoke())
    {
        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();
        // 或者 return query.AsExpandable().Where(pred).FirstOrDefault();
    }
}</code>

The key to implementing OR logic lies in the PredicateBuilder of the LINQKit library. It allows dynamic predicate construction and can be used in conjunction with Entity Framework. Predicates start with Predicate.False and are dynamically modified based on the IdentifierType flag.

The Expand() method is crucial because Entity Framework does not support calling expressions. AsExpandable() activates LINQKit's expression accessor and converts the expression into a form that Entity Framework can handle. Without it, an "Invoke" exception will occur.

Another approach is to use Pete Montgomery's predicate builder, which performs the same functionality without the need for Expand().

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