Coding ADO.NET Objects: Connection, Command, DataReader

A Simplified View of ADO.Net Objects Ado.Net Data Provider Connection Adapter Command Reader Dataset Data Consumer WinForm WebForm SQL Updates Results of SQL Selects

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.

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, 2011: –= "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\SalesDB2011.accdb“ For SQL Server Express –"Data Source=David-PC\sqlexpress;Initial Catalog=testSQL08;Integrated Security=True"

Connection Object Example 1 – Jet Database: –string strConn ="Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\\salesDB.mdb"; – OleDbConnection objConn = new OleDbConnection(strConn); –objConn.Open(); Example 2 – Access 2011: – string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; –OleDbConnection objConn = new OleDbConnection(strConn); –objConn.Open(); Object Browser: System.Data.OleDB/oledbConnection/New Basic Methods: –Open, Close –BeginTransaction

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. –ExecuteScalar: Executes the query, and returns the first column of the first row in the result set returned by the query. Typically used to execute a SQL aggregate function such as SUM, MAX, MIN, etc. –ExecuteNonQuery: Change the data in a database without using a DataSet by executing SQL’s UPDATE, INSERT, or DELETE statements.

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.

ExecuteReader Example string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); objConn.Open(); string strSQL = "select * from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL,objConn); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); if (objDataReader.HasRows) { MessageBox.Show("Data reader contains rows"); } else { MessageBox.Show("No row reurns"); } Note: No NEW in the Dim statement when define a dataReader object.

Import NameSpace The “using” statement must appear before all other declarations in a file and cannot appear inside a class or module declaration. –using System.Data.OleDb;

Read Records in a DataReader string Results=""; while (objDataReader.Read()==true) { Results+=objDataReader["cid"] + " " + objDataReader["Cname"] + Environment.NewLine; } textBox1.Text = Results.ToString(); Note: Use index to read a column - objDataReader[0] Note: The Read method will (1) read a record, (2) advance pointer, (3) return true/false to indicate if more records exist.

DataReader Example string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); objConn.Open(); string strSQL = "select * from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL,objConn); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); string Results=""; while (objDataReader.Read()==true) { Results += objDataReader["cid"] + " " + objDataReader["Cname"] + Environment.NewLine; } textBox1.Text = Results.ToString();

Add Items from a DataReader to a Listbox string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select cid from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); objConn.Open(); OleDbDataReader objDataReader; objDataReader=objComm.ExecuteReader(); while (objDataReader.Read()==true) { listBox1.Items.Add(objDataReader["cid"]); // listBox1.Items.Add(objDataReader.GetString(0)); } Note1: Listbox is not bound. 2. SelectedItem

Display Selected Customer Info in Textbox private void listBox1_SelectedIndexChanged(object sender, EventArgs e) { string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select * from customer where cid='" + listBox1.SelectedItem + "'"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); objConn.Open(); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); objDataReader.Read(); textBox1.Text = objDataReader["Cname"].ToString(); textBox2.Text = objDataReader["rating"].ToString(); objConn.Close(); }

Using a Parallel ArrayList to Store Cname and Rating ArrayList –Add –Clear –Remove –IndexOf Must add this reference: –using System.Collections;

Using a Parallel ArrayList ArrayList CnameList = new ArrayList(),RatingList = new ArrayList(); private void Form1_Load(object sender, EventArgs e) { string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select * from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); objConn.Open(); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); while (objDataReader.Read() == true) { listBox1.Items.Add(objDataReader["cid"]); CnameList.Add(objDataReader["cname"]); RatingList.Add(objDataReader["rating"]); } objConn.Close(); }

Use Listbox SelectedIndex private void listBox1_SelectedIndexChanged(object sender, EventArgs e) { textBox1.Text = CnameList[listBox1.SelectedIndex].ToString(); textBox2.Text = RatingList[listBox1.SelectedIndex].ToString(); }

Use SQL to Retrieve Summaries string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select count(cid) as custCount from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); objConn.Open(); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); if (objDataReader.Read() == true) { MessageBox.Show(objDataReader["custCount"].ToString()); } objConn.Close(); Note: Alias select count(cid) as custCount

Using Command’s ExecuteScalar to Run SQL Aggregates string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select count(cid) as custCount from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); objConn.Open(); textBox1.Text = objComm.ExecuteScalar().ToString(); objConn.Close();

ExecuteScalar Example private void radioButton_CheckedChanged(object sender, EventArgs e) { string strSQL; if (radioButton1.Checked) { strSQL = "select count(pid) as prodCount from product;"; } else if (radioButton2.Checked) { strSQL = "select max(price) as maxPrice from product;"; } else { strSQL = "select min(price) as minPrice from product;"; } string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); OleDbCommand objComm = new OleDbCommand(strSQL, objConn); objConn.Open(); textBox1.Text = objComm.ExecuteScalar().ToString(); objConn.Close(); }

Testing for Null: DBNull.Value if ( objDataReader["rating"]==DBNull.Value) { textBox2.Text = "NA"; } else { textBox2.Text = objDataReader["rating"].ToString(); }

Error Handling string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select cid from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); try { objConn.Open(); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); while (objDataReader.Read() == true) { listBox1.Items.Add(objDataReader["cid"]); } catch (System.Exception ex) { MessageBox.Show(ex.Message); } objConn.Close(); Note: Where do declare objConn?

