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Debugging and Error Handling

Debugging methods available in the ID

Error-handling techniques available in C#

Errors

Visual Studio IDE reports errors as soon

as it is able to detect a problem

Syntax errors

Language rule violation

Syntax error – extraneous semicolon

Quick

info

Error message

does not always

state the correct

problem

Run-Time Errors

Just because your program reports no syntax

errors does not necessarily mean it is running

correctly

One form of run-time error is a logic error

Program runs, but produces incorrect results

May be off-by-one in a loop

Sometime users enter incorrect values

Finding the problem can be challenging

Debugging in C#

Desk check

Many IDEs have Debuggers

Debuggers let you observer the run-time

behavior

You can break or halt execution

You can step through the application

You can evaluate variables

You can set breakpoints

Debug menu offers debugging options

Debugging in C# (continued)

Debug menu options

Debugging in C# (continued)

Select Start

Debugging

and number

of options to

run your

program

doubles

Debug menu options during debugging mode

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Breakpoints

Markers placed in an application, indicatingthe program should halt execution when itreaches that point

Break mode Examine expressions

Check intermediate results

Use Debug menu to set Breakpoint F9 (shortcut)

Toggles

Breakpoints (continued)

Red glyph placed on the breakpoint line

Breakpoint set

Break Mode

In Break mode, Debugger displays Locals

window

All variables and their values are shown

Locals window at the breakpoint

Break Mode (continued)

Breakpoint location

Debugging in C#

Continue Takes the program out of break mode and

restores it to a run-time mode

If more than one breakpoint set, Continuecauses the program to execute from the haltedline until it reaches the next breakpoint

Stepping through code Execute code line by line and see the

execution path

Examine variable and expression values asthey change

Stepping Through Code

Step Into (F11)

Program halts at the first line of code inside

the called method

Step Over (F10)

Executes the entire method called before it

halts

Step Out (Shift+F11)

Causes the rest of the program statements in

the method to be executed and then control

returns to the method that made the call

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Watches

Can set Watch windows during debugging

sessions

Watch window lets you type in one or more

variables or expressions to observe while

the program is running

Watch window differs from Locals window,

which shows all variables currently in scope

Quick Watch option on Debug menu lets you

type a single variable or expressionQuickWatch window

Watches (continued)

Bugs, Errors, and Exceptions

Bugs differ from exceptions Bugs, also called "programmer mistakes," should

be caught and fixed before application released

Errors can 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 stacktrace of methods with the method that raisedthe exception listed first

Bugs, Errors, and Exceptions

(continued)

Stack

trace

Figure 11-13 Unhandled exception raised by incorrect input string

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 try…catch…finally program constructs

What is an exception error?

An exception is a problem that arises during

the execution of a program.

A C# exception is a response to an

exceptional situation that arises while a

program is running, such as an attempt to

divide by zero.

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What is an exception error?

Exceptions provide a way to transfer control

from one part of a program to another.

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Exception handling

C# exception handling is built upon four

keywords:

try: A try block identifies a block of code for which

particular exceptions will be activated. It's

followed by one or more catch blocks.

catch: A program catches an exception with an

exception handler at the place in a program

where you want to handle the problem. The catch

keyword indicates the catching of an exception.

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Exception handling

C# exception handling is built upon four

keywords:

finally: The finally block is used to execute a

given set of statements, whether an exception is

thrown or not thrown.

For example, if you open a file, it must be closed

whether an exception is raised or not.

throw: A program throws an exception when a

problem shows up. This is done using a throw

keyword.

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DEBUGGING IN VS AND VCE

Both VS and VCE(express) allow you to build

applications in two configurations:

Debug (the default) and

Release.

DEBUGGING IN VS AND VCE

In debug configuration and execute it in debug

mode, more is going on than the execution of your

code. Debug builds maintain symbolic information

about your application, so that the IDE knows

exactly what is happening as each line of code is

executed

In release configuration, application code is

optimized. However, release builds also run faster;

and when you have finished developing an

application, you will typically supply users with

release builds because they won’t require the

symbolic information thatdebug builds include

Debug Menu and Toolbar

Breakpoints

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Breakpoints

Breakpoints

Toggle Breakpoints On/Off by clicking

in Editor's gray left margin indicator

Debugging in Break Mode

Breakpoints

Monitoring Variable Content

Monitoring variable content is just one

example of how VS and VCE help you a

great deal by simplifying things.

The easiest way to check the value of a

variable is to hover the mouse over its name

in the source code while in break mode.

A yellow tooltip showing information about

the variable appears, including the variable’s

current value.

Viewing Current Values During

Program Execution

Place mouse pointer over variable or

property to view current value

Monitoring Variable Content

Autos (VS only): Variables in use in the

current and previous statements (Ctrl+D, A)

Locals: All variables in scope (Ctrl+D, L)

Watch N: Customizable variable and

expression display (where N is 1 to 4, found

on Debug WindowsWatch)

Immediate and

CommandWindows

The Command (VS only) and Immediate

windows (found on the DebugWindows menu)

enable you to execute commands while an

application is running.

