2: Exceptions and User Interfaces

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2 Exceptions and User Interfaces
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Overview

• User Interface Design
• Input Validation
• ImageList and ToolBar Controls
• ListView Control
• TreeView Control
• Structured Exception Handling

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2.1 User Interface Design

• User interface interacts with the user
• Analyze needs of users
• consider first time users, intermediate users,
  and experienced users
• Task-Based Approach
• design program in terms of tasks it will perform
• use-case scenarios describe interactions between
  program and users

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Use Case Display Detail Information

• Steps
• User enters full or partial description of an
  item
• Program locates and displays a list of items
  matching the description
• User selects an item from the list
• Program displays detailed information on the item

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Designing Forms

• Avoid clutter
• Use AcceptButton and CancelButton
• Restrict user's choices
• Use TabControl to partition groups of controls
• Use AutoScroll to add scroll bars to a form
• Use TreeView for hierarchical information
• Use ListView for multicolumn information

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Using Microsoft Office as a Model

• Menu tips
• Use drop-down menus
• Duplicate button commands with menu commands
• Include File and Help menu commands
• Include Edit, View, and Windows submenus
• Use elipsis (...) when launching a submenu
• Use standard shortcut keys
• Wizards
• lead users through series of predetermined steps
• simplify users' decisions

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2.2 Input Validation

• General principles
• try to anticipate and prevent errors
• error checking can be at each tier of a
  Three-tier application
• restrict user's ability to input bad data

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Four Validation Approaches

• Trap user keystrokes within each input control
• Prevent user from moving away from an input
  control when invalid input found
• Validate each control separately as user moves
  away
• Perform validation after user inputs values into
  all form controls

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Trapping Keystrokes

• KeyPress event
• e.Handled property
• e.KeyChar property
• Private Sub txtPatientID_KeyPress( _
• ByVal sender As System.Object, _
• ByVal e As System.Windows.Forms.KeyPressEventArgs
  ) _
• Handles txtPatientID.KeyPress
• Examples
• If Not Char.IsLetter(e.KeyChar) Then _
• e.Handled True
• If Char.IsControl(e.KeyChar) Then Exit Sub

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Keyboard Validation

• Hands-on example
• User enters a patient ID number
• Program validates using KeyPress and TextChanged
  event handlers

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Handling the Validating Event

• Leave event
• user moves away
• Validating event
• code checks contents of control
• Validated event
• fired after Validating event handler finds no
  error
• CausesValidation property
• determines whether a Validating event will be
  fired

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Name and Age Input

• Hands-on example
• User enters last name and age
• validates both values
• Person class checks Age range
• Displays events as they are fired

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ErrorProvider Control

• Hands-on tutorial
• Displays exclamation mark icon next to controls
  having invalid input
• Only one ErrorProvider needed on a single form

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Calling SetError

• Call SetError if you detect bad data in the
  control
• Private Sub txtLastName_TextChanged(ByVal sender
  _
• As Object, ByVal e As System.EventArgs) _
• Handles txtLastName.TextChanged
•  
• If txtLastName.Text.Length 0 Then
• errProvider.SetError(CType(sender, Control), _
• "Last name cannot be blank")
• End If
• End Sub

Pass a blank value to hide the error icon
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Handling Two Events

• Handle both TextChanged and Leave to reduce
  duplicate code
• Private Sub txtLastName_TextChanged(ByVal sender
  _
• As Object, ByVal e As System.EventArgs) _
• Handles txtLastName.TextChanged, _
• txtLastName.Leave
•   etc.

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2.3 ImageList and ToolBar Controls

• ImageList control stores images in a form
• ToolBar control is a container for buttons
• joined to an ImageList control
• ButtonClick event handler
• Private Sub tlbMain_ButtonClick( _
• ByVal sender As System.Object, _
• ByVal e As System.Windows.Forms._
• ToolBarButtonClickEventArgs) _
• Handles tlbMain.ButtonClick
• e.Button.Text property
• e.Button.Tag property

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Simple ToolBar

• Hands-on tutorial SimpleToolBar
• Demonstrates ImageList and ToolBar controls
• Output

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2.4 ListView Control

• Common in MS-Windows applications
• Flexibility and power
• rows and columns
• columns can be resized and moved
• click events
• text alignment options
• list items stored in a collection
• user can edit list items

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ListView (cont)

• Creating Column Headings
• use Detail view
• Columns property page
• Sample
• lvwContacts.Columns.Add("Name", 150,
  HorizontalAlignment.Left)

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ListView (cont)

• ListViewItem class
• defines appearance, behavior, and data associated
  with each row in a ListView
• table format, large icons, small icons, or list
• Creating a ListViewItem
• Adding columns
• using the SubItems collection
• Changing font styles

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Contacts ListView

• Hands-on tutorial ListView1
• Fills a ListView control with employee contact
  information

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Useful ListView Techniques

• Responding to ItemCheck event
• fired when user clicks on an item's CheckBox
• Selecting items
• fired when user selects one or more items
• Removing items
• call the Items.Remove method
• Other ListView features
• (tables listing properties and events)

