Chapter 3 : Error Handling for the User Interface Slide 2 9:06:37 AM 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. Slide 3 9:06:37 AM STEP BY STEP 3_1 Slide 4 9:06:37 AM STEP BY STEP 3_1 Slide 5 9:06:37 AM UNDERSTANDING EXCEPTIONS • The FCL provides two categories of exceptions – ApplicationException Represents exceptions thrown by the applications – SystemException Represents exceptions thrown by the CLR Slide 6 9:06:37 AM HANDLING EXCEPTIONS • The try Block Slide 7 9:06:37 AM HANDLING EXCEPTIONS • The catch Block DivideByZeroExceptionArithmeticException OverflowException FormatException Slide 8 9:06:37 AM HANDLING EXCEPTIONS Slide 9 9:06:37 AM HANDLING EXCEPTIONS Slide 10 9:06:37 AM HANDLING EXCEPTIONS • The throw Statement [...]... 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... 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 Using the ErrorProvider Component and Other Validation Techniques The Validating Event and Sticky Form – The CausesValidation property of the btnExit control to false – Declare the following variable outside a method block in the class: private bool... Add the following code to the Click event handler of the Exit button: The Validating Event and Sticky Form • Attach the following event handling code to the Validating events of both the txtMile controls 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. 1415926 535 …... 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 The ErrorProvider Component • The ErrorProvider component can... 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 Definitions public 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.. .The throw Statement The throw Statement HANDLING EXCEPTIONS • The finally Block – 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... (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 – The ref keyword specifies by reference – The out keyword means a called method will initialize... randomObject.Next() • Returns a number from 0 to Int32.MaxValue – Int32.MaxValue = 2,147,4 83, 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();// phát sinh 1 số trong [0; 2,147,4 83, 647 value = rand.Next( 6 );// phát... 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... …) { //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 ) { double maximumValue = x; if ( y > maximumValue ) maximumValue = y; if ( z > maximumValue ) maximumValue = z; return maximumValue; } // end method Maximum User- defined method Maximum (p 195) public void DetermineMaximum() { Console.WriteLine( . 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 EXCEPTIONS • The try Block Slide 7 9:06:37 AM HANDLING EXCEPTIONS • The catch Block DivideByZeroExceptionArithmeticException OverflowException FormatException Slide 8 9:06:37 AM HANDLING EXCEPTIONS Slide. EXCEPTIONS • The FCL provides two categories of exceptions – ApplicationException Represents exceptions thrown by the applications – SystemException Represents exceptions thrown by the CLR Slide