Chapter 3 Error Handling for the User Interface

Chapter 3 :
Error Handling for the User
Interface
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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
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STEP BY STEP 3_1
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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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HANDLING EXCEPTIONS
•
The try Block
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HANDLING EXCEPTIONS
•
The catch Block
DivideByZeroExceptionArithmeticException
OverflowException
FormatException
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HANDLING EXCEPTIONS
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HANDLING EXCEPTIONS
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HANDLING EXCEPTIONS
•
The throw Statement
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The throw Statement
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The throw Statement

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HANDLING EXCEPTIONS
•
The finally Block
–
The finally block contains code that always
executes, whether or not any exception occurs.
Ex: page _ _ _
See Demo
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VALIDATING USER INPUT
•
Field-Level Validation
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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.)
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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.
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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
•
The ErrorProvider component can set a small
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
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The ErrorProvider Component
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Using the ErrorProvider Component and
Other Validation Techniques
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The Validating Event and Sticky Form
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The CausesValidation property of the btnExit 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 txtMile controls
Methods
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Math Class Methods
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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…
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Math Class Methods
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Math Class Methods