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Removing ListView Items

• Hands-on example ListView2
• User selects items with checkboxes
• removes all selected items
• Editing capability is automatic

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2.5 TreeView Control

• Suited to hierarchical information
• Each element is a node
• type TreeNode
• belongs to the Nodes collection
• Travel Tree example

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Basic TreeView Techniques

• Add nodes
• With tvwTravel
• .Nodes.Add("Air")
• .Nodes.Add("Land")
• .Nodes.Add("Water")
• End With
• Expand the tree
• tvwTravel.ExpandAll() 'expand all subtrees
• aNode.Expand() 'expand one node's subtree

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Basic TreeView Techniques

• Insert multiple nodes
• Dim nodeArray(2) As TreeNode
• nodeArray(0) New TreeNode("Air")
• nodeArray(1) New TreeNode("Land")
• nodeArray(2) New TreeNode("Water")
• tvwTravel.Nodes.AddRange(nodeArray)
• Use BeginUpdate and EndUpdate to suspend and
  resume drawing of the tree while adding nodes

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Basic TreeView Techniques

• Get most recently selected node
• Dim aNode As TreeNode tvwTravel.SelectedNode
• Sort a tree
• tvwTravel.Sorted True
• Remove a node
• tvwTravel.Nodes.Remove(tvwTravel.SelectedNode)
• Remove a subtree
• tvwTravel.SelectedNode.Nodes.Clear()
• Insert node just before the selected node
• With tvwTravel
• Dim index As Integer .SelectedNode.Index
• .Nodes.Insert(index, New TreeNode("Space"))
• End With

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Contact Categories

• Hands-on tutorial TreeViewContacts
• Fills a TreeView with a list of categories for
  name and email contacts
• user can insert and remove entries
• tree maintains itself in alphabetical order

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Disk Directory TreeView

• Hands-on example DirectoryTree
• Displays a disk directory in a TreeView control
• System.IO.DirectoryInfo class
• Uses recursion
• method calls itself

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2.6 Structured Exception Handling

• A runtime error is the result of a thrown
  exception
• specifically, an unhandled exception
• severe error that would cause the program to
  behave erratically if not dealt with
• Two types of exceptions
• those caused by faulty programming logic
• those caused by events outside the programmer's
  control
• Better than ad-hoc error handling methods
• which were unreliable because they relied on
  skills of individual programmers

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What is Structured Exception Handling?

• Programs can respond to exceptions
• allow program to resume gracefully after an error
• called handling an exception
• Common Language Runtime (CLR)
• internal mechanism for exception handling
• Interaction between two parts of a program
• one part of a program can only detect an error
• another part knows what to do about it

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Base Exception Classes

• SystemException
• also known as runtime exceptions
• typically caused by programmer errors
• ApplicationException
• application programs derive new exception types
  from this class
• IOException
• caused by file and stream I/O errors

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Integer Conversion Example

• User inputs an integer into a TextBox
• Calls Convert.ToInt32
• which throws a System.FormatException

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IOException Class Hierarchy
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Try...Catch...Finally Statement

• Syntax
• Try
• try-block
• Catch optional filters
• catch-block
• additional Catch blocks...
• Finally
• finally-block
• End Try

Optional
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Alternate Execution Paths
The Finally block is optional
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Integer Conversion Example

• Handles exception thrown by ToInt32 method
• Try
• Dim n As Integer
• n Convert.ToInt32(txtInput.Text)
• stsMessage.Text "OK"
• Catch
• stsMessage.Text "That's not an Integer!"
• End Try

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Averaging Test Scores

• Hands-on example
• User inputs a series of test scores
• Program calculates average
• handles exceptions, identifies invalid score
• Error message example

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Handling File Exceptions

• Hands-on example
• User enters a file name
• Operations
• program attempts to read the file
• program attempts to write to the file
• throws exceptions

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Propagating Exceptions

• Happens when exception is not caught immediately
• Exception chains backward through method calls
• Becomes unhandled if no Catch clause is found

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Business Tier

• Throw exceptions at this level
• Catch exceptions at this level
• Do not generate any output to the user
• let the Interface layer do that

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Designing Your Own Exception Types

• Do this when...
• your exception needs to hold custom information
• exception class name is self-documenting
• Inherit from ApplicationException
• Example
• Public Class PropertyException
• Inherits System.ApplicationException
• Private mBadValue As String
• Sub New(ByVal message As String, _
• ByVal badVal As String)...
• Public ReadOnly Property BadValue() As String...

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Testing PropertyException

• Hands-on example
• User inputs property values, which are assigned
  to a Student object.
• Exceptions caught
• PropertyException
• InvalidCastException

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Exception-Handling Tips

• Prevent exceptions before they are thrown
• Entire Try block might not execute
• Catch specific exceptions first, catch general
  exceptions last
• Throw predefined .NET exceptions when possible
• Your exception classes should inherit from
  ApplicationException

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The End