Command Object’s ExecuteNonQuery Method To run SQL: –Insert –Delete –Update The ExecuteNonQuery method also returns a value indicating the number of records affected by the SQL statement.

Use ExecuteNonQuery to Insert A New Record Create unbound text boxes to enter new customer record. string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQLInsert; strSQLInsert = "Insert into Customer values ('"; strSQLInsert += textBox1.Text + "','" + textBox2.Text + "','"; strSQLInsert += textBox3.Text + "','" + textBox4.Text + "')"; OleDbCommand objCommInsert= new OleDbCommand(strSQLInsert, objConn); objConn.Open(); MessageBox.Show(strSQLInsert.ToString()); try { objCommInsert.ExecuteNonQuery(); } catch (Exception ex) { MessageBox.Show(ex.Message); } objConn.Close();

Use ExecuteNonQuery to Delete A New Record 1.Create a listbox with CIDs 2.Delete the selected record and remove the CID from the listbox. string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "delete from customer where cid = '" + listBox1.SelectedItem + "'"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); try { int affectedRecords; objConn.Open(); affectedRecords= objComm.ExecuteNonQuery(); MessageBox.Show(affectedRecords.ToString() + "records deleted"); listBox1.Items.RemoveAt(listBox1.SelectedIndex); } catch (System.Exception ex) { MessageBox.Show(ex.Message); } objConn.Close();

Use ExecuteNonQuery to Update A New Record 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.

create CID listbox: private void Form4_Load(object sender, EventArgs e) { string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "select cid from customer;"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); try { objConn.Open(); OleDbDataReader objDataReader; objDataReader = objComm.ExecuteReader(); while (objDataReader.Read() == true) { listBox1.Items.Add(objDataReader["cid"]); } catch (System.Exception ex) { MessageBox.Show(ex.Message); } objConn.Close(); }

Update customer rating: private void button1_Click(object sender, EventArgs e) { string newRating; if (radioButton1.Checked) { newRating = "A"; } else if (radioButton2.Checked) { newRating = "B"; } else { newRating = "C"; } string strConn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\\SalesDB2011.accdb"; OleDbConnection objConn = new OleDbConnection(strConn); string strSQL = "Update customer set rating = '" + newRating + "' where cid='" + listBox1.SelectedItem + "'"; OleDbCommand objComm = new OleDbCommand(strSQL, objConn); try { objConn.Open(); objComm.ExecuteNonQuery(); } catch (System.Exception ex) { MessageBox.Show(ex.Message); } objConn.Close(); }

Working with ODBC Data Source Enter: using System.Data.Odbc; Use: –OdbcConnection: Use DSN Example of connection string: – string strConn = "DSN=test32SQLDS"; –OdbcCommand –OdbcDataReader

using System.Data.Odbc; private void Form5_Load(object sender, EventArgs e) { string strConn = "DSN=test32SQLDS"; OdbcConnection objConn = new OdbcConnection(strConn); string strSQL = "select cid from customer;"; OdbcCommand objComm = new OdbcCommand(strSQL, objConn); try { objConn.Open(); OdbcDataReader objDataReader; objDataReader = objComm.ExecuteReader(); while (objDataReader.Read() == true) { listBox1.Items.Add(objDataReader["cid"]); } catch (System.Exception ex) { MessageBox.Show(ex.Message); } objConn.Close(); }

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.

Setting a Field to Null with a Update Statement string strSQL = "Update customer set city = null where cid='" + listBox1.SelectedItem + "'";

Working with SQL Server Database Enter: using System.Data.SqlClient; Use: –SqlConnection: –Example of connection string: – string strConn = "Data Source=David- PC\\SQLEXPRESS;Initial Catalog=TestSQLServer;Integrated Security=True;Pooling=False"; –SqlCommand –SqlDataReader

Working with SQL Server Data Source private void Form5_Load(object sender, EventArgs e) { string strConn = "Data Source=David-PC\\SQLEXPRESS;Initial Catalog=TestSQLServer;Integrated Security=True;Pooling=False"; SqlConnection objConn = new SqlConnection(strConn); string strSQL = "select cid from customer;"; SqlCommand objComm = new SqlCommand(strSQL, objConn); try { objConn.Open(); SqlDataReader objDataReader; objDataReader = objComm.ExecuteReader(); while (objDataReader.Read() == true) { listBox1.Items.Add(objDataReader["cid"]); } catch (System.Exception ex) { MessageBox.Show(ex.Message); } objConn.Close(); }