Chapter 6 Debugging and error handling

1
Debugging and Error Handling
Chapter 6
Ebook: Beginning Visual C# 2010, chapter 7
Reference: CSharp How to Program, part D
Contents
 Introduction
 Debugging
 Error Handling
Slide 2
Introduction
 Errors in program: a program can have three types of
errors:
 Compile-time errors: The compiler will find syntax errors and
other basic problems
 Run-time errors: A problem can occur during program
execution, such as trying to divide by zero, which causes a
program to terminate abnormally
 Logical errors: A program may run, but produce incorrect
results, perhaps using an incorrect formula
1-3
Debugging
 Debugging is the process of finding and correcting logic
errors in applications
 Applications are executed in two ways: with debugging
enabled (F5) or without debugging enabled (Ctrl+F5)
 Techniques
 Debugging in nonbreak mode
 Outputting debugging information: Console.WriteLine(),
Debug.WriteLine()
 Debugging in break mode

 Using Breakpoints
 Some windows in Debug\Windows: Autos, Locals, Watch
Slide 4
Error Handling
 An exception is an indication of a problem that occurs
during a program's execution
 format error,
 arithmetic overflow,
 out-of range array subscripts,
 division by zero,
 invalid method parameters
 running out of available memory,
 …
 Your program should be able to handle these exceptional
situations. This is called exception handling
Slide 5
Error Handling (cont.)





int tu = Convert.ToInt32( txtNumerator.Text );
int mau = Convert.ToInt32( txtDenominator.Text );
int result = tu / mau;
lblOutput.Text = result.ToString();
Slide 6
2
Exception Handling: try…catch

try{
// code that may cause exception
}
catch ( ExceptionTypeA e ){
// statement to handle errors occurring
// in the associated try block
}
catch ( ExceptionTypeB e ){
// statement to handle errors occurring
// in the associated try block
}

Slide 7
Exception Handling:
try…catch finally

try{
// code that may cause exception
}
catch ( ExceptionTypeA e ){
// statement to handle errors occurring
// in the associated try block
}
//…
finally{
// statements always excuted
}

Slide 8
Exception Types

Slide 9
 Some common exception classes
Exception Class

Cause

DivideByZeroException

An attempt was made to divide by zero.

FormatException

The format of an argument is wrong.

IndexOutOfRangeException

An array index is out of bounds.

InvalidCastExpression

An attempt was made to cast to an invalid class.
OutOfMemoryException

Not enough memory to continue execution.

StackOverflowException

A stack has overflown.

Example: try…catch

private void divideButton_Click( object sender, System.EventArgs e ){
lblOutput.Text = "";
try
{
int x = Convert.ToInt32( txtX.Text );
int y = Convert.ToInt32( txtY.Text );
int result = x / y;
lblOutput.Text = result.ToString();
}
catch ( FormatException ) {
MessageBox.Show( "You must enter two integers“, "Invalid Number Format",
MessageBoxButtons.OK, MessageBoxIcon.Error );
}
catch ( DivideByZeroException er ) {
MessageBox.Show( er.Message, "Attempted to Divide by Zero",
MessageBoxButtons.OK, MessageBoxIcon.Error );
}
}
Slide 10
The throw statement
 A throw statement explicitly generates an exception in code
 There are two ways you can use the throw statement
 Rethrow the exception in a catch block:
catch(Exception e)
{
// Add code to create an entry in event log
throw;
}
 Throw explicitly created exceptions
string strMessage = “EndDate should be greater than the StartDate”;

ArgumentOutOfRangeException exp =
new ArgumentOutOfRangeException(strMessage);
throw exp;
Slide 11
.NET Exception hierarchy
 The FCL provides two categories of exceptions
 ApplicationException : represents exceptions thrown by the
applications
 SystemException : represents exceptions thrown by the CLR

Slide 12
Exception
ApplicationException SystemException
3
Programmer-defined Exception
classes
 Creating customized exception types
 Should derive from class ApplicationException
 Should end with “Exception”
 Should define three constructors
 A default constructor
 A constructor that receives a string argument
 A constructor that takes a string argument and an Exception
argument
Slide 13
Programmer-defined Exception
classes: Example
Programmer-defined Exception
classes: Example (cont.)
class NegativeNumberException : ApplicationException

{
public NegativeNumberException() : base( "Phai nhap vao so khong am" )
{
}
public NegativeNumberException( string message ) : base( message )
{
}
public NegativeNumberException( string message, Exception inner )
: base( message, inner )
{
}
}

Programmer-defined Exception
classes: Example (cont.)
private void squareRootButton_Click( object sender, System.EventArgs e ) {
outputLabel.Text = "";
try
{
double input = Double.Parse(inputTextBox.Text);
if ( input < 0 )
throw new NegativeNumberException( "Square root of negative number not permitted" );
double result = Math.Sqrt(input);
outputLabel.Text = result.ToString();
}
// BẮT lỗi định dạng số không hợp lệ
catch ( FormatException error ) {
MessageBox.Show( error.Message, "Invalid Operation", MessageBoxButtons.OK,
MessageBoxIcon.Error );
}

