CHARLES UNIVERSITY IN PRAGUE faculty of mathematics and physics C# 3.0 and.NET 3.5: A Brief Overview Pavel Ježek

C# 3.0 Features Implicitly Typed Local Variables Extension Methods Lambda Expressions Object Initializers Collection Initializers Anonymous Types Expression Trees Query Expressions C# 3.0 compiler and.NET 3.5 libraries are part of Visual Studio 2008 Pavel Ježek C# 3.0 and.NET 3.5

Implicitly Typed Local Variables Examples: var i = 5; var s = "Hello"; var d = 1.0; var numbers = new int[] {1, 2, 3}; var orders = new Dictionary (); Are equivalent to: int i = 5; string s = "Hello"; double d = 1.0; int[] numbers = new int[] {1, 2, 3}; Dictionary orders = new Dictionary (); Errors: var x; // Error, no initializer to infer type from var y = {1, 2, 3}; // Error, collection initializer not permitted var z = null; // Error, null type not permitted Pavel Ježek C# 3.0 and.NET 3.5

Extension Methods Declaration: public static partial class Extensions { public static int ToInt32(this string s) { return Int32.Parse(s); } } Usage: string s = "1234"; int i = s.ToInt32(); // Same as Extensions.ToInt32(s) Instance methods take precedence over extension methods Extension methods imported in inner namespace declarations take precedence over extension methods imported in outer namespace declarations Pavel Ježek C# 3.0 and.NET 3.5

Extension Methods (2) Declaration: public static partial class Extensions { public static T[] Slice (this T[] source, int index, int count) { if (index < 0 || count < 0 || source.Length – index < count) throw new ArgumentException(); T[] result = new T[count]; Array.Copy(source, index, result, 0, count); return result; } } Usage: int[] digits = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; int[] a = digits.Slice(4, 3); // Same as Extensions.Slice(digits, 4, 3) Pavel Ježek C# 3.0 and.NET 3.5

Lambda Expressions Example of C# 2.0 anonymous method: class Program { delegate T BinaryOp (T x, T y); static void Method (BinaryOp op, T p1, T p2) { Console.WriteLine(op(p1, p2)); } static void Main() { Method(delegate(int a, int b) { return a + b; }, 1, 2); } C# 3.0 lambda expressions provide further simplification: class Program { delegate T BinaryOp (T x, T y); static void Method (BinaryOp op, T p1, T p2) { Console.WriteLine(op(p1, p2)); } static void Main() { Method((a, b) => a + b, 1, 2); } Pavel Ježek C# 3.0 and.NET 3.5

Lambda Expressions (2) Expression or statement body Implicitly or explicitly typed parameters Examples: 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 arg); Func f1 = x => x + 1; // Ok Func f2 = x => x + 1; // Ok Func f3 = x => x + 1; // Error – double cannot be // implicitly converted to int Pavel Ježek C# 3.0 and.NET 3.5

Lambda Expressions (3) Lambda expressions participate in inference process of type arguments of generic methods In initial phase, nothing is inferred from arguments that are lambda expressions Following the initial phase, additional inferences are made from lambda expressions using an iterative process Pavel Ježek C# 3.0 and.NET 3.5

Lambda Expressions (4) Generic extension method example: public class List : IEnumerable, … { … } public static class Sequence { public static IEnumerable Select ( this IEnumerable source, Func selector) { foreach (T element in source) yield return selector(element); } Calling extension method with lambda expression: List customers = GetCustomerList(); IEnumerable names = customers.Select(c => c.Name); Rewriting extension method call: IEnumerable names = Sequence.Select (customers, c => c.Name); T type argument is inferred to Customer based on source argument type Sequence.Select (customers, c => c.Name) c lambda expression argument type is infered to Customer Sequence.Select (customers, (Customer c) => c.Name) S type argument is inferred to string based on return value type of the lambda expression Sequence.Select (customers, (Customer c) => c.Name) Pavel Ježek C# 3.0 and.NET 3.5

Lambda Expressions: Expression Trees Permit lambda expressions to be represented as data structures instead of executable code Lambda expression convertible to delegate D (assignment causes code generation) is also convertible to expression tree (abstract syntax tree) of type System.Linq.Expressions.Expression (assignment causes expression tree generation) Expression trees are immutable IL_0000: ldarg.0 IL_0001: ldc.i4.2 IL_0002: add IL_0003: ldc.i4.s 10 IL_0005: mul IL_0006: stloc.0 IL_0007: br.s IL_0009 IL_0009: ldloc.0 IL_000a: ret LambdaExpression BinaryExpression ParameterExpression(“value”) ConstantExpression(2) ConstantExpression(10) Func f = Expression > e = value => (value + 2) * 1

