Error handling and debugging in vb

Error-Handling and Debugging
• errors can be grouped into three categories:
1. Syntax errors
2. Run-time errors
3. Logic errors
• Syntax errors occur when you mistype a command or leave out
an expected phrase or argument.
• Visual Basic detects these errors as they occur and even
provides help in correcting them.
• You cannot run a Visual Basic program until all syntax errors
have been corrected.
• A syntax error (compiler error) is a programming mistake that
violates the rules of Visual Basic, such as a misspelled
property or keyword.


• Run-time errors are usually beyond your program's control.
Examples include:
• when a variable takes on an unexpected value (divide by zero),
when a drive door is left open, or when a file is not found. Visual
Basic allows you to trap such errors and make attempts to correct
them.
• Logic errors are the most difficult to find. With logic errors, the
program will usually run, but will produce incorrect or unexpected
results.
• A logic error is a human error — a programming mistake that makes
the program code produce the wrong results
• The Visual Basic debugger is an aid in detecting logic errors.
•

• Some ways to minimize errors:
• Design your application carefully. More design time
means less debugging time.
• Use comments where applicable to help you
remember what you were trying to do.
• Use consistent and meaningful naming conventions
for your variables, objects,and procedures.

• Run-Time Error Trapping and Handling
• Run-time errors are trappable.
• That is, Visual Basic recognizes an error has occurred
and enables you to trap it and take corrective action.
• Error trapping is enabled with the On Error statement:
1. On Error GoTo label
2. On Error GoTo 0
3. On Error GoTo line#
4. On Error Resume Next

1. On Error GoTo label
• ddThe Visual Basic On Error GoTo statement is the
foundation of handling trappable errors.
• When you execute an On Error GoTo Label statement
in your code, execution is transferred to the code
starting at Label if a trappable error has occurred.

• The best way to explain how to use error trapping is to look at
an outline of an example procedure with error trapping.
Sub SubExample()
. . [Declare variables, ...]
• . On Error GoTo HandleErrors
• . . [Procedure code]
• . Exit Sub
HandleErrors:
• . [Error handling code]
End Sub

• Once you have set up the variable declarations, constant
definitions, and any other procedure preliminaries, the On
Error statement is executed to enable error trapping.
• Your normal procedure code follows this statement.
• The error handling code goes at the end of the procedure,
following the HandleErrors statement label.
• This is the code that is executed if an error is encountered
anywhere in the Sub procedure.
• Note you must exit (with Exit Sub) from the code before
reaching the HandleErrors line to avoid inadvertent
execution of the error handling code.

• On Error GoTo 0
• Since the error handling code is in the same procedure
where an error occurs, all variables in that procedure are
available for possible corrective action.
• If at some time in your procedure, you want to turn off
error trapping, that is done with the following
statement:On Error GoTo 0
• The Err object returns, in its Number property
(Err.Number), the number associated with the current
error condition.
• The Error() function takes this error number as its
argument and returns a string description of the error.

• Once an error has been trapped and some action taken, control must be
returned to your application.
• That control is returned via the Resume statement. There are three
options:
• Resume Lets you retry the operation that caused the
error. That is, control is returned to the line
where the error occurred. This could be
dangerous in that, if the error has not been
corrected (via code or by the user), an infinite
loop between the error handler and the
procedure code may result.
• Resume Next Program control is returned to the line
immediately following the line where the error
occurred.
• Resume label Program control is returned to the line labeled label.

• On Error Resume Next
• The On Error Resume Next statement provides an
easy way to disregard errors, if you want to do so.
• Once you execute this statement, execution continues
with the next line of code if the current line generates
an error, and the error is disregarded.

• Debugging In Visual Basic
• Visual Basic offers a powerful suite of debugging options—
notably the ability to single-step through your code as it’s
executing.
• The standard way to debug is to place a breakpoint at a
particular line in your code, and when execution reaches that
line, it will halt and Visual Basic will enter the Debug state,
giving you access to your code and variables.
• You can examine the contents of those variables and work
through your code line by line, watching program execution
behind the scenes.
• You place a breakpoint in code by moving the text insertion
caret to that line and either selecting Toggle Breakpoint in the
Debug menu or pressing F9.

• To move through your program step-by-step, you can
select these stepping options in the Debug menu:
• Step Into—Single-step through the code, entering
called procedures if encountered.
• Step Over —Single-step through the code, stepping
over procedure calls.
• Step Out— Step out of the current procedure.

• immediate window
• You can print directly to the immediate window while an
application is running.
• Sometimes, this is all the debugging you may need. A few
carefully placed print statements can sometimes clear up
all logic errors, especially in small applications.
• To print to the immediate window, use the Print method:
• print[List of variables separated by commas or semi-
colons]
• Eg: ?a;b or print a; b

• Locals Window
• The locals window shows the value of any variables
within the scope of the current procedure.
• As execution switches from procedure to procedure,
the contents of this window changes to reflect only
the variables applicable to the current procedure.
• Repeat the above example and notice the values of A
and B also appear in the locals window.

• Watch Expressions
• The Add Watch option on the Debug menu allows you to
establish watch expressions for your application.
• Watch expressions can be variable values or logical
expressions you want to view or test. Values of watch
expressions are displayed in the watch window.
• In break mode, you can use the Quick Watch button on the
toolbar to add watch expressions you need.
• Simply put the cursor on the variable or expression you want
to add to the watch list and click the Quick Watch button.
• Watch expressions can be edited using the Edit Watch option
on the Debug menu.

Error handling and debugging in vb

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