Handling Exceptions and Stored Data

1
Chapter 11

• Handling Exceptions and Stored Data

Microsoft Visual C .NET From Problem Analysis
to Program Design
2
Chapter Objectives

• Learn about exceptions, including how they are
  thrown and caught
• Become aware of and use exception-handling
  techniques to include trycatchfinally clauses
• Explore the many exception classes and learn how
  to write and order multiple catch clauses
• Instantiate objects of the file stream classes
  and use their members to create and process text
  files

3
Chapter Objectives (continued)

• Be introduced to technologies used for accessing
  databases
• Learn how to access and update databases using
  ADO.NET classes

4
Exceptions

• Some circumstances are beyond programmers
  control
• You have assumed nothing unusual would occur
• Have probably experienced unhandled exceptions
  being thrown
• While you browsed Web pages
• While you were developing applications using C
• Unless provisions are made for handling
  exceptions, your program may crash or produce
  erroneous results
• Unhandled exception

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Exceptions (continued)
Normally do not want to try to debug application
while it is running
Click No
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Unhandled Exception

• Message displayed when you are creating console
  application and unhandled exception occurs

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Unhandled Exception (continued)

• Selecting DebuggtStart to run application in
  Visual Studio
• Yellow arrow marks the error (erroneous code
  highlighted)

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Raising an Exception

• Error encountered no recovery
• Raise or throw an exception
• Execution halts in the current method and the
  Common Language Runtime (CLR) attempts to locate
  an exception handler
• Exception handlerblock of code to be executed
  when a certain type of error occurs
• If no exception handler is found in current
  method, exception is thrown back to the calling
  method

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Bugs, Errors, and Exceptions

• Bugs differ from exceptions
• Bugs, also called "programmer mistakes," should
  be caught and fixed before application released
• Errorscan be created because of user actions
• Example
• Entering wrong type of data produces unhandled
  exception when ParseInt( ) called
• Details button in Visual Studio lists a stack
  trace of methods with the method that raised the
  exception listed first

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Bugs, Errors, and Exceptions (continued)
Stack trace
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Exception-Handling Techniques

• If event creates a problem frequently, best to
  use conditional expressions to catch and fix
  problem
• Execution is slowed down when CLR has to halt a
  method and find an appropriate event handler
• Exception-handling techniques are for serious
  errors that occur infrequently
• Exceptions classes integrated within the FCL
• Used with the trycatchfinally program
  constructs

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TryCatchFinally Blocks

• Code that may create a problem is placed in the
  try block
• Code to deal with the problem (the exception
  handler) is placed in catch blocks
• Catch clause
• Code to be executed whether an exception is
  thrown or not is placed in the finally block

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try // Statements catch
(ExceptionClassName exceptionIdentifier)
// Exception handler statements //
additional catch clauses finally //
Statements
Notice square brackets indicate optional entry
One catch clause required
finally clause optional
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TryCatchFinally Blocks (continued)

• Generic catch clause
• Omit argument list with the catch
• Any exception thrown is handled by executing code
  within that catch block
• Control is never returned into the try block
  after an exception is thrown
• Using a trycatch block can keep the program from
  terminating abnormally

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Use of Generic Catch clause
Example 11-2Uses a generic catch block
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What caused these exceptions to be thrown?
Never quite sure what causes the exception to be
thrown when a generic catch clause is used!
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Exception Object

• When an exception is raised, an object is created
  
• Object has properties and behaviors (methods)
• Catch clause may list an exception class
• Catch without exception type does not give
  you access to an object
• Base exception classException
• Message property returns a string describing
  exception
• StackTrace property returns a string that
  contains the called trace of methods

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Exception Object (continued)

• catch (System.Exception e)
• Console.Error.WriteLine("Problem with
  scores - "
• "Can not compute
  average")
• Console.Error.WriteLine(e.Message)

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Exception Classes

• ApplicationException and SystemException classes
  form the basis for runtime exceptions

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Exception Classes (continued)

• ApplicationException
• Derive from this class when you write your own
  exception class
• User program must throw the exceptionnot the CLR
  
