Dynamic linq Building Expression

Question

I need to create a dynamic linq expression for a dynamic search.The basic search is working but it fails to work with collection. I am able to get the book\'s title and auth

Answer 1

Based on your description I'm not sure that you need Expression. Creating an Expression with a complex object model is quite difficult. Do you really need to create dynamic expression or you simply need to create a dynamic query? If the object model is fixed then you don't need Expression.

I suggest first of all to clean your object model:

• Rename the Books class to Book (this class represents a Book not a list of books)
• Rename property Page to Pages (this property returns a list of pages)

Now you can write a dynamic where using just LINQ and one or more helper functions one for each property that you need to search. For example to search for Heading you can write:

        private static bool SearchByHeading(Book b, string heading)
        {
            if (string.IsNullOrEmpty(heading))
                return true;
            else
                return b.Pages.Any(p => p.Heading == heading);
        }

Here you can also see why your previous code didn't work. The expression to search for a given Heading is book.Pages.Any(p => p.Heading == x) and not book.Pages.Heading == x.

In any case given one or more functions like this you can rewrite your code with something like:

using System.Collections.Generic;
using System.Linq;

namespace XMLStorageAndFilter
{
    public class Book
    {
        public Book()
        {
            Pages = new List();
        }
        public string Title { get; set; }
        public Author Author { get; set; }
        public List Pages { get; set; }
    }
    public class Author
    {
        public string FirstName { get; set; }
    }
    public class Page
    {
        public string Heading { get; set; }
    }

    public class Program2
    {
        static void Main()
        {
            Page page = new Page();
            page.Heading = "Heading1";
            Book bok = new Book();
            bok.Title = "Title1";
            bok.Author = new Author() { FirstName = "FirstName1" };
            bok.Pages.Add(page);
            List testList = new List();
            testList.Add(bok);

            var searchResult = Search(testList, 
                title: "Title1",
                author: "FirstName1",
                heading: "Heading1");
        }

        private static IEnumerable Search(IEnumerable books, string author = null, string title = null, string heading = null)
        {
            return books
                .Where((b) => SearchByAuthor(b, author))
                .Where((b) => SearchByHeading(b, heading))
                .Where((b) => SearchByTitle(b, title))
                .ToList();
        }

        private static bool SearchByAuthor(Book b, string author)
        {
            if (string.IsNullOrEmpty(author))
                return true;
            else
                return b.Author.FirstName == author;
        }

        private static bool SearchByTitle(Book b, string title)
        {
            if (string.IsNullOrEmpty(title))
                return true;
            else
                return b.Title == title;
        }

        private static bool SearchByHeading(Book b, string heading)
        {
            if (string.IsNullOrEmpty(heading))
                return true;
            else
                return b.Pages.Any(p => p.Heading == heading);
        }
    }
}

I have skipped search values when null or empty, just an example. This code has also the advantage that is verified at compile time.