ado.net 2.0

1UCN Technology: Computer Science: Autumn 2012

ADO.NET 2.0

• Architecture• DataReader • DataSet• Connection factory• Config file


Data access

Swap?


Data Providers

• OLE DB:A COM object, that maps the OLE DB api to the DBMS' apiIs Microsoft propritary.

• ODBC: Works similar to a printer driver undependent of DBMS and OS

• .NET Data provider:A set of classes that implements a set of interfaces and abstract classes

• Some are provided by Microsoft and are installed with the .Net framework.

• Others are written by the DBMS provider


Microsoft Data Providers

• Data Provider Namespace Assembly• OLE DB System.Data.OleDb System.Data.dll• Microsoft SQL Server System.Data.SqlClient System.Data.dll• Microsoft SQL Server Mobile System.Data.SqlServerCe System.Data.SqlServerCe.dll• ODBC System.Data.Odbc System.Data.dll• Oracle System.Data.OracleClient System.Data.OracleClient.dll

MS Access


Other providers

• Data Provider Website• Firebird Interbase http://www.mono-project.com/Firebird_Interbase• IBM DB2 Universal Database http://www-306.ibm.com/software/data/db2• MySQL http://dev.mysql.com/downloads/connector/net/1.0.html• PostgreSQL http://www.mono-project.com/PostgreSQL• Sybase http://www.mono-project.com/Sybase• Oracle (proprietary) http://www.oracle.com/technology/tech/windows/odpnet/

index.html


Classes and Interfaces

• Object Base Class Implemented Interfaces

• Connection DbConnection IDbConnection - Connection• Command DbCommand IDbCommand - SQL command• DataReader DbDataReader IDataReader - Forward reader• DataAdapter DbDataAdapter IDataAdapter - Used with Dataset• Parameter DbParameter IDataParameter - Parameters for

Command• Transaction DbTransaction IDbTransaction - Transaction


Two ways of DB access

• Connected:

– Open connection.

– Read/Write access (select, insert, update and delete) using a Command object.

– When reading (select) a DataReader object is returned. A DataReader is an iterator (cursor) into the result table.

– Close Connection.

• Disconnected:

– Fill a DataSet object (a copy of a part of the database) using a DataAdapter.

– DataAdapter wraps SQL-statement(s).

– A DataSet object contains DataTable objects.

– DataTable objects contain collections of rows and columns.


Connection vs. Connectionless

• Connection:– Open Connection– Execute DB operations– Close Connection– Working on actual (live)

data– Other applications can not

access data.

Domæneklasser

SQL

Dataklasser

DB


Domæneklasser

SQL

Dataklasser

DB

• Connectionless:– Create a copy of a part of

the database– Execute DB operations on

the copy– Other applications may

change date– The copy may be come

inconsistent.

Connection vs. Connectionless

• Data are changed in the local copy:– at update it is checked if the data in the

database have been modified by others– in that case the update is rejected

(ConcurrencyException).


Overview of using databases

• 4 steps:

1. Open connection to database

2. Execute SQL for updating DB or fetching records

3. Handle data

4. Close connection


Step 1: Open Connection

• Connection are opened according to connection string info– here is a connection to a MS Access database opened– The database is located here: @"c:\AnyPlace\MyDb.mdb"– @ means that escape characters are discarded

<%@ import namespace="System.Data"%><%@ import namespace="System.Data.OleDb"%>

string sConnection;sConnection = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source="+@"c:\AnyPlace\MyDb.mdb");

OleDbConnection dbConn;dbConn = new OleDbConnection(sConnection);dbConn.Open();

connection


Open Connection

• Connections are opened using a connection string– May be found using Visual Studio:

// Create and open a connection. SqlConnection cn = new SqlConnection(); cn.ConnectionString = "Data Source=PCM06073\\SQLEXPRESS;”

+”Initial Catalog=vw;Integrated Security=True;"; cn.Open(); ShowConnectionStatus(cn);

// or should it be? cn.ConnectionString = “Server=PCM06073\\SQLEXPRESS;” +”Initial Catalog=vw;Integrated Security=True;"; cn.Open();

Data Sourceor

Server?

Server is preferable.


Connection Strings

• Connection strings are product specific (DB specific) anf often very well-documented.

• Help may be found at:– www.connectionstrings.com – www.able-consulting.com/ADO_conn.htm

• In many cases VS can help.


Step 2: Get / fetch data

• Data are fetched by creating a Command object and use it to execute a SQL statement.

• Data can be stored in either a DataReader object or a DataSet object.


Differences between DataReader and DataSet/DataAdapter

• DataReader can only be used for reading data.• It can only be traversed once (forward).• DBCommand can update the database by ExecuteNonQuery. This

update is executed immediately.