• SystemException
• Most runtime exceptions derive from this class
• SystemException class adds no functionality to
  classes
• Includes no additional properties or methods

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SystemException Classes
Over 70 classes derive from the SystemException
class
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System.DivideByZeroException

• Derived class of System.ArithmeticException class
  
• Thrown when an attempt to divide by zero occurs
• Only thrown for integral or integer data types
• Floating-point operands do not throw an exception
• Result reported as either positive infinity,
  negative infinity, or Not-a-Number (NaN)
• Follows the rules from IEEE 754 arithmetic

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Filtering Multiple Exceptions

• Can include multiple catch clauses
• Enables writing code specific to thrown exception
• Should be placed from most specific to the most
  generic
• If Exception class is included, it should always
  be placed last

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Custom Exceptions

• Derive from the ApplicationException class

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Custom Exceptions (continued)

• Throwing a programmer-defined exception
• Exception object is instantiated when "an
  exceptional condition occurs
• Can be any condition, but should be one that
  happens infrequently
• After object is instantiated, object is thrown

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Throwing an Exception

• static double GetResults(double value1, double
  value2)
• if (value2 lt .0000001) // Careful comparing
  floating-point values
• // for
  equality
• FloatingPtDivisionException e new
• FloatingPtDivisionException
• ("Exception type Floating
  point division by zero")
• throw e
• return value1 / value2

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Input Output (IO) Exceptions

• System.IO.IOException
• Direct descendent of Exception
• Thrown when a specified file or directory is not
  found
• Thrown when program attempts to read beyond the
  end of a file
• Thrown when there are problems loading or
  accessing the contents of a file

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Input Output (IO) Exceptions (continued)
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File Streams

• System.IO namespace
• Several abstract classes for dealing with files
• Stream, TextWriter, and TextReader
• Stream classes provide generic methods for
  dealing with input/output
• IO.Stream class and its subclassesbyte-level
  data
• IO.TextWriter and IO.TextReaderdata in a text
  (readable) format
• StreamReader and StreamWriter derived classes of
  IO.TextWriter and IO.TextReader

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File Streams (continued)
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File Streams (continued)

• StreamWriter class for write data to text file
• Includes implementations for Write( ) and
  WriteLine( )
• StreamReader class to read or and from text files
  
• Includes implementations of Read( ) and ReadLine(
  )
• System.IO namespace
• using System.IO

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File Streams (continued)

• StreamWriter outputFile new StreamWriter("someOu
  tputFileName")
• StreamReader inputFile new StreamReader("someInp
  utFileName")
• outputFile and inputFile represents the file
  stream objects
• Actual file names are someOutputFileName and
  someInputFileNameinside double quotes
• Place file extension such as .dat, .dta, or .txt
  onto the end of actual file name when it is
  created

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File Streams (continued)

• Use Write( ) or WriteLine( ) with the
  instantiated stream object
• outputFile.WriteLine("This is the first line in a
  text file")
• Use Read( ) or ReadLine( ) with the instantiated
  stream object
• string inValue inputFile.ReadLine( )

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File Streams (continued)
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File Streams (continued)

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File Streams (continued)
Static methods are called using class name
instead of object name
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Writing Text Files

• Enclosed attempts to access text files inside
  trycatch blocks
• Constructor for StreamWriter class is overloaded
  
• Can include Boolean variable indicating whether
  file should be appended to or overwritten if it
  already exists
• New file is created by default
• Values are placed in the file in a sequential
  fashion

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Writing Text Files SayingGUI Application
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Writing Text Files SayingGUI Application
(continued)

• Three event-handler methods included
• Form-load event handler, an object of the
  StreamWriter class is instantiated  
• Included in a trycatch clause
• Button click event-handler method retrieves the
  string from the text box and writes the text to
  the file
• Also enclosed in a trycatch clause
• Form closing event closes the file and releases
  resources associated with file
• Also enclosed in a trycatch clause

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Writing Text Files SayingGUI Application
(continued)
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Writing Text Files SayingGUI Application
(continued)
If a path is not specified for the file name, the
bin\debug subdirectory for the current project is
used
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Reading Text Files

