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Understanding linq



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  • 2. INDEX Introduction to LINQ Why LINQ? Language features supporting the LINQ project Getting started with standard query operators LINQ to Objects Beyond basic in-memory queries Getting started with LINQ to SQL Peeking under the covers of LINQ to SQL Introducing LINQ to XML
  • 3. INTRODUCTION TO LINQ Linq is short for Language Integrated Query. We use the term language-integrated query to indicate that query is an integrated feature of the developers primary programming languages (for example, Visual C#, Visual Basic). Component of .NET Framework 3.5 It is a set of methods that are defined by the Enumerable and Queryable classes.
  • 4. LANGUAGE INTEGRATED QUERY (LINQ) VB C# Others… .NET Language-Integrated Query LINQ enabled data sources LINQ enabled ADO.NET LINQ LINQ LINQ LINQ LINQ To Objects To DataSets To SQL To Entities To XML <book> <title/> <author/> <price/> </book> Objects Relational XML
  • 5. QUERY WITHOUT LINQ Objects using loops and conditions foreach(Customer c in customers) if (c.Region == "UK") ... Databases using SQL SELECT * FROM Customers WHERE Region=UK‘ XML using XPath/XQuery //Customers/Customer[@Region=UK]
  • 6. ADO WITHOUT LINQSqlConnection c = new SqlConnection(…);c.Open();SqlCommand cmd = new SqlCommand( @”SELECT c.Name, c.Phone Query in FROM Customers c quotes WHERE c.City = @p0”); Arguments looselycmd.Parameters[“@po”] = “London”; boundDataReader dr = c.Execute(cmd);While(dr.Read()){ Results loosely string name = r.GetString(0); bound string phone = r.GetString(1); Datetime date = r.GetDateTime(2);} Compiler cannot help catchr.Close(); mistakes
  • 7. LIMITATIONSo Several steps and verbose code are required.o Database queries bypass all kinds of compile-time checks.o The SQL we write for a given DBMS is likely to fail on a different one.
  • 8. ADVANTAGES OF USING LINQ Works with any data source. Compile-time syntax checking, and debugging support. IntelliSense, Design time support. Same basic syntax for different types of data sources. Providing designer tools that create object-relational mappings.
  • 9. THE SYNTAX Starts with from Zero or more from, join, let, where, or from id in source orderby{ from id in source | join id in source on expr equals expr [ into id ] | let id = expr | Ends with where condition | select or group by orderby ordering, ordering, … } select expr | group expr by key[ into id query ] Optional into continuation
  • 11. LANGUAGE FEATURES SUPPORTING THE LINQ PROJECT Lambda expressions Extension methods Initialization of objects and collections in expression context Local types inference Anonymous types Lazy evaluation +Query Expressions = LINQ 
  • 12. LAMBDA EXPRESSIONS Express the implementation of a method and the instantiation of a delegate from that method; they have the form: c => c + 1 which means a function with an argument, that returns the value of the argument incremented by one The parameters of a lambda expression can be explicitly or implicitly typed.
  • 13. Examples of lambda expressions x => x + 1 // Implicitly typed, expression body x => { return x + 1; } // Implicitly typed, statement body (int x) => x + 1 // Explicitly typed, expression body (int x) => { return x + 1; } // Explicitly typed, statement body (x, y) => x * y // Multiple parameters () => Console.WriteLine() // No parameters A lambda expression is a value, that does not have a type but can be implicitly converted to a compatible delegate type delegate R Func<A,R>(A arg); Func<int,int> f1 = x => x + 1; // Ok Func<int,double> f2 = x => x + 1; // Ok Func<double,int> f3 = x => x + 1; // Error – double cannot be //implicitly converted to int
  • 14. EXTENSION METHODS Extend classes that you could not modify. They are defined as static methods of other classes that take at least one parameter, and the first parameter has the type of the class it extends, but preceded by the keyword this
  • 15. EXTENSION METHODS (C#) Extension method namespace MyStuff { public static class Extensions { public static string Concatenate(this IEnumerable<string> strings, string separator) {…} } } Brings extensions into scope using MyStuff; string[] names = new string[] { "Jenny", "Daniel", "Rita" }; obj.Foo(x, y) string s = names.Concatenate(", ");  XXX.Foo(obj, x, y) IntelliSense!
