Advanced Interactive Programming Objectoriented Programming Error Handling

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Advanced Interactive ProgrammingObject-oriented
ProgrammingError Handling
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Outline

• Collection
• Group discussions on OOP
• Discussion on OOP concepts
• Learning from each other
• Error handling
• Compile errors
• Run time errors
• Logic errors

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Collection
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Collection Concept

• A way of grouping a set of related items
• A collection usually consists of a group of
  objects of different types, known as its elements
• Example collections include set, array and list
• Some are ordered and others unordered
• Iteration and enumeration utility classes
• Provide mechanisms to walk through all objects in
  a collection
• Provide methods to manipulate elements of a
  collection

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Forms and Controls Collections

• VB provides default form and control collection
  objects for managing and accessing their elements
• Collection members have index values
• These collections are 0-based, i.e. the index
  value starts from 0
• Collections have a Count property

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Example Forms Collections
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The Generic Collection Class

• Unlike the default Forms and Controls collections
• Used for grouping programmers own objects
• Generic collections are 1-based
• The generic collection has a single property
  Count
• It must first be declared and bound to a variable
  name

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Generic Collection Methods

• The Add method is used to add members to the
  collection
• The Remove method is used to remove members from
  the collection
• The Item method is used to return single members
  of the collection when furnished an index value

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A Generic Collection Example
Set Queue New Collection Set Account New
AccountCls Account.AssignOwner (Fred) Queue.Add
(Account) Set Account New AccountCls Account.Ass
ignOwner (Mary) Queue.Add (Account) Print
Queue.Count 2 Print Queue(1).Owner Fred Print
Queue(2).Owner Mary
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Collection in OOP

• Very common because we need to manage a set of
  objects they might all have the same class or
  derived from different subclasses of the same
  superclass.
• More functionalities, for example, performing
  type or existence testing when adding an object
  or retrieving objects in terms of other
  attributes keys, the textual index.
• Extensive use of Iterator or Enumerator class to
  manipulate individual objects

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References

• See MSDN library
• Topic Form Object, Forms Collection and Controls
  Collection
• Collection Object in Visual Basic
• Using Properties and Collections to Create Object
  Models

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Seminar and Discussion on Object Oriented
Programming
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Schedule

• Grouping
• Group discussions
• Question answering and explanation if necessary
• Approx. 30 minutes

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Topic One

• Explain and discuss what you understand by the
  following concepts with your group member. It
  should include their relationships, usage in VB,
  benefits, etc.
• Object
• Class
• Interface
• Class libraries

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Topic Two

• Explain and discuss what you understand by the
  following concepts with your group member. It
  should include their relationships, usage in VB,
  benefits, etc.
• Encapsulation
• Inheritance
• Class hierarchy
• Implementation inheritance
• Interface inheritance
• Polymorphism
• Implementation inheritance based polymorphism
• Interface inheritance based polymorphism

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Topic Three

• Explain and discuss what you understand by the
  following concepts with your group members.
• Procedural programming
• Visual programming
• Event-oriented programming
• Object-oriented programming
• Are they all the same or different thing?

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• Error Handling

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Error Types

• Compile (including syntax) errors, which result
  from incorrectly constructed code.
• Such as incorrect usage of keywords, punctuation,
  terms, conventions, operators, control
  structures, etc.
• Runtime errors, which occur while the application
  is running when a statement attempts an operation
  that is impossible to carry out.
• Statements are syntactically correct
• Must run before being detected
• Logic errors, which occur when an application
  doesn't perform the way it was intended.

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Error Handling Purposes

• Make programs robust and fault-tolerant
• Recover from fatal errors to avoid consequences
• Terminate programs gracefully if not possible
  to recover
• Different strategies are used to tackle different
  types of errors
• Syntax checker and compiler for compile errors
• Error handling statements for run-time errors
• Debugging tools for logic errors

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Handling Runtime Errors
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Using On Error Statement

• Turns the error trapping process on
• Alerts Visual Basic to watch for any run-time
  errors in the code listed below the On Error
  statement
• If an error occurs, the On Error statement sends
  program control to the error-handling routine
• Turns the error trapping process off
• Nested On Error statements

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On Error Statement Syntax

• On Error GoTo line
• Line is either a line label or a line number that
  identifies the position of the procedures error
  handler
• Line label or number must be in the same
  procedure
• The error handler is the set of instructions that
  tells the application how to handle the trapped
  errors
• On Error GoTo 0
• Disable an error trap
• Raised error are still detected but not directed
  to the error handler

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Procedure Structure with On Error Trapping

• Private Sub statement
• Dim statements, etc.
• On Error GoTo line
• statements
• Exit Sub
• line
• error-handling instructions
• End Sub

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Error Handling Example
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The Err Object