The Command window enables you to perform

VS operations manually (such as menu and

toolbar operations).

The Immediate window enables youto execute

additional code besides the source code lines

being executed, and to evaluate expressions.

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Immediate and

CommandWindows

Error-handling techniques

available in C#

Debugging and Error Handling

UNDERSTANDING

EXCEPTIONS

An exception occurs when a program

encounters any unexpected problems.

Your program should be able to handle these

exceptional situations and, if possible,

gracefully recover from them. This is called

exception handling.

STEP BY STEP 3_1

STEP BY STEP 3_1

UNDERSTANDING

EXCEPTIONS

The FCL provides two categories of

exceptions

ApplicationException Represents exceptions

thrown by the applications

SystemException Represents exceptions thrown

by the CLR

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try block

A try block contains code that requires common cleanup orexception-recovery operations.

The cleanup code should be put in a single finally block.

The exception recovery code should be put in one or morecatch blocks. Create one catch block for each kind of type you want to handle.

A try block must have at least one catch or finallyblock.

catch block A catch block contains code to execute in response to

an exception.

If the code in a try block doesn’t cause an exception tobe thrown, the CLR will never execute the code in any ofits catch blocks.

You may or may not specify a catch type inparantheses after catch :

The catch type must be of type System.Exception or a typethat derived from System.Exception

If there is no catch type specified, that catch block handles anyexception. This is equivalent to having a catch block thatspecifies System.Exception as a catch type.

CLR searches for a matching catch type from top to bottom.

If CLR cannot find any catch type that matches the exception, CLRcontinues searching up the callstack to find a catch type.

catch block Once the catch block that matches the exception isfound, you have 3 choices:1. Re-throw the same exception, notifying the higher-up call stack

of the exception

2. Throw a different exception, giving richer exception informationto code higher-up in the call stack

3. Let the code continue from the bottom of the catch block

In choices 1-2, an exception is thrown and code startslooking for a catch block whose type matches theexception thrown

In choice 3, the finally block is executed

You can also specify a variable name likecatch(Exception e) to access information specificto the exception.

finally block

The CLR does not completely eliminate memory leaks. Why?

Even though GC does automatic memory clean-up, it only cleansup if there are no references kept on the object.

Even then there may be a delay until the memory is required.

Thus, memory leaks can occur if programmers inadvertently keepreferences to unwanted objects.

C# provides the finally block, which is guaranteed to executeregardless of whether an exception occurs. If the try block executes without throwing, the finally block executes.

If the try block throws an exception, the finally block still executesregardless of whether the exception is caught.

This makes the finally block ideal to release resources fromthe corresponding try block.

finally block Local variables in a try block cannot be accessed in the

corresponding finally block, so variables that must beaccessed in both should be declared before the try block.

Placing the finally block before a catch block is a syntax

error.

A try block does not require a finally block, sometimes noclean-up is needed.

A try block can have no more than one finally block.

Avoid putting code that might throw in a finally block.

Exception handling will still work but the CLR will not keep theinformation about the first exception just thrown in the corresponding tryblock.

Try Block - General Form

The try Block

try

{

statements that may cause error

}

catch [ExceptionType VariableName ]

{

statements for action when an exception occurs

}

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HANDLING EXCEPTIONS

The catch Block

DivideByZeroExceptionArithmeticExceptionOverflowExceptionFormatException

HANDLING EXCEPTIONS

HANDLING EXCEPTIONS HANDLING EXCEPTIONS

The throw Statement

The throw Statement The throw Statement

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HANDLING EXCEPTIONS The finally Block: contains code that always executes,

whether or not any exception occurs.

Example 11. class Program {

2. public static void division(int num1, int num2)

3. {

4. float result=0.0f;

5. try

6. {

7. result = num1 / num2;

8. }

9. catch (DivideByZeroException e)

10. {

11. Console.WriteLine("Exception Error !! \n divid by zero !!");

12. // Console.WriteLine("Exception caught: {0}", e);

13. }

14. finally

15. {

16. Console.WriteLine("Result: {0} ", result);

17. }

18. }

19. static void Main(string[] args)

20. {

21. division(10,0);

22. Console.ReadLine();

23. } }50

Example 2

1. catch (DivideByZeroException ex2)

2. {

3. // …..

4. }

5. catch (FormatException ex1)

6. {

7. //……

8. }51

Try

ex1.Message

Try

Exception only

Instead of

DivideByZeroException

Or

FormatException

Practice

52

Use

IndexOutOfRangeException

&

FormatException

&

Exception

VALIDATING USER INPUT

Field-Level Validation

1. Enter (Occurs when a control is entered.)

2. GotFocus (Occurs when a control receives

focus.)

