C# 3.0

Tom Roeder

CS215 2006fa

Version 3

From PDC 2005 preview compiler available LINQ: language-integrated query

High level points: adds native query functionality to C# adds better functional programming

Several changes required to make this work implicitly typed variables extension methods

Implicitly typed variables var i = 5;

var s = "Hello";var d = 1.0;var orders = new Dictionary<int,Order>();

Type of the variable induced from expression must include initializer can’t be null. Why not?

What happens if “var” is a class in scope? Works in for loops

Extension methods

Can add methods to other classes any methods (although look static) only from static classes

When import a namespace that has extensions, then added to classes once imported, called as usual

Local methods take precedence first local for normal method, then extension

Extension methodspublic static class Extensions { public static int ToInt32(this string s) { return Int32.Parse(s);

} public static T[] Slice<T>(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; }

}

Extension methodsusing N1;namespace N1

{public static class E{

public static void F(this object obj, int i) { }public static void F(this object obj, string s)

{ }}

}class A { }class B

{public void F(int i) { }

}class C

{public void F(object obj) { }

}

Extension methodsclass X

{static void Test(A a, B b, C c) {

a.F(1); a.F("hello");

b.F(1);b.F("hello");c.F(1);c.F("hello");

}}

Extension methods

Not for properties, events, operators currently under consideration

Equivalent to calling the static method difference from actually extending the class?

How could we use this for Polynomials? Disadvantages?

implicitness security

Lambda expressions

Generalized function syntax x . x + 1 in C# 3.0, have x => x + 1

From anonymous delegate syntax: delegate(int x) { return x + 1;}

Can have implicitly typed variables Can have more than one variable Can have expression or statement body

NB: no statement bodies in preview compiler

Lambda expressions

Can be converted to delegate type if parameters and body match delegate R Func<A,R>(A arg);Func<int,int> f1 = x => x + 1;Func<int,double> f2 = x => x + 1;Func<double,int> f3 = x => x + 1;

Participate in type inference If expression body, get expression trees

Lambda expressions

Type inferencepublic static IEnumerable<S> Select<T,S>(

this IEnumerable<T> source,Func<T,S> selector)

{ foreach (T element in source)

yield return selector(element);}

If call Select(customers, c => c.Name); T, S mapped to appropriate types

Lambda expressions

Given the codedelegate R Func<A,R>(A arg); static Z F<X,Y,Z>(X value, Func<X,Y> f1, Func<Y,Z> f2) {

return f2(f1(value));}

what happens when get the following?F("1:15:30", s => TimeSpan.Parse(s), t => t.TotalSeconds)

Intializers

Can initialize fields like attribute fields new C(1, 2, name=“my class”); works if public field or if property with set can be nested (eg. Rectangle with two Points)

Collection initializers List<int> digits = new List<int> { 0, 1}; Must implement System.Generic.ICollection<T>

Object initializers var a = new Point { X = 0, Y = 1 };

Anonymous types

var x = new {p1 = 10, p2 = “name”}; x is of anonymous type type can’t be referred to by name in program

structural type equivalence two anonymous types can be compatible

implicitly typed arrays var a = new[] { 1, 10, 100, 1000 }; must have consistent types or have implicit conversions

Query expressions

Adds querying to language important for interaction with DB but also with built-in data structures leave query planning to data structure designer

Implemented using above functionality anonymous types and variables useful lambda expressions make it cleaner.

Query expressions

eg.var x = from s in students where s.City == “Ithaca" select s; defines an expression translates to invocation of Where method on students

if select contains expression, calls Select uses lambda expressions as parameters we saw Select above

Other clauses

groupby from c in customers group c.Name by c.Country c.GroupBy(c => c.Name, c => c.Country);

orderbyfrom c in customersorderby c.Nameselect new { c.Name, c.Phone }

multiple from clausesfrom c in customerswhere c.City == "London"from o in c.Orderswhere o.OrderDate.Year == 2005select new { c.Name, o.OrderID, o.Total }

Other clauses

inner join? can be done with multiple generators just use a different object internally not as efficient?

maybe need more syntax? don’t have full spec yet, just examples

Query expression pattern

As with collections: not an interface just need to implement the methods necessary for backwards compatibility methods may be implemented by extension

Methods named as above Any class that implements pattern can be

accessed via the new syntax

Sequences

A collection IEnumerable<T> given iterator, can be viewed as a sequence

New extension operators Take/TakeWhile Skip/SkipWhile Reverse Concat Intersect/Union/Except

Expression trees

Data structure that represents expression type Expression<D> if D is delegate type

eg. Expression<Func<T>> e = x => x + 1; can be used to parse the function

Spec not currently available Idea

take an expression tree and convert to SQL

Database Integration

Final step don’t think about the database types as separate allow databases to back classes!

How would you do this in C#? Queries are now in the language

eg. var x = from s in sourceswhere s.fileName == “Polynomial.cs”select s;

DLinq Attributes

[Table(Name="DVDTable")]public class DVD{

[Column(Id = true)]public string Title;

[Column]public string Rating;

}

Connections Revisited public class MyDVDs : DataContext{

public Table<DVD> DVDs;public MyDVDs(string connection) :

base(connection) {}}

creates the connection to a database MyDVDs db = new MyDVDs(“dvd.mdf”);var x = from d in db.DVDs

where d.Rating == “G” select d;

DLinq Query Expressions

var x = <some query> the query is not executed immediately why not?

Convert to Query<DVD> holds the actual query as an Expression tree convert to SQL at execution time. When? eg. when need IEnumerator for foreach

Changes

db.SubmitChanges() try to update the DB with our changes may fail: why?

other applications may have made modifications what should we do?

db.RejectChanges() throw away all changes since last updated

Concurrency control optimistic in general

Transactions

Transaction: unit of work isolated from other units can be made to look like happening serially

using(TransactionScope ts = new TransactionScope()) {db.SubmitChanges();ts.Complete();

}

creates a new transaction for these updates if transaction succeeds, all changes accepted if transaction aborts, no object rollback

XLinq

Instant XML intro language for defining markup nested tags and attributes eg <name instructor=“true”>Tom Roeder</name>

XML used as a document language often want to extract information from XML eg. in web search

What is the relationship to DBs?

C# XML Programming

Current model Read in XML document via various methods everything based in XMLDocument

XLinq model XElement

no document requirements: can be created anywhere can be queried as in DLinq

built up functionally

XLinq Query Example from c in contacts.Elements("contact")

where (string) c.Element("address").Element("state") == "WA"orderby (string) c.Element("name")select (string) c.Element("name");

What is the difference with DLinq? why all the casts? how to fix?

LINQ team looking into schema support adds metadata to XML queries

C# 3.0 Conclusions

query ability useful but many things are adding functional programming increases the

possible styles of C# programming additions apparently not made on general

principles but to support a particular narrow model of programming.

Nonetheless: try it out