• An intrinsic object with global scope. There is
  no need to create an instance of it in your code.
• When a run-time error occurs, the properties of
  the Err object are filled with information that
  uniquely identifies the error and information
  that can be used to handle it.
• The properties of the Err object are set by the
  generator of an error Visual Basic, an object,
  or the programmer.
• The Err object's properties are reset to zero or
  zero-length strings ("") after an Exit Sub, Exit
  Function, Exit Property or Resume Next statement
  within an error-handling routine

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The Err Object

• Properties and methods

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Internal Runtime Error Codes
Code Message 3 Return without
GoSub 5 Invalid procedure call 6 Overflow 7
Out of memory 9 Subscript out of
range 10 This array is fixed or temporarily
locked 11 Division by zero 13 Type mismatch
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Using the Err Object
Different errors
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Different code
Detailed information
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Resume Statement

• Used to specify where in a procedure, function or
  class execution continues after an error is
  handled
• Provides three
• choices

On Error Goto label statements Exit Sub /
Function label error handling code
somelabel
An error raised here
Resume Next statement
Resume statement
Resume somelabel statement
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Call Stack

• A call stack is a list of procedures that were
  called in order to get to the current procedures

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Error Handling in a Call Stack

• Error handling become complex when multiple calls
  are made
• Programmers must decide which procedure handles
  which errors
• An error trap and an error handler must be used
  in pairs. If a error is raised but no error
  handler, a fatal error happens and the program
  terminate disgracefully

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Rethrowing Errors

• Errors can be rethrowed (reraised) after being
  trapped
• Many reasons, mainly different strategies for
  different errors
• Using the Raise method of the err object

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Rethrowing Error Example
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Handling Logic Errors
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Preventing Errors

• Following conventions - prefixes
• Use comments and explicit variable declarations
• Use the syntax checker
• Proper scoping
• etc.

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Break Mode

• VB IDE operates in different modes
• Break mode, design mode and run mode
• Break mode for debugging purpose
• Execution suspended
• Variable values and program logic can be checked
• Edit code and continue execution

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Break Mode

• Many ways to start break mode
• Stop statement in code
• Debug object Assert method
• Automatically start the Immediate Window

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Using Immediate Window

• Start Immediate Window, click the Immediate
  Window item in the View Menu at design mode
• View debugging output while the program is
  running.

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Immediate Window

• Evaluate any expression, variable, or object and
  view the value that is returned
• Query the value of a variable or a property while
  running an application. While execution is
  halted, assign or change the variable a new value
  as you would in code.
• Call and execute procedures

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Immediate Window Example
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The Debug Object

• Debug object Assert method for testing conditions
  
• e.g. Debug.Assert(xgt0 And xlt9)
• Use the Print method of Debug to display the
  values of expressions and variables in the
  Immediate Window
• Testing purpose only, will not be compiled in the
  executable

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Alternative Debugging Strategies

• Trace debugging using Print or MsgBox
  statements to trace and verify the execution flow
• Log files using Write or Print statements
• Problems
• Manually evaluating and checking expressions and
  values of programs in Immediate Window is not
  scalable and manageable for complex applications

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The VB Debugger

• Debugging tools cannot diagnose or fix errors,
  but help analyze
• How execution flows from one part of the
  procedure to another
• How variables and property settings change as
  statements are executed
• A debugger help you look inside your application
  to determine what happens, what and where
  something went wrong

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Debugging Tools in VB Debugger
Debug Tool Bar Debug Menu
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Using Step Into, Over, etc.

• Select the Step Into/Over in Debug menu to step
  through code one statement or one procedure at a
  time
• Display variable values by positioning mouse
  pointer over the variable
• Use Debug.print to print variable or property
  values in the Immediate Window

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Step through Bank Example
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Working with Break Points

• Defines a line in the Code window where Visual
  Basic suspends execution of the application
• Setting a break point by selecting a line and the
  Toggle Breakpoint item in the Debug menu or
  directly clicking margin indicator bar
• Break Points remain in your code until you clear
  them
• To clear a break point, select it and press F9 or
  the Toggle Break Point button
• Clear all Break Points by pressing CtrlShiftF9

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Example
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Locals Window

• Select Locals Window in View menu to start the
  Locals Window
• Displays the current value and data type of all
  local variables
• You can inspect or change these values at Break
  Points

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Locals Window Example
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Watch Window

• Add either at design time or in break mode
• Monitor both local and module variables
• Watch different behaviour

Use the Add Window
Use Drag and drop
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Quick Watch

• Highlight an expression or a variable
• Select Quick Watch in Debug menu
• Click Add to add the expression to Watch Window

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Debugger and Error Handlers

• Hard to debug code when error handlers are
  involved
• Disable error handlers using Break on All Errors
  in the General tab
• Any run time error also places IDE in break mode

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Summary

• Collection
• OOP seminar
• Error handling