EF Core Linqkit Predicate Builder

LinqKit.Microsoft.EntityFrameworkCore allows you to construct a lambda expression tree dynamically that performs an or-based or and-based predicate. The need for dynamic predicates is the most common in a typical business application.

Let's consider the following example that implements a keyword-style search where we want to match all of the specified keywords in a book title or, in other words, the book title contains all the given keywords.

public static void Example2()
{
    using (var context = new BookStore())
    {
        string[] keywords = { "Introduction", "Machine Learning" };

        var predicate = PredicateBuilder.New<Book>(true);

        foreach (string keyword in keywords)
        {
            string temp = keyword;
            predicate = predicate.And(b => b.Title.Contains(temp));
        }

        var query = context.Books.Where(predicate);

        foreach (var book in query.ToList())
        {
            Console.WriteLine(book.Title);
        }
    }
}

The PredicateBuilder.New() creates an object called ExpressionStarter, which acts for all intents and purposes as an Expression<Func<T, bool>> object.

Let's run the above code, and you will see the following output.

Introduction to Machine Learning

Now consider another example where we want to get all those books that contain some or all of a given set of keywords.

public static void Example3()
{
    using (var context = new BookStore())
    {
        string[] keywords = { "Introduction", "Machine Learning" };

        var predicate = PredicateBuilder.New<Book>(true);

        foreach (string keyword in keywords)
        {
            string temp = keyword;
            predicate = predicate.Or(b => b.Title.Contains(temp));
        }

        var query = context.Books.Where(predicate);

        foreach (var book in query.ToList())
        {
            Console.WriteLine(book.Title);
        }
    }
}

Let's run the above code, and you will see the following output.

Introduction to Microeconomics
Introduction to Computing
Advanced Topics in Machine Learning
Introduction to Machine Learning
Introduction to AI