Chapter 3 Error Handling for the User Interface

Slide 139:06:37 AM HANDLING EXCEPTIONS – The finally block contains code that always executes, whether or not any exception occurs... This cancels the Validating event, leaving the foc

Chapter 3 :

Error Handling for the User

Interface

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

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STEP BY STEP 3_1

STEP BY STEP 3_1

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UNDERSTANDING EXCEPTIONS

• The FCL provides two categories of exceptions

thrown by the applications

the CLR

HANDLING EXCEPTIONS

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HANDLING EXCEPTIONS

DivideByZeroException ArithmeticException FormatException OverflowException

HANDLING EXCEPTIONS

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HANDLING EXCEPTIONS

HANDLING EXCEPTIONS

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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

Ex: page _ _ _ See Demo

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.)

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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.

The CausesValidation Property

• The default value of the CausesValidation

property for a control is true for all controls

• 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

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The ErrorProvider Component

icon next to a field when it contains an error

• When the user moves the mouse pointer over

the icon, an error message pops up as a ToolTip

The ErrorProvider Component

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Using the ErrorProvider Component and

Other Validation Techniques

The Validating Event and Sticky Form

– The CausesValidation property of the btnExi t control

to false

– Declare the following variable outside a method

block in the class:

private bool closingFlag = false;

– Add the following code to the Click event handler of the Exit button :

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The Validating Event and Sticky Form

• Attach the following event handling code to the

Validating events of both the txt Mile controls

Methods

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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…

Math Class Methods

Max( -2.3, -12.7 ) is -2.3

Sqrt( x ) Sqrt( 900.0 ) is 30.0

Sqrt( 9.0 ) is 3.0 Tan( x ) Tan( 0.0 ) is 0.0

• Contains the code of what the method does

• Contains the return value if necessary

– For uses call elsewhere in program

• Pass parameters if needed

//Contains the code of what the method does

//Contains the return value

}

User-defined method Maximum (p 196)

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

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User-defined method Maximum (p 195)

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

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 (p 219)

• 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

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Reference and value parameters

void SquareRef( ref int x )

Value of z after Square: 5

Value of z after SquareRef: 25

Value of z after SquareOut: 25

Random Number Generation

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int value;

value = rand Next();// phát sinh 1 số trong [0;

2,147,483,647 value = rand Next( 6 );// phát sinh 1 số trong [0; 5] value = rand Next( 1, 7 );// phát sinh 1 số trong [1,6]

Finish