Ralf Laemmel - Not quite a sales pitch for C# 3.0 and .NET's LINQ - 2008-03-05

Not quite a sales pitch for C# 3.0 and .NET's LINQ

Ralf Lämmel

Ex-Microsofty (2005-2007, XML/Data Programmability team)

Software Languages Team

CS Department, University of Koblenz-Landau

Germany

 :=

Why LINQ? Because: Data != Objects Aka the Bermuda Triangle of data processing Objects XML Relations

What’s LINQ?

http://msdn.microsoft.com/netframework/future/linq/

“ The LINQ Project is a codename for a set of extensions to the .NET Framework that encompass language-integrated query, set, and transform operations. It extends C# and Visual Basic with native language syntax for queries and provides class libraries to take advantage of these capabilities.”

Database access in pre-LINQ era SqlConnection c = new SqlConnection(…); c.Open(); SqlCommand cmd = new SqlCommand( @“SELECT c.Name, c.Phone FROM Customers c WHERE c.City = @p0” ); cmd.Parameters[“@po”] = “London”; DataReader dr = c.Execute(cmd); while (dr.Read()) { string name = r.GetString(0); string phone = r.GetString(1); DateTime date = r.GetDateTime(2); } r.Close(); Queries in quotes Arguments loosely bound Results loosely typed Compiler cannot help catch mistakes

Database access with LINQ to SQL public class Customer { public int Id; public string Name; public string Phone; … } Table<Customer> customers = db.Customers; var contacts = from c in customers where c.City == "London" select new { c.Name, c.Phone }; Classes describe data Tables are real objects Query is natural part of the language Results are strongly typed

XML access in pre-LINQ era public static double XmlOrder2Total(XmlDocument doc) { static string ons = "http://www.vertical.com/Order"; double total = 0.0; XmlElement o = (XmlElement)doc.GetElementsByTagName("Order", ons).Item(0); foreach (XmlElement x in o.GetElementsByTagName("Item", ons)) { XmlNode priceNode = x.GetElementsByTagName("Price", ons).Item(0); XmlNode quantityNode = x.GetElementsByTagName("Quantity", ons).Item(0); double priceValue = Double.Parse(priceNode.InnerText); int quantityValue = Int32.Parse(quantityNode.InnerText); total += priceValue * quantityValue; } return total; } Imperative aggregation Low-level namespace model Verbose query API Tedious conversions

XML access with LINQ to XML public static double XmlOrder2Total(XElement o) { static XNamespace ons = "http://www.vertical.com/Order"; return (from item in o.Elements(ons + "Item") select (double)item.Element(ons + "Price") * (int)item.Element(ons + "Quantity") ).Sum(); } Functional aggregation Designated type for namespaces SQL/XQuery like query syntax Concise accessors

Language INtegrated Query (LINQ) LINQ provides one programming model for all types of data (objects, SQL, XML, DataSets) LINQ enabled data sources LINQ To Objects Objects LINQ To XML <book> <title/> <author/> <price/> </book> XML LINQ enabled ADO.NET LINQ To Datasets LINQ To SQL LINQ To Entities Relational Others… VB C# .NET Language-Integrated Query

.NET through the ages

The evolution of C# C# 1.0 C# 2.0 C# 3.0 Components on a Managed Runtime Generics Language Integrated Query

C# 3.0 design goals

Integrate objects, relational data, and XML

And

Increase conciseness of language

Add functional programming constructs

Remain 100% backwards compatible

…

C# 3.0 language innovations var contacts = from c in customers where c.City == "Redmond" select new { c.Name, c.Phone }; var contacts = customers .Where(c => c.City == "Redmond") .Select(c => new { c.Name, c.Phone }); Extension methods Lambda expressions Query expressions Object initializers Anonymous types Local variable type inference

Local variable type inference Not to be confused with Pascal

Object initializers Field or property assignments

Anonymous types class ??? { public string Name; public int Age; } ???

Anonymous types class ??? { public string Name; public string Phone; } public class Customer { public string Name; public Address Address; public string Phone; … } public class Contact { public string Name; public string Phone; } Customer c = GetCustomer(…); Contact x = new Contact { Name = c.Name, Phone = c.Phone }; Customer c = GetCustomer(…); var x = new { c.Name, c.Phone }; Customer c = GetCustomer(…); var x = new { Name = c.Name, Phone = c.Phone }; Projection style initializer

Lambda expressions – motivating example Delegate type (function type) Higher-order function

Lambda expressions (not yet) Non-anonymous function

Lambda expressions (C# 1.0/2.0)

Lambda expressions (C# 3.0 sugar) Lambda expression

Lambda expressions (w/ type inference) Lambda expression

Extension methods Static method with “this” argument Invoke It like an instance method

Query expressions

Queries translate to method invocations

Where, Join, OrderBy, Select, GroupBy, …

from c in customers where c.City == "Redmond" select new { c.Name, c.Phone }; customers .Where(c => c.City == "Redmond" ) .Select(c => new { c.Name, c.Phone });

Query expressions from id in source { from id in source | join id in source on expr equals expr [ into id ] | let id = expr | where condition | orderby ordering , ordering , … } select expr | group expr by key [ into id query ] Starts with from Zero or more from , join , let , where , or orderby Ends with select or group by Optional into continuation

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>

Extension methods and queries namespace System.Linq { public static class Enumerable { public static IEnumerable<T> Where<T>(this IEnumerable<T> source, Func<T, bool> predicate) { … } public static IEnumerable<S> Select<T, S>(this IEnumerable<T> source, Func<T, S> selector) { … } … } } using System.Linq; Extension methods IEnumerable<string> contacts = customers.Where(c => c.State == "WA").Select(c => c.Name); Brings extensions into scope obj.Foo(x, y)  ???.Foo(obj, x, y) IntelliSense!

Expression trees

LINQ architecture System.Linq.Enumerable Delegate based Source implements IEnumerable<T> System.Linq.Queryable Expression tree based Source implements IQueryable<T> SQL DataSets Objects Others…

LINQ to SQL Architecture Enumerate SQL Query or SProc Rows Objects SubmitChanges() DML or SProcs Application LINQ to SQL from c in db.Customers where c.City == "London" select c.CompanyName SELECT CompanyName FROM Customer WHERE City = 'London' db.Customers.Add(c1); c2.City = “Seattle"; db.Customers.Remove(c3); INSERT INTO Customer … UPDATE Customer … DELETE FROM Customer …

Summary – LINQ contributions