1. Coding ADO.NET Objects: Connection, Command, DataReader

2. A Simplified View of ADO.Net Objects
Data Provider
Data Consumer
Adapter
Dataset
WinForm
Connection
Command
WebForm
Reader

3. Connection with a Connection Object
A connection object represents a unique session with a data source.
Property:
Connection string: string used to open a database.
Data source, OLE DB provider, password, if any, security, etc.
Methods:
Use the Open/Close method to open/close a connection.

4. Connection String
Containing information about database, OLE DB provider, password, if any, security, etc.
For Jet database:
="Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb“
For Access 2007:
= "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\SalesDB2007.accdb“
For SQL Server Express
"Data Source=DCHAO100L\sqlexpress;Initial Catalog=testSQL08;Integrated Security=True"

5. Connection Object Basic Methods: Open, Close Example 1: Example 2:
dim strConn as string ="Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb"
dim objConn as new OledbConnection(strConn)
objConn.open()
Example 2:
dim objConn as new OledbConnection()
objConn.ConnectionString=strConn
Object Browser: System.Data.OleDB/oledbConnection/New
Basic Methods:
Open, Close

6. Command Object
The command object allows us to execute a SQL statement.
Properties:
CommandType: Text, stored procedure, tableDirect
CommandText: SQL statement, procedure name, table name
Connection
Other properties:
Parameters
Basic Methods:
ExecuteReader: Creates a DataReader object that contains the results of the query.
ExecuteNonQuery: Change the data in a database without using a DataSet by executing SQL’s UPDATE, INSERT, or DELETE statements.

7. DataReader Object It is read-only and forward-only cursor.
Basic Methods:
Read: Reads the current record and advances the pointer to the next record. It returns False if no more record in the DataReader.
Close: Closes the dataReader.
Property:
HasRows – Boolean, indicates whether this System.Data.Common.DbDataReader contains one or more rows.

8. ExecuteReader Example
dim strConn as string= "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\SalesDB2007.accdb“
dim objConn as new OledbConnection(strConn)
dim strSQL as string = "select * from customer;"
dim objComm as new OledbCommand(strSQL,objConn)
dim Results as string
objConn.open()
dim objDataReader as oledbDataReader
objDataReader=objComm.executeReader()
Note: No NEW in the Dim statement when define a dataReader object.

9. Import NameSpace
The Imports statement must appear before all other declarations in a file and cannot appear inside a class or module declaration.
Imports System.Data.OleDb
Public Class Form1
Note: To import a nameSpace for all forms/modules in a project:
Use Project Property Page: Imports

10. Read Records in a DataReader
dim Results as string
do while objDataReader.Read()=true
Results+=objDataReader("cid") + “ “ + objDataReader("Cname") + vbCrLF
loop
Textbox1.text=Results
Note: objDataReader.Item(0)
Note: objDataReader.Item(“cid”)
Note: The Read method will (1) read a record, (2) advance pointer, (3) return true/false to indicate if more records exisit.

11. Add Items from a DataReader to a Listbox
Dim strConn As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\SalesDB2007.accdb“
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select cname from customer;"
Dim objComm As New OleDbCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
LISTBOX1.Items.Add(objDataReader("cid"))
Loop

12. Display Selected Customer Info in Textbox
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
Dim strConn As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\SalesDB2007.accdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select * from customer where cid='" & ListBox1.SelectedItem & "';"
Dim objComm As New OleDbCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
objDataReader.Read()
TextBox1.Text = objDataReader("cname")
TextBox2.Text = objDataReader("rating")
objDataReader.Close()
End Sub

13. Testing for Null IsDBNull Function
If objDataReader.Read = True Then
If IsDBNull(objDataReader("custCount")) Then
TextBox1.Text = 0
Else
TextBox1.Text = objDataReader("custCount").ToString
End If
Note: The keyword “Nothing” tests if an object is null.