• StreamReader class enables lines of text to be
  read from a file
• Constructor for StreamReader is overloaded
• Can specify different encoding schema or an
  initial buffer size
• Can use members of parent or ancestor classes or
  static members of the File class
• To avoid programming catch for FileNotFoundExcepti
  on or DirectoryNotFoundException, call
  File.Exists(filename)

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Reading Text FilesFileAccessApp Application

• Read from text files in sequential fashion

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Reading Text FilesFileAccessApp Application
(continued)
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Database Access

• Example DBMS include SQL server, Oracle, and
  Access
• Many DBMSs store data in tabular format
• Data in tables are related through common data
  field keys
• Typically use a query language to program
  database access
• Structured query language (SQL)
• ActiveX Data Objects (ADO.NET).NET data access
  technology for accessing data in databases

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ADO.NET

• Includes number of classes that can be used to
  retrieve, manipulate, and update data in
  databases
• Can work with databases in a disconnect manner
• Database table(s) can be retrieved to a temporary
  file
• To retrieve data first you must connect to the
  database
• ADO.NET uses a feature called data providers to
  connect, execute commands, and retrieve results
  from a database

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Data Providers

• Microsoft SQL Server
• Applications using SQL Server 7.0 or later
• Oracle
• Applications using Oracle data sources
• Object Linking and Embedding Database (OLE DB)
• Applications that use Microsoft Access databases
• Open Database Connectivity (ODBC)
• Applications supported by earlier versions of
  Visual Studio

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Data Providers (continued)

• Classes is encapsulated into a different
  namespace by provider
• Four core classes make up each data provider
  namespace
• ConnectionEstablishes a connection to a data
  source
• CommandExecutes a command against a data source
• DataReaderPerforms a forward-only (sequential)
  access of the data in the data source
• DataAdapterPopulates a dataset and updates the
  database

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Connecting to the Database (Microsoft Access DBMS)

• Add using directive
• using System.Data.OleDb
• Instantiate an object of connection class
• Send connection string that includes the actual
  database provider and the data source (name of
  the database)
• string sConnection
• sConnection "ProviderMicrosoft.Jet.OLEDB.4.0"
  
• "Data Sourcemember.mdb"
• OleDbConnection dbConn
• dbConn new OleDbConnection(sConnection)
• dbConn.Open()

Enclose in try catch block
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Retrieving Data from the Database

• One way to retrieve records programmaticallyissue
  an SQL query
• Object of OleDbCommand class used to hold SQL
• string sql
• sql "Select From memberTable Order By
  LastName Asc, "
• "FirstName Asc" // Note the
  two semicolons
• OleDbCommand dbCmd new OleDbCommand()
• dbCmd.CommandText sql // set command SQL
  string
• dbCmd.Connection dbConn // dbConn is
  connection object

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SQL Queries

• Select From memberTable Order By LastName Asc,
  FirstName Asc
• Asterisk () selects all fields (columns) in
  database
• Can replace by field name(s)
• Asc(ascending) returns in ascending order by
  LastName duplicate last names ordered by first
  name
• Retrieves all rows (records)
• Where clause can be added
• Select PhoneNumber From memberTable Where
  FirstName 'Gary' AND LastName 'Jones'

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Retrieving Data from the Database

• Select StudentID, FirstName, LastName,
  PhoneNumber From memberTable

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Processing Data

• Can retrieve one record at a time in memory
• Process that record before retrieving another
• OR can store the entire result of the query in
  temporary data structure similar to an array
• Disconnect from the database
• ADO.NET includes data reader classes (by
  provider)
• Used to read rows of data from a database

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Retrieving Data Using a Data Reader

• OleDbDataReader class
• READ-ONLY access
• Forward retrieval (sequential access)
• Results returned as query executes
• Sequentially loop through the query results
• Only one row is stored in memory at a time
• Useful to accessing data from large database
  tables
• Declare an object of the OleDbDataReader class
• Call ExecuteReader( ) method

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Retrieving Data Using a Data Reader (continued)

