1. Part B – Structured Exception Handling
Lecture Set 8
Validating Data and Handling Exceptions
Part B – Structured Exception Handling

2. Objectives
To understand how to us the Structured Exception handling tools of Visual Basic
Describe the Exception hierarchy and name two of its subclasses.
Describe the use of Try…Catch statements to catch specific exceptions as well as all exceptions.
Learn how to throw exceptions and how to catch them and take appropriate steps to prevent software failure
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3. Objectives (continued)
Describe the use of the properties and methods of an exception object.
Describe the use of Throw statements.
Describe the three types of data validation that you’re most likely to perform on a user entry.
Describe two ways that you can use generic validation methods in a method that validates all of the user entries for a form.
Describe the use the Validating event and the use of masked text boxes for data validation.
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4. Structured Exception Handling
Run-time errors will cause a program to terminate because of an exception being thrown
Exceptions can be thrown for several reasons
Numeric overflow errors
Type conversion errors
Division by zero errors
Database errors of all sorts
Null reference exceptions
Create structured exception handlers to prevent a program from terminating
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5. The Exception Class Hierarchy
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7. Why are Unhandled Exceptions Bad?
In a nutshell – software that fails for any reason is unacceptable
Software has to be written to accept any input from the user (or from external events) and react appropriately
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9. The System.Exception Class
All exceptions are derived from the System.Exception class
Properties
The Message property contains an informational message
The Source property is a string containing the name of the application causing the error
The StackTrace property returns a string containing the error location
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10. Do We Need Structured Exception Handling?
Can’t we program without this?
Can’t we program without structured alternatives and repetitions
Can’t we program without classes?
Can’t we program without validation structures and tools?
Can’t I write good code without having to worry about the accessibility of my data?
Advantages to having all this structured “stuff”
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13. Try-Catch (Another example)

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15. Structured Exception Handlers (Syntax)
try
// Place executable statements that might throw
// (cause) an exception in this block.
catch
// This code runs if the statements in the Try
// block throw an exception.
finally
// This code always runs immediately before
// the Try block or Catch block exits.
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19. What’s in a Try-Catch Statement?
The try block contains the code that may raise (throw) an exception or multiple exceptions
The catch block contains the code that actually handles the exception thrown (additional catch blocks may handle additional exceptions)
Note that if multiple procedures are called in the try block, the catch block catches all exceptions that are not handled by the called procedures
Variables declared within a try … catch block are local to that block
Code for the most specific exceptions must be placed first and for the least specific exceptions the code comes last
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20. Execution Flow of a Structured Exception Handler (VB example)

22. Notes on the Last Slide
Always catch code for the most specific exceptions first. Why?
The Finally code block is executed whether the exception occurs and is caught or not
The ToDecimal and ToInt32 methods (of the Convert class) do not do any rounding and can lead to FormatExceptions in certain cases
This is as opposed to CDec and CInt, the intrinsic functions that do rounding (which can lead to type conversion errors)
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23. Throwing Exceptions
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24. A Function that Throws a FormatException
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25. Throwing Exceptions for Special Purposes
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26. When to Throw an Exception
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27. Code To Validate One Data Entry Item
What data validation do you need to perform?
Ensure that an entry has been made
In other words ensure that the user has not skipped over the item
Ensure that the entry is in the proper format
Ensure that the entry range is valid
Note use of the Select method for a Textbox
Moves control back to the given Textbox
Where parameters are used, Select also helps the user get things correct by showing what the incorrect entry
See also, pp in text
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28. Three Functions for Validating Textbox Entries
These functions are generic ONLY in the sense that they will work on any Textbox
However, the functions that check for Decimal ranges and formats are not generic as to the types of the values whose ranges or formats are to be checked
SO – beware – read the code on pp
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29. Validating Multiple Entries
This material concerns the writing of a function to validate multiple (three in this case) textbox entries on a single form.
You can take your choice – separate validations for each box or one per form (or worse, one long compound condition)
Read pp in text
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