ADO.NET (and more LINQ) Entity Framework (and more LINQ) Verification (CodeContracts) TPL (Task Parallel Library) Other.NET/C# topics

ADO.NET To måder at tilgå DB på: Connected Åbn connection Læse-/Skrivetilgang (select, insert, update og delete) via Command-objekt Ved læsetilgang (select) returneres et DataReader-objekt Luk Connection Disconnected Fyld et DataSet-objekt (kopi af dele af db) vha. en DataAdapter DataAdapter indpakker SQL-statement(s) DataSet-objekt indeholder DataTable-objekter DataTable-objekter indeholder collections af rækker og kolonner

Connection vs. Connectionless Connection: Open Connection Udfør db-operationer Luk Connection Der arbejdes på aktuelle data Andre applikationer lukkes evt. ude

Connectionless: –Tag en kopi af et databaseudsnit –Udfør db-operationer på kopien –Andre applikationer kan ændre data –Der arbejdes evt. på en uaktuel kopi af data Connection vs. Connectionless Data ændres i den lokale kopi: –ved update checkes om andre har ændret dataene i kopien –i så fald afvises opdateringen (ConcurrencyException)

Hvis DB skal opdateres Tilføj et commandobjekt til dataadapteren InsertCommand UpdateCommand DeleteCommand Gøres vha. parametriserede queries, fx: da.InsertCommand = new OleDbCommand("INSERT INTO ” + Ansat (løn, navn, stilling) VALUES (?, ?, ?)",dbconn); da.Update(ds, ”Ansat”); 1. parameter: sql 2. parameter: connection Her opdateres DB. Kan fejle (ConcurrencyException)

Transaction - Definition A transaction is an operation on data in the database. A transaction may be composed of several database operations, but is viewed as a logical unit of work A transaction must be done completely or not done at all A transaction must have the ACID properties: A: Either it is done in total or it is not done at all (Atomicity) C: The database moves from one consistent state to an other consistent state (Consistency) I: If more operations are accessing the same data, they are not to disturb each other – they must execute as if they executed alone (Isolation) D: When a transaction completes, its changes to the database are permanent (Durability)

Transactions – example: T1 and T2 are executing concurrently T1: Transfers N DKKs from account X to account Y: read_item(X); X:= X-N; write_item(X); read_item(Y); Y:= Y+N; write_item(Y); T2: Deposits M DKK on account Y: read_item(Y); Y:= Y+M; write_item(Y); Any possible problems? time

Transactions – Problems We want several transactions to execute concurrently (Why?) Three types of problems: lost update uncommitted dependency (temporary update) inconsistent analysis (incorrect summary) Crash during execution of a transaction must be handled

Lost Update

Uncommitted Dependency

Inconsistent Analysis

SQL Support for Transactions By default any SQL statement is considered to be atomic, that is: a transaction. Transactions involving more than one SQL statement must be opened by BeginTransaction() and terminated by either Commit() or Rollback(). It is possible to specify the isolation level of a transaction: Read Uncommitted Read Committed (Default on MS SQL Server) Repeatable Read Serializable

Isolation Levels READ UNCOMMITTED In this isolation level, dirty reads are allowed. One transaction may see uncommitted changes made by some other transaction. READ COMMITTED Data records retrieved by a query are not prevented from modification by some other transaction. Non-repeatable reads may occur, meaning data retrieved in a SELECT statement may be modified by some other transaction when it commits. In this isolation level, read locks are acquired on selected data but they are released immediately whereas write locks are released at the end of the transaction.

Isolation Levels REPEATABLE READ All data records read by a SELECT statement cannot be changed; however, if the SELECT statement contains any ranged WHERE clauses, phantom reads can occur. In this isolation level, the transaction acquires read locks on all retrieved data, but does not acquire range locks. SERIALIZABLE This isolation level specifies that all transactions occur in a completely isolated fashion; i.e., as if all transactions in the system had executed serially, one after the other. The DBMS may execute two or more transactions at the same time only if the illusion of serial execution can be maintained. At this isolation level, phantom reads cannot occur.

Entity Framework Data menu >> Add New Data Source… select Database, Entity Data Model, … VS generates persistent model ― entities, collections, etc. var query = from doctor in _db.Doctors orderby doctor.LastName, doctor.FirstName select doctor; foreach (Doctor d in query) { string name = string.Format("{0}, {1}", d.LastName, d.FirstName); lstDoctors.Items.Add(name); } var query = from doctor in _db.Doctors orderby doctor.LastName, doctor.FirstName select doctor; foreach (Doctor d in query) { string name = string.Format("{0}, {1}", d.LastName, d.FirstName); lstDoctors.Items.Add(name); } SchedulingEntities _db = new SchedulingEntities(); From Joe Hummel

Specifikationer: Code Contracts Metoder specificeres vha. pre- og postbetingelser: Prebetingelse: Et udsagn om programmets tilstand (værdier af variable) som skal være sandt inden metoden må kaldes. Postbetingelse: Et udsagn om programmets tilstand (værdier af variable) som skal være sandt når metoden returnerer. Invarianter specificerer krav, som går på tværs af metoder.

Eksempel: Dictionary (Map – værdibaseret container, lagrer par af (key, value)) public interface Dictionary { invariant // Basic queries public int count(); ensures (count()==0) ==> public boolean has(Object key); requires public Object valueFor(Object key); // Derived queries ensures \result == public boolean isEmpty(); // Commands requires ensures ensures ensures public void put(Object key, Object value); requires ensures ensures public void remove(Object key); } Her specifikation som kommentarer Understøttes i.NET af CodeContracts

TPL: Task Parallel Library Vi får næppe bedre performance pga. øget clock- frekvens (More’s lov holder ikke længere). Derimod får vi flere processorer – fire er allerede ved at være standard på almindelige lap-tops. Derfor bliver parallelprogrammering mere og mere interessant. TPL er.NET 4’s bud en API til parallelprogrammering.

Task using System.Threading.Tasks; Task T = new Task( code ); T.Start(); if… while… using System.Threading.Tasks; Task T = new Task( code ); T.Start(); if… while… tells.NET that task *can* start, then returns immediately tells.NET that task *can* start, then returns immediately original code task "code" Program “forks” and now 2 code streams are executing concurrently ― original, and T. computation to perform… From Joe Hummel

Language support TPL takes advantage of.NET language features Lambda expressions Closures Parallel.For(0, N, (i) => { C[i] = A[i] + B[i]; } ); Parallel.For(0, N, (i) => { C[i] = A[i] + B[i]; } ); int[] A, B, C;. for(i=0; i<N; i++) { C[i] = A[i] + B[i]; } int[] A, B, C;. for(i=0; i<N; i++) { C[i] = A[i] + B[i]; } lambda expression close over data (by ref!) From Joe Hummel