lecture 11: programmatic database access with ado.net

Lecture 11:

Programmatic Database Access with ADO.NET

11-2MicrosoftIntroducing CS using .NETJ# in Visual Studio .NET

Objectives

“Programmatic database access typically involves executing SQL queries using classes from the language's framework. In .NET, the Active Data Objects (ADO) classes in the FCL are the primary means of database programming. ADO.NET is a vendor-neutral, object-oriented, SQL-based approach…”

• Architecture of ADO.NET• Basic database access• Application design• Updating a database• DataSets


Part 1

• Architecture of ADO.NET…


Relational technology

• ADO.NET is designed to access relational databases• Example:

– Sales database with customers, orders, and products


Architecture

• ADO.NET architecture based on data providers

– data providers encapsulate DB-specific details

Data Provider

ADO.NET

.NET Programmer

DB


Existing data providers

• .NET currently ships with 4 data providers:– one for Microsoft SQL Server– one for Oracle– one for older OLEDB technology (used for ADO, VB6)– one for older ODBC (Open Database Connectivity) technology

• More third-party providers are available…– Oracle's own provider: http://otn.oracle.com/tech/windows/odpnet/

– DB2: http://www7b.software.ibm.com/dmdd/downloads/dotnetbeta/

– MySQL: http://www.mysql.com/, http://crlab.com/mysqlnet/

– etc.


ADO.NET object model

• ADO.NET is an object-oriented approach• Classes are spread across a number of FCL namespaces

– some are provider-neutral, others are provider-specific

SQL Server

System.Data.Common

System.Data

System.Data.SqlClient

System.Data.OleDb

other DBs, e.g. MS Access

provider-neutral

ODBC (Open Database Connectivity)

System.Data.Odbc


Part 2

• Basic database access…


Overview of database access

• Three steps:

1. open connection to database

2. execute SQL to retrieve records / update DB

3. close connection


import System.Data.*;import System.Data.OleDb.*;

String sConnection;sConnection = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source=Sales.mdb";

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

MessageBox.Show(dbConn.get_State().toString());


String sConnection;sConnection = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source=Sales.mdb";

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

MessageBox.Show(dbConn.get_State().toString());

(1) Open connection

• Connections are opened based on connection string info– here we open a connection to a MS Access 2000 database– "Sales.mdb" must exist in same dir as .EXE (e.g. bin\Debug)

connection


Building connection strings

• Connection strings are vendor-specific• Connection strings are not well-documented• Where to turn for help?

– www.connectionstrings.com– www.able-consulting.com/ADO_conn.htm


(2) Retrieve records

• Retrieve records via SQL Select query– read-only access via DataReader & field names

String sql, fn, ln; Customer c;sql = "Select * From Customers Order By LastName Asc, FirstName Asc;";

OleDbCommand dbCmd;OleDbDataReader dbReader;

dbCmd = new OleDbCommand(sql, dbConn);dbReader = dbCmd.ExecuteReader();

while ( dbReader.Read() ) // retrieve records one-by-one… {

fn = String.valueOf(dbReader.get_Item("FirstName")); ln = String.valueOf(dbReader.get_Item("LastName")); c = new Customer(fn, ln); this.listBox1.get_Items().Add(c);

}

String sql, fn, ln; Customer c;sql = "Select * From Customers Order By LastName Asc, FirstName Asc;";

OleDbCommand dbCmd;OleDbDataReader dbReader;

dbCmd = new OleDbCommand(sql, dbConn);dbReader = dbCmd.ExecuteReader();

while ( dbReader.Read() ) // retrieve records one-by-one… {

fn = String.valueOf(dbReader.get_Item("FirstName")); ln = String.valueOf(dbReader.get_Item("LastName")); c = new Customer(fn, ln); this.listBox1.get_Items().Add(c);

}

data reader recordrecordrecord

field name of data value in current record

command connection


(3) Close connection

• Be sure to close reader and connection…– to flush pending updates (in general)– so others can access DB (connections are limited resources)

dbReader.Close();dbConn.Close();dbReader.Close();dbConn.Close();