• DataAdapter is the connection between DataSet and database.• Data are fetched to the DataSet, might be modified and sent back to

the database.• Updates are executed on a local copy. Concurrency problems must

be handled. • Possible to traverse forward and backward.• A DataSet can contain multiple tables.


Step 2: Get records Method 1: DataSet

• Get records by SQL Select query and DataAdapter– Data can be read and changed

string sql;sql = " Select lastname, firstname" + " From employee" + " Order By lastname Asc, firstname Asc;";

OleDbCommand dbCmd;dbCmd = new OleDbCommand(dbConn, sql);adapter.SelectCommand = dbCmd;dataset =new DataSet("MyTable");adapter.Fill(dataset); dataset

recordrecordrecord


Step 2: Get records Method 2: DataReader

• Get records via SQL Select query– read-only access to the database

string sql;sql = " Select lastname, firstname" + " From employee" + " Order By lastname Asc, firstname Asc;";

OleDbCommand dbCmd;dbCmd = new OleDbCommand(sql, dbConn);

OleDbDataReader dbReader;dbReader = dbCmd.ExecuteReader(); data reader

recordrecordrecord


What is achieved?

• We have created a connection to a database.

• The connection is placed in the connection object.

• We have done a search by using a SQL-statement.

• The search was executed by using a command object.

• The result of the search was stored in a DataSet or as here a DataReader object.

• Now it is possible to get the data from this object for viewing, passing on to client or handle in other ways.

Example: ..\vwReader


Factory

• Problem: Unlike ODBC, knowledge of the implementation might be spread to many places in the application.That's because of the set of dedicated classes

• The solution is to use a factory (from factory pattern)• In ADO.NET 2.0, it is implementet in the framework.

• Use factory to create a Connection and a Command object

Example: \ReaderWithConfig


Use factory to create Connection and Command objects

• Here is no references in the source code to concrete classes• provider and connectionString may be placed in the config file

string provider = "System.Data.OleDb";string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=firma.mdb";DbProviderFactory factory= DbProviderFactories.GetFactory(provider);DbConnection conn=factory.CreateConnection();conn.ConnectionString=connectionString;conn.Open();Console.WriteLine(conn.State);

DbCommand command = factory.CreateCommand();command.CommandText = "SELECT * FROM employee";command.Connection = conn;DbDataReader dataReader = command.ExecuteReader();while (dataReader.Read()){

Console.WriteLine(dataReader[1]);}


Connection strings

• Connection strings depends on the provider and are not well documented

• Where to get help?– www.connectionstrings.com– www.carlprothman.net/Default.aspx?tabid=81– Providers documentation

• Visual Studio can generate some of them


.Config fil• Configuration files are used several places in .Net• i.e in ASP.NET for setting site properties and security• In Remoting for setting remote object properties• In ADO.NET to set provider and connection string• You might also use own application dependent properties• In a .Net application the config file should be called

<full assemblyname.config>, i.e test.exe.config or test.dll.config

• If you call it app.config in Visual Studio, it automatically copied and renamed when the application is built.

• In ASP.NET it must be named web.config. You might define one in different sub folders, but the security part (authentification e.g.) must be in the root folder of the site.


.config fil and ADO.NET

• Example of SQLExpress connection string:

<?xml version="1.0" encoding="utf-8" ?><configuration> <configSections> </configSections> <connectionStrings> <add name="TestConnection" connectionString="Data Source=pcm06463\sqlexpress;Initial Catalog=dbtest; User ID=test;Password=" providerName="System.Data.SqlClient" /> </connectionStrings></configuration>

Machine name

Nessesary for sqlexpress

Database

Internal name


Get provider and connection string

• From ConfigurationManager:

.... string provider = ConfigurationManager.ConnectionStrings["TestConnection"].ProviderName; string connStr = ConfigurationManager.ConnectionStrings["TestConnection"].ConnectionString;.....

Example: \ReaderWithConfig

Example: \ReaderWithConfig2


Parameters in SQL statements• Makes it possible to use the same SQL-statement in multible

situations.• But harder to debugge

string sqlString = "INSERT INTO datotest Values (@dato)"; SqlParameter sqlParam = new SqlParameter("@dato", System.Data.SqlDbType.DateTime); sqlParam.Value = DateTime.Now;

SqlCommand comm = _conn.CreateCommand(); comm.CommandText = sqlString; comm.Parameters.Add(sqlParam);

int result = comm.ExecuteNonQuery();ExecuteNonQuery is

used for insert, delete and update


Transaction - Definition

September 2010UCN T&B - CBH (FEN) Databases/Transactions

26

• 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?

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


Transactions

• In C#: Bank Transactions

Example: demos\Transactions\BankTransaction1


Multiple Databases

• Moving from (badly designed) database to another (better designed) database:

Example: demos\flytDataApp

ado.net 2.0

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