• To position the OleDbDataReader object onto the
  row of the first retrieved query result, use
  Read( ) method of the OleDbDataReader class
• Read( ) also used to advance to the next record
• Think about what is retrieved as one-dimensional
  table consisting of the fields from that one row
• Fields can be referenced using actual ordinal
  index
• Fields can also be referenced using the table's
  field names as indexers to the data reader object

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Retrieving Data Using a Data Reader (continued)

• First call to dbReader.Read( ) retrieves first
  row
• dbReader0 refers to 1234
• dbReader1 refers to Rebecca
• dbReader"FirstName" also refers to "Rebecca"

Field name must be enclosed in double quotes when
used as indexers
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Retrieving Data Using a Data Reader (continued)

• Member aMember
• OleDbDataReader dbReader
• dbReader dbCmd.ExecuteReader( ) //
  dbCmdOleDbCommand object
• while (dbReader.Read( ))
• // retrieve records 1-by-1...
• aMember new Member(dbReader"FirstName".To
  String( ),
• dbReader"LastName".ToString( ))
• this.listBox1.Items.Add(aMember)
• dbReader.Close() // Close the Reader object
• dbConn.Close() // Close the Connection object

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Retrieving Data Using a Data Reader (continued)
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Retrieving Data Using a Data Reader (continued)
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Retrieving Data Using a Data Reader DBExample
Application
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Updating Database Data

• Data Reader enables read-only access to database
• Several ways to change or update database
• Can write Insert, Delete, and Update SQL
  statements and then execute those queries by
  calling OleDbCommand.ExecuteNonQuery( ) method
• Can instantiate objects of dataset and data
  adapter classes
• Use data adapter object to populate dataset
  object
• Adapter class has Fill( ) and Update( ) methods

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Updating Database Data (continued)

• Not required to keep a continuous live connection
• Can create temporary copy in memory of the
  records retrieved using a dataset
• Interaction between dataset and actual database
  is controlled through data adapter
• Each of the different data providers has its own
  dataset and data adapter objects
• System.Data.OleDbAccess database

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Using Datasets to Process Database Records

• Instantiate a connection object using connection
  string
• Not necessary to call Open( ) method
• Select records (and fields) from database by
  executing SQL Select
• Instantiate object of Dataset class (for a table)
• DataSet memberDS new DataSet()
• Instantiate an object of DataAdapter class
• OleDbDataAdapter memberDataAdap new
  OleDbDataAdapter( )

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Command Builder Class

• Class that automatically generates SQL for
  updates
• Must set the SelectCommand property of the
  OleDbDataAdapter class
• private OleDbCommandBuilder cBuilder
• cBuilder new OleDbCommandBuilder(memberDataAdap)
  
• memberDataAdap.SelectCommand dbCmd
• CommandBuilder object only used for datasets that
  map to a single database table

See slide 53 dbCmd set the SQL Select
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Filling the Dataset using the Data Adapter

• After instantiating objects of data adapter,
  dataset, and command builder classes
• Using data adapter Fill( ) method to specify name
  of table to use as the data source
• memberDataAdap.Fill(memberDS, "memberTable")
• To show contents of table, presentation user
  interface layer is needed
• Grid control works well

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Creating a DataGrid to Hold the Dataset

• Place DataGrid control object on Windows Form
• DataGrid object can be selected from ToolBox
• Able to navigate around in data grid
• Can make changes by editing current records
• Can insert and delete new records
• To tie DataGrid object to dataset,
  SetDataBinding( ) method is used
• this.dataGrid1.SetDataBinding(memberDS,
  "memberTable")

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Updating the Database

• Load the database into a DataGrid object and make
  changes
• Flush the changes back up to live database using
  the Update( ) method of DataAdapter class
• memberDataAdap.Update(memberDS, "memberTable")

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Updating the Database DataSetExample Application
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Updating the Database DataSetExample Application
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Chapter Summary

• Exceptions
• How to throw and caught exceptions
• Exception-handling techniques
• trycatchfinally clauses
• Exception classes
• Use multiple catch clauses
• File stream classes for sequential access
• ADO.NET classes to access database records