  • 16. Example of Extension Method: public static int VowelCount(this String source) { int count = 0; List<char> vowels = new List<char>() { a, e, i, o, u, A‘,E, I, O, U }; foreach (char c in source) { if (vowels.Contains(c)) count++; } return count; } string name = “extension”; int count = name.VowelCount();
  • 17. INITIALIZATION OF OBJECTS AND COLLECTIONS IN EXPRESSION CONTEXTObject initializers let you assign values to any accessible fields orproperties of an object at creation time without having to explicitlyinvoke a constructor.Example:private class Cat{ // Auto-implemented properties. public int Age { get; set; } public string Name { get; set; }}Cat cat = new Cat { Age = 10, Name = "Fluffy" };
  • 18. Collection Initializers Collection initializers let you specify one or more element intializers when you initialize a collection class that implements IEnumerable. Two simple collection intializers List<int> digits = new List<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; List<int> digits2 = new List<int> { 0 + 1, 12 % 3, MakeInt() }; The following collection initializer uses object initializers to initialize objects of the Cat class List<Cat> cats = new List<Cat> { new Cat(){ Name = "Sylvester", Age=8 }, new Cat(){ Name = "Whiskers", Age=2 }, new Cat(){ Name = "Sasha", Age=14 } };
  • 19. Anonymous Types, Implicitly Typed variables An anonymous type has no name and is generated by the compiler based on the initialization of the object being instantiated. var results = from employee in employees join contact in contacts on employee.Id equals contact.Id select new { employee.FirstName, employee.LastName, contact.City }; the part at “select new”, it’s not using any class name there to initialize, but it’s there and it’s anonymous That is the purpose of the var keyword. It infers the type of the object based on the data type with which it has been intialized
  • 20. LOCAL VARIABLE TYPE INFERENCE (C#) int i = 666; string s = "Goodbye"; double d = 3.14; int[] numbers = new int[] {1, 2, 3}; Dictionary<int,Order> orders = new Dictionary<int,Order>(); var i = 666; var s = "Goodbye"; var d = 3.14; var numbers = new int[] {1, 2, 3}; var orders = new Dictionary<int,Order>(); “The type on the right hand side”
  • 21. • Variable type inferred from initialiser Integer String var x = 666 var s = “Bye" Double var d = 3.14 var numbers = new Integer() {1, 2, 3} var orders = new Dictionary(Of Integer, Order)()
  • 22. LAZY EVALUATION LINQ follows a lazy evaluation model Queries execute not when constructed, but when enumerated. This means you can build up a query in as many steps as you like, and it wont actually hit the server until you eventually start consuming the results. var query = db.Customers.Where (c => c.Name.StartsWith ("A")); query = query.Where (c => c.Purchases.Count() >= 2); var result = query.Select (c => c.Name); foreach (string name in result) //Only now is the query executed! Console.WriteLine (name);
  • 23. C# 3.5 LANGUAGE INNOVATIONS var contacts = Query expressions from c in customers where c.City == "Hove"Local variable select new { c.Name, c.Phone };type inference Lambda expressions var contacts = customers .Where(c => c.City == "Hove") .Select(c => new { c.Name, c.Phone });Extension methods Anonymous Object types initializers
  • 25. GETTING STARTED WITHSTANDARD QUERY OPERATORS LINQ Query operators are an extension to the .NET Framework Class Library. These are a set of extension methods to perform operations in the context of LINQ queries. These operators are at the heart of the LINQ foundation and they are the real elements that make LINQ possible.
  • 26. Some of the clauses for working with LINQ
  • 28. LINQ EXAMPLE - QUERYING AN ARRAY//Create an array of integers int[] myarray = new int[] { 49, 28, 20, 15, 25, 23, 24, 10, 7, 34 };//Create a query for odd numbersvar oddNumbers = from i in myarray where i % 2 == 1 select i;//Compose the original query to create a query for odd numbersvar sorted = from i in oddNumbers orderby i descending select i;//Display the results of the queryforeach (int i in oddNumbers) Console.WriteLine(i);
  • 29. Query translates to method invocation.Where, Join, OrderBy, Select, GroupBy, … from c in customers where c.City == "Hove" select new { c.Name, c.Phone }; customers .Where(c => c.City == "Hove") .Select(c => new { c.Name, c.Phone });
