Introduction to Linq

Introduction to Linq

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  The LINQ Project C# VB Others… .NET Language Integrated Query Standard DLinq XLinq Query (ADO.NET) (System.Xml) Operators <book> <title/> <author/> <year/> <price/> </book> Objects SQL WinFS XML
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  Language Integrated Query 2
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  How does itwork? Standard query operators Query expressions Extension methods Lambda expressions Expression trees Local variable type inference 3
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  C# 3.0 LanguageInnovations Query var contacts = expressions from c in customers where c.State == "WA" Local variable select new { c.Name, c.Phone }; type inference Lambda expressions var contacts = customers .Where(c => c.State == "WA") .Select(c => new { c.Name, c.Phone }); Extension methods Anonymous Object types initializers 4
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  Lambda Expressions and ExtensionMethods 5
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  Object Initializers Embedded public class Rectangle objects { private Point p1 = new Point(); private Point p2 = new Point(); Read-only properties public Point P1 { get { return p1; } } public Point P2 { get { return p2; } } } Rectangle r = new Rectangle { P1 = { X = 0, Y = 1 }, P2 = { X = 2, Y = 3 } No “new Point” }; Rectangle r = new Rectangle(); r.P1.X = 0; r.P1.Y = 1; r.P2.X = 2; r.P2.Y = 3; 6
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  Local Variable TypeInference int i = 5; string s = "Hello"; double d = 1.0; int[] numbers = new int[] {1, 2, 3}; Dictionary<int,Order> orders = new Dictionary<int,Order>(); var i = 5; var s = "Hello"; var d = 1.0; var numbers = new int[] {1, 2, 3}; var orders = new Dictionary<int,Order>(); “var” means same type as initializer 7
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  Anonymous Types publicclass Customer { public string Name; public Address Address; public class Contact public string Phone; { class ??? public List<Order> Orders; public{string Name; … public string Phone; Name; public string } } public string Phone; Customer c = GetCustomer(…); } Contact x = new Contact { Name = c.Name, Phone = c.Phone }; Customer c = GetCustomer(…); var x = new { Name = c.Name, Phone = c.Phone }; Projection style Customer c = GetCustomer(…); initializer var x = new { c.Name, c.Phone }; 8
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  Query Expressions Language integrated query syntax Starts with from Zero or more from or where from id in source { from id in source | where condition } Optional [ orderby ordering, ordering, … ] orderby select expr | group expr by key [ into id query ] Ends with select Optional into or group by continuation 9
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  Query Expressions Queriestranslate to method invocations Where, Select, SelectMany, OrderBy, GroupBy from c in customers where c.State == "WA" select new { c.Name, c.Phone }; customers .Where(c => c.State == "WA") .Select(c => new { c.Name, c.Phone }); 10
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  C# 3.0 LanguageInnovations c => c.Name Lambda expressions Extension methods static void Dump(this object o); Local variable type inference var x = 5; Object initializers new Point { x = 1, y = 2 } Anonymous types new { c.Name, c.Phone } Query expressions from … where … Expression trees select Expression<T> 11
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  Standard Query Operators Restriction Where Projection Select, SelectMany Ordering OrderBy, ThenBy Grouping GroupBy Quantifiers Any, All Partitioning Take, Skip, TakeWhile, SkipWhile Sets Distinct, Union, Intersect, Except Elements First, FirstOrDefault, ElementAt Aggregation Count, Sum, Min, Max, Average Conversion ToArray, ToList, ToDictionary Casting OfType<T> 12
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  Deferred Query Execution Customer[] custs = SampleData.GetCustomers(); var query = from c in custs where c.City == "London" select c.Name; var query = custs.Where(c => c.City == "London").Select(c => c.Name); string[] names = query.ToArray(); custs names ID Name Phone Where Select c => c.City == "London" c => c.Name 13
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  DLinq For RelationalData Accessing data today Queries in SqlConnection c = new SqlConnection(…); quotes c.Open(); SqlCommand cmd = new SqlCommand( @"SELECT c.Name, c.Phone Loosely bound FROM Customers c arguments WHERE c.City = @p0"); cmd.Parameters.AddWithValue("@p0", "London“); DataReader dr = c.Execute(cmd); while (dr.Read()) { Loosely typed string name = dr.GetString(0); result sets string phone = dr.GetString(1); DateTime date = dr.GetDateTime(2); } dr.Close(); No compile time checks 14
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  DLinq For RelationalData Accessing data with DLinq Classes public class Customer { … } describe data public class Northwind: DataContext { Tables are like public Table<Customer> Customers; collections … } Strongly typed connection Northwind db = new Northwind(…); var contacts = from c in db.Customers Integrated where c.City == "London" query syntax select new { c.Name, c.Phone }; Strongly typed results 15
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  DLinq For RelationalData Language integrated data access Maps tables and rows to classes and objects Builds on ADO.NET and .NET Transactions Mapping Encoded in attributes Relationships map to properties Persistence Automatic change tracking Updates through SQL or stored procedures 16
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  XLinq For XMLData Programming XML today Imperative model XmlDocument doc = new XmlDocument(); XmlElement contacts = doc.CreateElement("contacts"); Document foreach (Customer c in customers) centric if (c.Country == "USA") { XmlElement e = doc.CreateElement("contact"); XmlElement name = doc.CreateElement("name"); No integrated name.InnerText = c.CompanyName; queries e.AppendChild(name); XmlElement phone = doc.CreateElement("phone"); phone.InnerText = c.Phone; Memory e.AppendChild(phone); <contacts> intensive contacts.AppendChild(e); <contact> <name>Great Lakes Food</name> } <phone>(503) 555-7123</phone> doc.AppendChild(contacts); </contact> … </contacts> 17
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  XLinq For XMLData Programming XML with XLinq Declarative model XElement contacts = new XElement("contacts", from c in customers where c.Country == "USA" Element select new XElement("contact", centric new XElement("name", c.CompanyName), new XElement("phone", c.Phone) Integrated ) queries ); Smaller and faster 18
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  XLinq For XMLData Language integrated query for XML Expressive power of XPath / XQuery But with C# or VB as programming language Leverages experience with DOM Element centric, not document centric Functional construction Text nodes are just strings Simplified XML namespace support Faster and smaller 19
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  The LINQ Project Language Integrated Query for .NET Native query syntax in C# 3.0 and VB 9.0 Standard Query Operators SQL-like queries for any .NET collection DLinq Query enabled data access framework XLinq Query enabled, smaller, faster XML DOM 20
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  Benefits Of LINQ Unified querying of objects, relational, XML Type checking and IntelliSense for queries SQL and XQuery-like power in C# and VB Extensibility model for languages / APIs 21