14. Null Value Example
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
Dim strConn As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\SalesDB2007.accdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select * from customer where cid = '" & ListBox1.SelectedItem & "'"
Dim objComm As New OleDbCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
objDataReader.Read()
TextBox1.Text = objDataReader("Cname")
TextBox2.Text = objDataReader("rating") ‘ Statement won’t work if Rating is null in database
End Sub

15. Error Handling Try objConn.Open() Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
Results += objDataReader("cid") + " " + objDataReader("Cname") + vbCrLf
Loop
TextBox1.Text = Results
objConn.Close()
Catch except As System.Exception
MessageBox.Show(except.Message)
End Try

16. Possible Database Errors
SQL syntax errors
Database connection not open
Null
Violate database constraints
Referential integrity
Field data type and length
Primary key constraint
Invalid data – database may have validation rules.

17. Use Command Object’s ExecuteNonQuery to Insert A New Record
Create unbound text boxes to enter new record.
Add an Insert button with the following handler
Dim strSQLInsert As String
strSQLInsert = "Insert into Customer values ('"
strSQLInsert = strSQLInsert & TextBox1.Text & "','" & TextBox2.Text & "','"
strSQLInsert = strSQLInsert & TextBox3.Text & "','" & TextBox4.Text & "')"
Dim objCommInsert As New OleDbCommand(strSQLInsert, objConn)
objCommInsert.ExecuteNonQuery()

18. ExecuteNonQuery Example
dim strSQLUpd as string="Update customer set rating = ‘A’”
strSQLUpd=strSQLUpd & " where cname='" & CnameList.selectedItem.text & "'“
Dim objCommUpd As New OleDbCommand(strSQLUpd, objConn)
objCommUpd.ExecuteNonQuery()
Note: How many records affected by this command?
ExecuteNonQuery method returns an integer indicating the number of record affected by the update command.
Messagebox.Show(objCommUpd.ExecuteNonQuery())

19. Demo Create a project that do the following tasks:
Use a DataReader to retrieve customer IDs and populate a listbox.
Select a new rating from radio buttons for the selected customer.
Update customer’s rating using the ExecuteNonQuery method of a Command object.
, fa n

20. Declare OleDB objects and create listbox
Imports System.Data.OleDb
Public Class Form3
Inherits System.Windows.Forms.Form
Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select cid from customer;"
Dim objComm As New OleDbCommand(strSQL, objConn)
Private Sub Form3_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
ListBox1.Items.Add(objDataReader("cid"))
Loop
objConn.Close()
End Sub

21. Update customer rating
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
objConn.Open()
Dim newRating As String
If RadioButton1.Checked = True Then
newRating = "A"
ElseIf RadioButton2.Checked Then
newRating = "B"
Else
newRating = "C"
End If
Dim strSQLUpd As String = "Update customer set rating = '" & newRating & "'"
strSQLUpd = strSQLUpd & " where cid='" & ListBox1.SelectedItem & "'"
Dim objCommUpd As New OleDbCommand(strSQLUpd, objConn)
objCommUpd.ExecuteNonQuery()
objConn.Close()
End Sub

22. Working with ODBC Data Source
Imports System.Data.Odbc
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim strConn As String = "DSN=MySalesDB"
Dim objConn As New OdbcConnection(strConn)
Dim strSQL As String = "select cname from customer;"
Dim objComm As New OdbcCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OdbcDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
ListBox1.Items.Add(objDataReader("cname"))
Loop
End Sub
End Class

23. Working with SQL Server Data Source
Imports System.Data.SqlClient
Public Class Form9
Private Sub Form9_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim strConn As String = "Data Source=DCHAO100L\sqlexpress;Initial Catalog=testSQL08;Integrated Security=True"
Dim objConn As New SqlConnection(strConn)
Dim strSQL As String = "select * from customer;"
Dim objComm As New SqlCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As SqlDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
ListBox1.Items.Add(objDataReader("cname"))
Loop
End Sub
End Class