Object Initializers Class representing a point: public class Point { private int x, y; public int X { get { return x; } set { x = value; } } public int Y { get { return y; } set { y = value; } } } New instance can be created using object initializer: var a = new Point { X = 0, Y = 1 }; Which is equivalent to: var a = new Point(); a.X = 0; a.Y = 1; Pavel Ježek C# 3.0 and.NET 3.5

Object Initializers (2) Class representing a rectangle: public class Rectangle { private Point p1, p2; public Point P1 { get { return p1; } set { p1 = value; } } public Point P2 { get { return p2; } set { p2 = value; } } } New instance can be created using object initializer: var r = new Rectangle { P1 = new Point { X = 0, Y = 1 }, P2 = new Point { X = 2, Y = 3 } }; Which is equivalent to: var r = new Rectangle(); var __p1 = new Point(); __p1.X = 0; __p1.Y = 1; r.P1 = __p1; var __p2 = new Point(); __p2.X = 2; __p2.Y = 3; r.P2 = __p2; Pavel Ježek C# 3.0 and.NET 3.5

Object Initializers (3) Class representing a rectangle with constructor that allocates p1 and p2: public class Rectangle { private Point p1 = new Point(); private Point p2 = new Point(); public Point P1 { get { return p1; } } public Point P2 { get { return p2; } } } New instance can be created using object initializer: var r = new Rectangle { P1 = { X = 0, Y = 1 }, P2 = { X = 2, Y = 3 } }; Which is equivalent to: var r = new Rectangle(); r.P1.X = 0; r.P1.Y = 1; r.P2.X = 2; r.P2.Y = 3; Pavel Ježek C# 3.0 and.NET 3.5

Collection Initializers Example: List digits = new List { 0, 1, 2}; Is equivalent to: List digits = new List (); digits.Add(0); digits.Add(1); digits.Add(2); List has to implement System.Collections.Generic.ICollection interface with the Add(T) method Pavel Ježek C# 3.0 and.NET 3.5

Combining Object and Collection Initializers Class representing a contact with name and list of phone numbers: public class Contact { private string name; private List phoneNumbers = new List (); public string Name { get { return name; } set { name = value; } } public List PhoneNumbers { get { return phoneNumbers; } } } List of contacts can be created and initialized with: var contacts = new List { new Contact { Name = "Chris Smith", PhoneNumbers = { " ", " " } }, new Contact { Name = "Bob Harris", PhoneNumbers = { " " } } }; Which is equivalent to: var contacts = new List (); var __c1 = new Contact(); __c1.Name = "Chris Smith"; __c1.PhoneNumbers.Add(" "); __c1.PhoneNumbers.Add(" "); contacts.Add(__c1); var __c2 = new Contact(); __c2.Name = "Bob Harris"; __c2.PhoneNumbers.Add(" "); contacts.Add(__c2); Pavel Ježek C# 3.0 and.NET 3.5

Auto-implemented Properties Backed up by a private field normally inaccessible to programmer (only via the property): class LightweightCustomer { public double TotalPurchases { get; set; } public string Name { get; private set; }// read-only public int CustomerID { get; private set; } // read-only } Pavel Ježek C# 3.0 and.NET 3.5

Anonymous Types Following expression: new { p1 = e1, p2 = e2, … pn = en } Can be used to define an anonymous type : class __Anonymous1 { private T1 f1 ; private T2 f2 ; … private Tn fn ; public T1 p1 { get { return f1 ; } set { f1 = value ; } } public T2 p2 { get { return f2 ; } set { f2 = value ; } } … public T1 p1 { get { return f1 ; } set { f1 = value ; } } } And create its instance using object initializer Different anonymous object initilizers that define properties with same names in the same order generate the same anonymous type: var p1 = new { Name = "Lawnmower", Price = }; var p2 = new { Name = "Shovel", Price = }; p1 = p2; Pavel Ježek C# 3.0 and.NET 3.5

Partial Methods Can appear only in partial classes or structs, and must be void, private and without out parameters: partial class A { string _name; partial void OnNameChanged(); public string Name { set { _name = value; OnNameChanged(); } partial class A { partial void OnNameChanged() { // Do something } Pavel Ježek C# 3.0 and.NET 3.5

Query Expressions – Examples Query expression: from c in customers where c.City == "London“ select c Gets translated to: customers.Where(c => c.City == "London") Pavel Ježek C# 3.0 and.NET 3.5

