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Using LINQ in C#

March 30, 2017
LINQ or Language
Integrated Query gives the capability to query objects and C# allows two distinctive ways to achieve this – You can either use LINQ query operators model or LINQ extension methods model with Lamba expressions on Func<> delegate.
We have – 

LINQ to Objects – Objects in memory eg:
Collections
·      
LINQ to Entities – eg: Databases
·      
LINQ to XML – eg: XML
·      
LINQ to Data sets – ADO.NET data sets

Example

We have a Book class and a Book repository that will hold a
list of Books. In the main program we will use LINQ to so some operations on
the Book list.
Book
public class Book
    {
        public string Title { get; set; }
        public float Price { get; set; }
    }
BookRepository –
returns a list of predefined Books
public class BookRepository
    {
        public IEnumerable<Book> GetBooks()
        {
            return new List<Book>
            {
                new Book() { Title = "ABC", Price = 20.50f },
                new Book() { Title = "DE", Price = 30},
                new Book() { Title = "RE", Price = 20},
                new Book() { Title = "YE", Price = 50},
                new Book() { Title = "WEE", Price = 34},
                new Book() { Title = "SDE", Price = 40}
            };
        }
    }
Main program
class Program
    {
        static void Main(string[] args)
        {
            var books = new BookRepository().GetBooks();
 
            //LINQ Query Operators
 
            var cheapBooks =
                from b in books
                where b.Price > 30
                orderby b.Title
                select b.Title;
 
            //LINQ Extension methods
            var cheaperBooks = books
                                .Where(b => b.Price > 30)
                                .OrderBy(b => b.Title)
                                .Select(b => b.Price);
            //returns one or 0 items.Single returns 1 and 
only1 item
            var singleBook = books.
                   SingleOrDefault(b => b.Title 
== "WEE");
 
            //returns first item from the match
            var firstItem = books.
                        FirstOrDefault(b => b.Price > 30);
 
            //returns last item from the match
            var lastItem = books.
                      LastOrDefault(b => b.Price > 30);
 
            //return skip some and next few
            var skipItems = books.
                                Skip(2).Take(3);
 
            //Count items
            var cnt = books.Count();
 
            //Max, Min, Sum
 
            var max = books.
                            Skip(2).Take(4).Max(b => b.Price);
 
           //var item = books.Aggregate(b => b.Price);
 
            foreach (var i in cheapBooks)
            {
                Console.WriteLine(i);
                //Console.WriteLine(i.Title);
            }
 
            foreach (var i in cheaperBooks)
            {
                Console.WriteLine(i);
                //Console.WriteLine(i.Title);
            }
 
            Console.WriteLine(singleBook.Title);
            Console.WriteLine(firstItem.Title);
            Console.WriteLine(lastItem.Title);
 
            foreach (var i in skipItems)
            {
                Console.WriteLine(i.Title);
                //Console.WriteLine(i.Title);
            }
 
            Console.WriteLine(cnt);
            Console.WriteLine(max);
        }

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