LINQ

• LINQ stands for Language Integrated Query. LINQ gives to capability to query objects.
• We can query, 
• Objects in memory eg :Collections)(LINQ to Objects), 
• Databases (LINQ to SQL), 
• XML(LINQ to XML) 
• ADO.NET Data Sets (LINQ to Data Sets). 

 class Book
    {
        public string Name { get; set; }
        public decimal Price { get; set; }
    }

class BookRepository
    {

        public IEnumerable GetBooks()
        {
            var boooks = new List()
            {
                new Book() { Name="Fundamentals of C#" , Price = 20 },
                new Book() { Name="ASP.NET MVC" , Price = 30},
                new Book() { Name="Entity Framework" , Price = 12 },
                new Book() { Name ="Java" , Price = 10 },
                new Book() { Name = "Web API" , Price = 15 },
                new Book() { Name="Java Script Basics" , Price= 25 }
            };

            return boooks;
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            var books = new BookRepository().GetBooks();
            var cheapbooks = new List();
            
            if(books != null)
            {
                cheapbooks = books.Where(book=>book.Price >12).ToList();
            }

            foreach (var book in cheapbooks)
            {
                Console.WriteLine(book.Name);
            }

            Console.Read();
        }
    }

Some of the more frequently used standard query methods and keywords

Method/Keywords	Syntax	Description
Where	Where(Predicate)	Filters a sequence of values based on a predicate.
OrderBy	OrderBy(Predicate)	Sorts the elements of a sequence in ascending order.
ThenBy	ThenBy(Predicate)	Performs a subsequent ordering of the elements in a sequence in ascending order.
Select	Select(Predicate)	Projects each element of a sequence into a new form.
Take	Take(int)	Returns a specified number of contiguous elements from the start of a collection/sequence.
Skip	Skip(int)	Bypasses a specified number of elements in a sequence and then returns the remaining elements.
FirstOrDefault	FirstOrDefault(Predicate)	Returns the first element of a sequence, or a default value if no element is found.
First	First(Predicate)	Returns the first element of a sequence.
Single	Single(Predicate)	Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.