Query Expressions – Examples Query expression: from c in customers where c.City == "London“ select c.Name Gets translated to: customers.Where(c => c.City == "London").Select(c => c.Name) Pavel Ježek C# 3.0 and.NET 3.5

Query Expressions – Examples Query expression: from c in customers orderby c.Name select new { c.Name, c.Phone } Gets translated to: customers.OrderBy(c => c.Name).Select(c => new { Name = c.Name, Phone = c.Phone }) Pavel Ježek C# 3.0 and.NET 3.5

Query Expressions – Examples Query expression: from c in customers where c.City == "London" from o in c.Orders where o.OrderDate.Year == 2005 select new { c.Name, o.OrderID, o.Total } Gets translated to: customers.Where(c => c.City == "London"). SelectMany(c => c.Orders. Where(o => o.OrderDate.Year == 2005). Select(o => new { Name = c.Name, OrderID = o.OrderID, Total = o.Total }) ) Pavel Ježek C# 3.0 and.NET 3.5

Query Expressions Query expressions or LINQ (Language INtergrated Queries) are the key feature of.NET 3.5 Query expressions are translated to method calls – works on classes like: delegate R Func (A arg); class C { public C Where(Func predicate); public C Select (Func selector); public C SelectMany (Func > selector); public O OrderBy (Func keyExpr); public O OrderByDescending (Func keyExpr); public C > GroupBy (Func keyExpr); public C > GroupBy (Func keyExpr, Func elemExpr); } … Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Creating Entity Classes Only instances of “entity classes”, i.e. “entities”, can be stored in/retrieved from databases [Table] // Table is named “Customer” public class Customer { public string CustomerID; // Transient data not stored in DB public string City; // Transient data not stored in DB } [Table(Name="Customers")] public class Customer { public string CustomerID; // Transient data not stored in DB public string City; // Transient data not stored in DB } [Table(Name="Customers")] public class Customer { [Column(Id=true)] // Part of database primary key public string CustomerID; [Column] public string City; } Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL - DataContext DataContext is equivalent of ADO.NET connection It retrieves data (objects) from the database and submits changes It translates requests (queries) for objects into SQL queries Using the DataContext to retrieve customer objects whose city is London: // DataContext takes a connection string DataContext db = newDataContext("c:\\northwind\\northwnd.mdf"); // Get a typed table to run queries Table Customers = db.GetTable (); // Query for customers from London var q = from c in Customers where c.City == "London" select c; foreach (var cust in q) { Console.WriteLine("id = {0}, City = {1}", cust.CustomerID, cust.City); } Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Strongly Typed DataContext public partial class Northwind : DataContext { public Table Customers; public Table Orders; public Northwind(string connection): base(connection) {} } Retrieving customer objects whose city is London more easily: Northwind db = new Northwind("c:\\northwind\\northwnd.mdf"); var q =from c in db.Customers where c.City == "London“ select c; foreach (var cust in q) { Console.WriteLine("id = {0}, City = {1}",cust.CustomerID, cust.City); } Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Query Execution Queries are not imperative – they are not executed immediately var q =from c in db.Customers where c.City == "London" select c; Only creates an instance of IQueryable type and assigns it to q When the application tries to enumerate the contents of a query, it gets executed (i.e. deferred execution): // Execute first time foreach (Customer c in q) Console.WriteLine(c.CompanyName); // Execute second time foreach (Customer c in q) Console.WriteLine(c.CompanyName); Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Query Execution Queries are not imperative – they are not executed immediately var q =from c in db.Customers where c.City == "London" select c; Only creates an instance of IQueryable type and assigns it to q When the application tries to enumerate the contents of a query, it gets executed (i.e. deferred execution): // Execute db.Log = Console.Out; foreach (Customer c in q) Console.WriteLine(c.CompanyName); Standard output: SELECT [t0].[EmployeeID], [t0].[LastName], [t0].[FirstName], [t0].[Title], [t0].[TitleOfCourtesy], [t0].[BirthDate], [t0].[HireDate], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[HomePhone], [t0].[Extension], [t0].[Photo], [t0].[Notes], [t0].[ReportsTo], [t0].[PhotoPath] FROM [dbo].[Employees] AS [t0] WHERE [t0].[City] Input NVarChar (Size = 6; Prec = 0; Scale = 0) [London] Pavel Ježek C# 3.0 and.NET 3.5