  • 31. LINQ TO OBJECTS Native query syntax in using System; using System.Query; C# and VB using System.Collections.Generic;  IntelliSense class app { static void Main() {  Autocompletion string[] names = = { "Allen", "Arthur", string[] names { "Burke", "Connor", "Frank", "Everett", "Bennett" }; Query Operators can "Albert", "George", be used against any IEnumerable<string> ayes };names "Harris", "David" = Func<string, bool>s[0] == A); s.Length == 5; .Where(s => filter = s => .NET collection Func<string, string> extract = s => s; IEnumerable<string> expr = (IEnumerable<T>) Func<string, string> s in ayes)s = s.ToUpper(); foreach (string item from project = names Console.WriteLine(item); == 5 where s.Length  Select, Where, IEnumerable<string> exprs= names orderby names[0] = "Bob"; .Where(filter) select s.ToUpper(); GroupBy, Join, etc. .OrderBy(extract) foreach (string item inin ayes) foreach (string item expr) .Select(project); Deferred Query Console.WriteLine(item); Console.WriteLine(item); Evaluation } foreach (string item in expr) } Console.WriteLine(item); Lambda Expressions } } Allen Arthur Arthur BURKE DAVID FRANK
  • 33. Querying non-generic collectionTrying to query an ArrayList using LINQ to Objects directly fails ArrayList books = GetArrayList(); var query = from book in books where book.PageCount > 150 select new { book.Title, book.Publisher.Name }; Nongeneric collections aren’t a big problem with LINQ once you know the trick. The trick is to use the Cast operator Querying an ArrayList is possible thanks to the Cast query operator ArrayList books = GetArrayList(); var query = from book in books.Cast<Book>() where book.PageCount > 150 select new { book.Title, book.Publisher.Name }; dataGridView.DataSource = query.ToList();
  • 34. Grouping by multiple criteria var query1 = from book in SampleData.Books group book by book.Publisher, book.Subject; var query2 = from book in SampleData.Books group book by book.Publisher group book by book.Subject; The trick is to use an anonymous type to specify the members on which to perform the grouping. var query = from book in SampleData.Books group book by new { book.Publisher, book.Subject };
  • 35. LINQ ARCHITECTUREvar query = from c in customers where c.City == "Hove" select c.Name;var query = customers.Where(c => c.City == "Hove").Select(c => c.Name);Source implements Source implementsIEnumerable<T> IQueryable<T> System.Linq.Enumerable System.Linq.Queryable Delegate based Expression tree based Objects SQL DataSets Others…
  • 37. WHAT IS LINQ TO SQL? LINQ to SQL is an O/RM (object relational mapping) implementation that ships in the .NET Framework. Which allows you to model a relational database using .NET classes. You can then query the database using LINQ, as well as update/ insert/ delete data from it. LINQ to SQL fully supports transactions, views, and stored procedures. It also provides an easy way to integrate data validation and business logic rules into your data model.
  • 38. THE DATACONTEXT, WHAT IS IT? DataContext will be created for each LinqToSQL-File we add to our solution. The DataContext is the main object through which we will communicate with our database. The properties of the DataContext class are the tables and stored procedures in the database we modelled/created.
  • 39. DLINQ RUNTIME SUPPORTfrom c in db.Customers db.Customers.Add(c1);where c.City == "London"select c.CompanyName Application c2.City = “Seattle"; db.Customers.Remove(c3); Enumerate Objects SubmitChanges() LINQ to SQL SQL Query Rows DML or SProc or SProcs SELECT CompanyName INSERT INTO Customer … FROM Customer UPDATE Customer … WHERE City = London DELETE FROM Customer …
  • 43. LINQ TO XML Large Improvement Over Existing Model Supports:  Creating XML  Loading & querying XML  Modifying & saving XML  Streaming, Schema, Annotations, Events
  • 44. LINQ TO XML New XML API implemented in v3.5 assembly  System.Xml.Linq.dll Namespaces  System.Xml.Linq  System.Xml.Schema  System.Xml.Xpath Can be used independently of LINQ
  • 45. KEY CLASSES IN SYSTEM.XML.LINQ System.Xml.Linq is a “DOM like” API  Manipulates an XML tree in memory Naturally work with both XML documents and fragments The two key classes in System.Xml.Linq
  • 46. LOADING XML CONTENT Loading Xml is performed with;  XElement.Load  XDocument.Load Both support loading from  URI, XmlReader, TextReader
  • 47. MODIFYING XML XML tree exposed by XElement is modifiable Modifications through methods such as:  XElement.Add()  XElement.Remove()  XElement.ReplaceWith() Modified tree can be persisted via  XElement.Save(), XDocument.Save()  Both supporting filename, TextWriter, XmlWriter.
  • 48. CREATING AN XML DOCUMENTXNamespace ns = ""; XDocument books = new XDocument( new XElement(ns + "bookstore", new XElement(ns + "book", new XAttribute("ISBN", isbn), new XElement(ns + "title", "ASP.NET Book"), new XElement(ns + "author", new XElement(ns + "first-name", a.FirstName), new XElement(ns + "last-name", a.LastName) ) ) ));books.Save(@"C:Books.xml");
  • 49. …MEANWHILE IN C# No XML Literals, But There’s Something to Close the Gap “Paste XML as XElement” Add-in  Add XML to Clipboard  Edit -> Past XML as XElement Included in VS2008 Samples  Help -> Samples