Part 3

• Data-driven application design…


Data Tier design

• Recall N-Tier design… GUI.exe DataAccess


public class DataAccess{ private String sConnection;

public DataAccess(String filename) { this.sConnection = String.Format("Provider=...;Data Source={0}{1}", System.AppDomain.get_CurrentDomain().get_BaseDirectory(), filename); }

public java.util.ArrayList getCustomers() throws System.Exception { . . . }


public class DataAccess{ private String sConnection;

public DataAccess(String filename) { this.sConnection = String.Format("Provider=...;Data Source={0}{1}", System.AppDomain.get_CurrentDomain().get_BaseDirectory(), filename); }

public java.util.ArrayList getCustomers() throws System.Exception { . . . }


Guaranteed close?

• Ensure reader / DB closed via try-catch-finally pattern– and *note* that we want to catch .NET System.Exception objects, not

Java java.lang.Exception objects…

OleDbConnection dbConn = null;OleDbDataReader dbReader = null;

try { dbConn = new OleDbConnection(…); dbConn.Open(); . . .}catch(System.Exception ex) { System.Diagnostics.Debug.WriteLine("DB error: " + ex.get_Message()); throw new System.Exception("Data Error!", ex);}finally { // always executes whether we succeed or throw exception… if (dbReader != null) dbReader.Close(); if (dbConn != null) dbConn.Close();}

OleDbConnection dbConn = null;OleDbDataReader dbReader = null;

try { dbConn = new OleDbConnection(…); dbConn.Open(); . . .}catch(System.Exception ex) { System.Diagnostics.Debug.WriteLine("DB error: " + ex.get_Message()); throw new System.Exception("Data Error!", ex);}finally { // always executes whether we succeed or throw exception… if (dbReader != null) dbReader.Close(); if (dbConn != null) dbConn.Close();}


Part 4

• Updating a database…


Executing action queries

• Use action queries when you need to modify a DB– updates– inserts– deletes

• Execute action queries via ExecuteNonQuery method…


Basic idea

• Open, execute, and close:

String sConnection, sql;sConnection = "...";sql = "...";

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

int rows;dbConn.Open(); rows = dbCmd.ExecuteNonQuery(); // returns # of rows affected…dbConn.Close();

if (rows != 1) // sanity check to make sure it worked... throw new System.Exception("Query ran but failed to update DB?!");

String sConnection, sql;sConnection = "...";sql = "...";

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

int rows;dbConn.Open(); rows = dbCmd.ExecuteNonQuery(); // returns # of rows affected…dbConn.Close();

if (rows != 1) // sanity check to make sure it worked... throw new System.Exception("Query ran but failed to update DB?!");


Dynamic SQL

• Most of the time you'll need to build SQL dynamically– i.e. based on input values from the user

• Example:– delete the selected customer…

String fn, ln; Customer c;

c = this.listBox1.get_SelectedItem();fn = c.firstName;ln = c.lastName;

sql = "...";

String fn, ln; Customer c;

c = this.listBox1.get_SelectedItem();fn = c.firstName;ln = c.lastName;

sql = "...";


Dynamic string building

• Dynamic SQL requires dynamic string building• Example:

– build dynamic SQL to delete selected customer…

– don't forget the delimiters for strings & dates!

String sql;

sql = String.Format("Delete From Customers Where " + "FirstName='{0}' And LastName='{1}';", fn, ln);

String sql;

sql = String.Format("Delete From Customers Where " + "FirstName='{0}' And LastName='{1}';", fn, ln);


Example

• Delete selected customer…

private void listBox1_SelectedIndexChanged(…){ String fn, ln, sql; int rows; Customer c; c = (Customer) this.listBox1.get_SelectedItem(); if (c == null) return; // nothing selected…

fn = c.firstName; ln = c.lastName; sql = String.Format("Delete From Customers Where " + "FirstName='{0}' And LastName='{1}';", fn, ln); . . . dbConn.Open(); rows = dbCmd.ExecuteNonQuery(); // delete! dbConn.Close();

if (rows != 1) throw new System.Exception("Query ran but failed to delete?!");

this.listBox1.get_Items().Remove(c); // update GUI! MessageBox.Show("Deleted!");}


fn = c.firstName; ln = c.lastName; sql = String.Format("Delete From Customers Where " + "FirstName='{0}' And LastName='{1}';", fn, ln); . . . dbConn.Open(); rows = dbCmd.ExecuteNonQuery(); // delete! dbConn.Close();

if (rows != 1) throw new System.Exception("Query ran but failed to delete?!");

this.listBox1.get_Items().Remove(c); // update GUI! MessageBox.Show("Deleted!");}


Oops!