3. Leave (Occurs when focus leaves a control.)

4. Validating (Occurs when a control is validating.)

5. Validated (Occurs when a control is finished

validating.)

6. LostFocus (Occurs when a control looses

focus.)

The Validating Event

Inside the Validating event, you can write code

to do the following:

Programmatically correct any errors or omissions

made by the user.

Show error messages and alerts to the user so

that the user can fix the problem

Use the Focus() method of the control to transfer

the focus back to the field.

Set the Cancel property of CancelEventArgs to

true. This cancels the Validating event, leaving

the focus in the control.

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VALIDATING USER INPUT

• 1. KeyDown

• 2. KeyPress

• 3. KeyUp

• The KeyPress event happens after the KeyDown

event but before the KeyUp event

• KeyPress event match keys include any alphabetic

and numeric characters (alphanumeric a–z, A–Z,

and 0–9), not raise this event include Ctrl, Alt, and

the function keys

VALIDATING USER INPUT EXAM private void textBoxAge_KeyPress(object sender,

KeyPressEventArgs e)

{

if ((e.KeyChar < 48 || e.KeyChar > 57) && e.KeyChar != 8)

e.Handled = true;

}

private void textBox1_KeyUp(object sender,

System.Windows.Forms.KeyEventArgs e)

{

if (e.Alt == true)

MessageBox.Show("The ALT key is still down");

}

Methods

Math Class Methods

The Math class

Allows the user to perform common math

calculations

Using methods

ClassName.MethodName( argument1,

arument2, … )

Constants

Math.PI = 3.1415926535…

Math.E = 2.7182818285…

Method Definitions

Writing a custom method

Header

ReturnType Properties Name( Param1, Param2, …)

Body

Contains the code of what the method does

Contains the return value if necessary

For uses call elsewhere in program

Pass parameters if needed

All methods must be defined inside of a class

Method Definitionspublic void MethodName ( )

{

// Contains the code of what the method does

}

public ReturnType methodName(Param1,

Param2, … )

{

//Contains the code of what the method

does

//Contains the return value

}

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User-defined method Maximum

public double Maximum( double x, double y, double z )

{

double maximumValue = x;

if ( y > maximumValue )

maximumValue = y;

if ( z > maximumValue )

maximumValue = z;

return maximumValue;

} // end method Maximum

User-defined method Maximum

public void DetermineMaximum()

{

Console.WriteLine( "Enter three floating-point values,\n"

+ " pressing 'Enter' after each one: " );

double number1 = Convert.ToDouble( Console.ReadLine() );

double number2 = Convert.ToDouble( Console.ReadLine() );

double number3 = Convert.ToDouble( Console.ReadLine() );

double result = Maximum( number1, number2, number3 );Console.WriteLine( "Maximum is: " + result );

}

Argument Promotion

Implicit Conversion

Object is converted to a needed type implicitly

Only done if complier knows no data will be lost

Explicit Conversion

Object is manually converted

Required if there could be a loss of data

Widening

Make an object that of a derived class and more complex

Narrowing

Make an object that of a base class and cause some data

loss

Argument PromotionType Can be Converted to Type(s)

bool object

byte decimal, double, float, int, uint, long, ulong, object, short or ushort

sbyte decimal, double, float, int, long, object or short

char decimal, double, float, int, uint, long, ulong, object or ushort

decimal object

double object

float double or object

int decimal, double, float, long or object

uint decimal, double, float, long, ulong, or object

long decimal, double, float or object

ulong decimal, double, float or object

object None

short decimal, double, float, int, long or object

ushort decimal, double, float, int, uint, long, ulong or object

Passing Arguments: Call-By-

Value vs. Call-By-Reference

Passing by value

Send a method a copy of the object

When returned are always returned by value

Set by value by default

Passing by reference

Send a method the actual reference point

Causes the variable to be changed throughout the program

When returned are always returned by reference

The ref keyword specifies by reference

The out keyword means a called method will initialize it

Reference and value

parameters

void SquareRef( ref int x )

{ x = x * x; }

void Square( int x )

{ x = x * x; }

void SquareOut( out int x )

{ x=5;

x = x * x; }

int z=5;

Square ( z );

int z=5;

SquareRef (ref z );

int z;

SquareOut (out z );

Value of z after Square: 5

Value of z after SquareRef: 25

Value of z after SquareOut: 25

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Random Number Generation Class Random

Within namespace System

randomObject.Next()

Returns a number from 0 to Int32.MaxValue

Int32.MaxValue = 2,147,483,647

randomObject.Next ( x )

Returns a value from 0 up to but not including x

randomObject.Next ( x, y )

Returns a number between x and up to but not including y

Class Random_ Example

Random rand = new Random();

int value;

value = rand.Next();

// [0; 2,147,483,647]

value = rand.Next( 6 );

// [0; 5]

value = rand.Next( 1, 7 );

// [1,6]