// BẮT lỗi nhập số âm
catch ( NegativeNumberException error ) {
MessageBox.Show( error.Message, "Invalid Operation", MessageBoxButtons.OK,
MessageBoxIcon.Error );
}
}
Validating user input
 When getting data from the user, you must ensure that the
entered data is valid
 You can use various techniques for validating data:
 By using standard controls such as combo boxes, list boxes,
radio buttons, check boxes, numericupdown, trackbar,
 By enabling or disabling data fields, depending on the state of
other fields
 By capturing the user’s keystrokes and analyze them for
validity
 By analyzing the contents of the data field as a whole and
warn the user of any incorrect values when the user attempts
to leave the field or close the window
Slide 17
Keystroke-level validation
Slide 18
 When pressing a key on a control, three events occur in
the following order:
 KeyDown event
 KeyPress event
 KeyUp event
4
KeyPressEventArgs class
 Properties

 KeyChar: returns the character value that corresponds to the
key
 Handled: indicates whether the event has been handled
 Example: not allow to input alphabets
private void txtX_KeyPress(object sender, KeyPressEventArgs e)
{
if ((e.KeyChar >= 'A' && e.KeyChar <= 'Z') ||
(e.KeyChar >= 'a' && e.KeyChar <= 'z'))
e.Handled = true;
}

Slide 19
KeyEventArgs class
Slide 20
 Properties
 Alt: returns true if the Alt key is pressed; otherwise, returns false
 Control: …
 Handled: indicates whether the event has been handled
 KeyCode: returns the keyboard code for the event
 KeyData: returns the key code for the pressed key, combination of
modifier keys (Ctrl, Shift, and Alt)
 KeyValue: returns the integer representation of the KeyData
 Modifiers: returns the modifier flags that indicate what
combination of modifier keys (Ctrl, Shift, and Alt) are pressed
 Shift: …
KeyEventArgs class (cont.)
 Example: catch Enter key
private void TextBox1_KeyDown( object sender, KeyEventArgs e )
{
if (e.KeyCode == Keys.Enter)

MessageBox.Show("You pressed Enter key");
}
 Example: catch keys combination
private void TextBox1_KeyDown( object sender, KeyEventArgs e )
{
if (e.Alt && e.KeyCode.ToString() == "F")
{
MessageBox.Show("You pressed Alt+F combination");
}
}
Slide 21
Field-level validation
Slide 22
 When the user enters and leaves a field, the events occur in
the following order:
 Enter: Occurs when a control is entered
 GotFocus: Occurs when a control receives focus
 Leave: Occurs when focus leaves a control
 Validating: Occurs when a control is validating
 Validated: Occurs when a control is finished validating
 LostFocus: Occurs when a control looses focus
 The Validating event is the ideal place to store the validating
logic for a field
The Validating event
Slide 23
 In the Validating event, to retain the focus in the current
control, forcing the user to fix the problem before
proceeding further
 To do this, you can use either of the following techniques
 Use the Focus() method of the control to transfer the focus

back to the field
 Set the Cancel property of CancelEventArgs to true to cancel
the Validating event
 When you want a control to respond, regardless of the
validation status of other controls, you should set the
CausesValidation property of that control to false
The Validating event (cont.)
 Example:
private void txtX_Validating(object sender, CancelEventArgs e)
{
try {
int x = Convert.ToInt32(txtX.Text);
if (x < 0)
{
MessageBox.Show("Please enter a value > 0");
txtX.SelectAll();
txtX.Focus(); // e.Cancel = true;
}
}
catch (Exception ex) {
MessageBox.Show(ex.Message);
txtX.SelectAll();
txtX.Focus(); // e.Cancel = true;
}
}
Slide 24
5
The ErrorProvider component
Slide 25
 You can display an error to alert the user about invalid data

input
 Using MessageBox.Show
 Using ErrorProvider component
 BlinkRate property
 BlinkStyle property
 ContainerControl property
 GetError() method
 Icon property
 SetError() method
 SetIconAlignment() method
 SetIconPadding() method
The ErrorProvider component
 Example:
private void txtX_Validating(object sender, CancelEventArgs e) {
try {
int x = Convert.ToInt32(txtX.Text);
if (x > 0)
errorProvider1.SetError(txtX, "");
else
{
errorProvider1.SetError(txtX, "Please enter a value > 0");
txtX.Focus(); // e.Cancel = true;
}
}
catch (Exception ex) {
errorProvider1.SetError(txtX, ex.Message);
txtX.Focus(); // e.Cancel = true;
}
}
Slide 26