• Try to delete "O'Dahl, Kathie"…• What happens?


Escaping delimiters

• Most common DB programming mistake:– forgetting to escape delimiter characters…

• Solution?– just replace ' with 2 in a row, i.e. ' '


fn = c.firstName; ln = c.lastName;

fn = fn.Replace("'", "''"); ln = ln.Replace("'", "''"); sql = String.Format("Delete From Customers Where " + "FirstName='{0}' And LastName='{1}';", fn, ln);


fn = c.firstName; ln = c.lastName;

fn = fn.Replace("'", "''"); ln = ln.Replace("'", "''"); sql = String.Format("Delete From Customers Where " + "FirstName='{0}' And LastName='{1}';", fn, ln);


Part 5

• DataSets…


DataSets

• DataSets are an in-memory data structure– easily filled with data from a database– easily passed around– easily displayed in a GUI app

DataSet

Name Price Stock

Ants $ 0.49 5000

Birds $ 4.49 500

Cats $29.95 100

Dogs $79.95 20

DBCommand ConnectionDataAdapter

• DataSet mirrors the database– data forms a temporary table called

"Table" within DataSet"Table"


Filling a DataSet

• DataAdapter object is used to fill a DataSet…• Example:

– fill DataSet with all product data

sql = "Select * From Products Order By Name Asc;"; . . .DataSet ds;OleDbDataAdapter adapter;ds = new DataSet();adapter = new OleDbDataAdapter(dbCmd);

dbConn.Open(); adapter.Fill(ds);dbConn.Close();

sql = "Select * From Products Order By Name Asc;"; . . .DataSet ds;OleDbDataAdapter adapter;ds = new DataSet();adapter = new OleDbDataAdapter(dbCmd);

dbConn.Open(); adapter.Fill(ds);dbConn.Close();

"Table"

Name Price Stock

Ants $ 0.49 5000

Birds $ 4.49 500

Cats $29.95 100

Dogs $79.95 20


DataGrid display

• DataSet can be bound to DataGrid control for easy display– one line of code!

. . .this.dataGrid1.SetDataBinding(ds, "Table");

. . .this.dataGrid1.SetDataBinding(ds, "Table");


DataGrid is a powerful control

• By default, DataGrid is read/write– user can modify data– user can add rows– user can delete rows

• However, all changes are local to DataSet– to flush changes back to DB, reconnect and update…


Flushing changes back to database

• Reconnect, and apply adapter's Update() method– use CommandBuilder object to generate necessary SQL for you

// retrieve existing data set from grid…ds = (DataSet) this.dataGrid1.get_DataSource();

. . .

OleDbCommandBuilder cmdBuilder;cmdBuilder = new OleDbCommandBuilder(adapter);

dbConn.Open(); adapter.Update(ds); // this will throw exception if update(s) conflict… dbConn.Close();

// retrieve existing data set from grid…ds = (DataSet) this.dataGrid1.get_DataSource();

. . .

OleDbCommandBuilder cmdBuilder;cmdBuilder = new OleDbCommandBuilder(adapter);

dbConn.Open(); adapter.Update(ds); // this will throw exception if update(s) conflict… dbConn.Close();


Summary

• Databases are a critical component of most business apps• SQL is the standard programming language for databases

• Database programming is based on framework classes– in .NET, those classes are called ADO.NET– the more you know about SQL the better

lecture 11: programmatic database access with ado.net

Documents

data system

data providers data

microsoftintroducing

data source

existing data

basic database access

architecture of adonet

overview of database