1. Error Handling
In VB.net
1

2. Error Types
In VB.net errors fall into one of two main
categories:
 Syntax Errors
 Runtime Errors
 Logic Errors
Each type of error has a number of methods
which can help the programmer identify
and fix the problem. This process is called
Debugging.
2

3. Syntax Errors
Syntax (or compiler) errors occur when you
write code that does not follow the rules of
the language.
 When the compiler recognises a syntax error, it
marks the error with a blue wavy line.
3

4. Debugging Syntax Errors
 Moving the cursor over the marked code
causes an error message to appear.
 Clicking on the error list, takes you directly to
the offending line
4

5. Runtime Errors
Runtime errors are the result of code that
despite having the correct syntax, cannot be
executed.
 if not handled, will generate an exception, that
will cause the program to halt.
This type of error is often caused by an
unexpected variable value e.g.
 missing or invisible
 wrong data type
Anticipate code that may result in runtime
errors and provide ways to avert crashes.
5

6. Debugging Runtime Errors
Using the debugger, accessed via the Debug menu
or toolbar.
 Setting a breakpoint- pause the program at a particular
line of code or when a variable changes value.
 Step Into - watch your program run line-by-line
 Rest the cursor over a variable to see its current value
 The Locals window - lists the current values of relevant
variables (before the line is executed)
 The Watches window - choose program variable
monitor, updated automatically as the program runs.
 The Command/Immediate window – try out
commands or change the value of variables while the
program is running.
• Type “?” followed by a variable or expression to see its value
• Use “=“ to assign a new value to a variable
6

7. Trapping Errors
To prevent your program crashing - include
error-trapping statements in your code.
 Validate user input
• If…..Then….Else….End If
 Activate error-trapping to correct the error
• On Error GoTo Label…..Resume
 Use structured exception handling
• Try….Catch….Finally
 Provide helpful error messages
7

8. Validation
Any data input by the user should be validated
to prevent the program crashing
 If….Then…..Else
8

9. Activate Error-Trapping
Turn error-trapping on with a statement, so
that if the system generates an error, the
program jumps to the code after the label.
 On Error GoTo Label
 Identify end of normal processing by adding
“Exit Sub” just above End Sub
 Between Exit Sub and End Sub, place the Label
indicated by adding a colon (:)
 Following the label, insert the code to correct
the error. Finish with a “Resume” statement
9

10. Activate Error-Trapping
The Resume statement has the following options:
 Resume - Re-execute the statement that caused the problem
 Resume Next - Continue the code following the problem
statement
 Resume Label2 – Go to the code following Label2
10

11. Structured Exception Handling
A simpler method of Error-trapping is now
available using the following format:
 Try
• Statements that may cause an error
 Catch condition1
• Code if condition1 is true
 Catch condition2
• Code if condition2 is true
 Finally (optional)
• Tidying-up statements (e.g. database error)
 End Try
11

12. Structured Exception Handling
This method provides the option of trapping
various types of errors, and allows different
messages to be given for each one.
It also avoids jumping up and down the
procedure required by the On Error
(unstructured) Method
12

13. Logic Errors
Logic Errors occur when the program
generates incorrect output or operates
incorrectly.
These errors can go unnoticed, and even
when they are detected, the source of the
error can be difficult to diagnose.
The only reliable method for identifying this
type of error is:
 Systematic and comprehensive Testing
 Check “expected output” against “actual output”
13