Expression Trees Permit lambda expressions to be represented as data structures instead of executable code Lambda expression convertible to delegate D (assignment causes code generation) is also convertible to expression tree (abstract syntax tree) of type System.Linq.Expressions.Expression (assignment causes expression tree generation) Expression trees are immutable Pavel Ježek C# 3.0 and.NET 3.5 IL_0000: ldarg.0 IL_0001: ldc.i4.2 IL_0002: add IL_0003: ldc.i4.s 10 IL_0005: mul IL_0006: stloc.0 IL_0007: br.s IL_0009 IL_0009: ldloc.0 IL_000a: ret LambdaExpression BinaryExpression ParameterExpression(“value”) ConstantExpression(2) ConstantExpression(10) Func f = Expression > e = value => (value + 2) * 1

LINQ to SQL – Advantage of Deferred Execution Partial query contruction: var q =from c in db.Customers where c.City == "London" select c; if (orderByLocation) { q =from c in q orderby c.Country, c.City select c; } else if (orderByName) { q =from c in q orderby c.ContactName select c; } foreach (Customer c in q) Console.WriteLine(c.CompanyName); Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Avoiding Deferred Execution Query can be converted to standard collection classes using ToList() or ToArray() Standard Query Operators – leads to immediate execution: var q =from c in db.Customers where c.City == "London" select c; // Execute once using ToList(), ToArray() var list = q.ToList(); foreach (Customer c in list) Console.WriteLine(c.CompanyName); Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Defining Relationships [Table(Name="Customers")] public class Customer { [Column(Id=true)] public string CustomerID;... private EntitySet _Orders; [Association(Storage="_Orders", OtherKey="CustomerID")] public EntitySet Orders { get { return this._Orders; } set { this._Orders.Assign(value); } } [Table(Name="Orders")] public class Order { [Column(Id=true)] public int OrderID; [Column] public string CustomerID; private EntityRef _Customer; [Association(Storage="_Customer", ThisKey="CustomerID")] public Customer Customer { get { return this._Customer.Entity; } set { this._Customer.Entity = value; } } Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Querying Across Relationships Query using the Orders property to form the cross product between customers and orders, producing a new sequence of Customer and Order pairs: var q =from c in db.Customers from o in c.Orders where c.City == "London" select new { c, o }; Another query producing the same result: var q =from o in db.Orders where o.Customer.City == "London" select new { c = o.Customer, o }; Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Modifying Entities Northwind db = new Northwind("c:\\northwind\\northwnd.mdf"); // Query for a specific customer string id = "ALFKI"; var cust = db.Customers.Single(c => c.CustomerID == id); // Change the name of the contact cust.ContactName = "New Contact"; // Delete an existing Order Order ord0 = cust.Orders[0]; // Removing it from the table also removes it from the Customer’s list db.Orders.Remove(ord0); // Create and add a new Order to Orders collection Order ord = new Order { OrderDate = DateTime.Now }; cust.Orders.InsertOnSubmit(ord); // Ask the DataContext to save all the changes – generates appropriate SQL command db.SubmitChanges(); Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Creating Simple Databases [Table(Name="DVDTable")] public class DVD { [Column(Id = true)] public string Title; [Column] public string Rating; } public class MyDVDs : DataContext { public Table DVDs; public MyDVDs(string connection) : base(connection) {} } // Creating a new database MyDVDs db = new MyDVDs("c:\\mydvds.mdf"); db.CreateDatabase(); // Replacing an existing one MyDVDs db = new MyDVDs("c:\\mydvds.mdf"); if (db.DatabaseExists()) { db.DeleteDatabase(); } db.CreateDatabase(); Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Inheritance [Table] [InheritanceMapping(Code = "C", Type = typeof(Car))] [InheritanceMapping(Code = "T", Type = typeof(Truck))] [InheritanceMapping(Code = "V", Type = typeof(Vehicle), IsDefault = true)] public class Vehicle { [Column(IsDiscriminator = true)] public string Key; [Column(Id = true)] public string VIN; [Column] public string MfgPlant; } public class Car : Vehicle { [Column] public int TrimCode; [Column] public string ModelName; } public class Truck : Vehicle { [Column] public int Tonnage; [Column] public int Axles; } Pavel Ježek C# 3.0 and.NET 3.5

LINQ to SQL – Inheritance Pavel Ježek C# 3.0 and.NET 3.5 Example queries: var q = db.Vehicle.Where(p => p is Truck); var q = db.Vehicle.OfType (); var q = db.Vehicle.Select(p => p as Truck).Where(p => p != null); foreach (Truck p in q) Console.WriteLine(p.Axles);

LINQ to SQL – Query Visualizer in VS static void Main(string[] args) { Northwnd db = new files\linq preview\data\northwnd.mdf"); var q = from c in db.Customers where c.City == "London" select c; } // Breakpoint Pavel Ježek C# 3.0 and.NET 3.5