Debugging And Error Handling
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Basic |
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Visual Basic has built in Error Scanning to check for syntax errors.
For example, if you forget to put the "Then" keyword at
the end of an If statement, you get an error message saying, "Expected:
Then or GoTo".
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TYPES OF ERRORS:
There are three types of error that can occur:
1. Compile-Time.
2. Run-Time
3. Logical.
Visual Basic's error scanning only detects errors made
on a single line. A missing "End If" is an example
of a compile-time error, and would be detected when the program
is compiled before a run. Errors can also arise during the execution
of a program and are called run-time errors.
An example of a run-time error might be the wrong data type
provided for a field in a database or trying to divide by zero.
Logical errors are the hardest to detect as they follow the
rules of the programming language, but behave in a manner that was
not intended. Logical errors occur through incorrect logic such
as using a logical "And" instead of a logical "Or"
in a condition.
VISUAL BASIC DEBUGGER:
Make sure the Debug toolbar is visible by putting a check
against "Debug" in the "View", "Toolbars"
menu item. When the program is running, you can pause it so you
can examine values or set test value by pressing the "Break"
button on the toolbar.
The toolbar buttons are described in the rest of this tutorial.
INTERMEDIATE PANE
You can examine variables directly in the Intermediate Pane
with the Print command, and set values with the assignment operator.
Typing, Print Me.Left would return the left position of the
form, and typing me.left = 1000 would set the left position
of the form to 1000.
You can also print to the Intermediate Pane from Visual basic with
the Debug object's Print method.
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| Debug.Print Me.ScaleWidth - shpBall.Left + shpBall.Width |
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The Debug object's Assert method may be used to enter "Break
Mode" should condition not be true.
The following will go into Break Mode if a Shape is positioned
outside the right hand side of the Form.
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| Debug.Assert shpBall.Left + shpBall.Width & Me.ScaleWidth |
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Breakpoints and Watches are two useful features
of the debugger, that allow you to halt the program at a certain
point or observe variables to track the logic.
BREAKPOINT
The Toggle Breakpoint command is used to add/remove breakpoints
where the cursor is positioned in the code window. Breakpoints cause
the program to enter "Break Mode", and display
the block of code on screen in order you may examine values and
step through the code. Breakpoints may also be added by clicking
on the "Margin Indicator Bar" on the left of the
code window.
STEPPING THROUGH THE CODE
The Debug Step commands allow you to step through the program
in order to visually trace the flow of execution. The Step Into
command executes the current statement. If the current statement
is a procedure or function, the next statement will be the first
line of the procedure or function. The Step Over command
executes the current statement. If the current statement is a procedure
or function, the procedure or function is executed as a single statement.
The Step Out command executes the remaining statements in a
procedure or function.
THE LOCALS WINDOWS:
The Locals Window displays the value and data types of all
variables in the module or form. You can also inspect the state
of the Form's properties with the "Me" entry.
THE WATCH WINDOW:
Variables are add to the watch window by either right-clicking
the variable and selecting "Add Watch" or by selecting
"Add Watch" from the "Debug" menu.
"Break When Value Is True" causes the program to
enter "Break Mode", should the value be True, and Break
When Value Changes causes the program to enter "Break Mode"
whenever the value changes.
QUICK WATCH:
Variables may be added to the watch window by highlighting
the variable and selecting "Quick Watch" from the
"Debug" menu.
Clicking "Add", adds the variable to the Watch
window.
CALL STACK
The Call Stack window allows you to see the list of procedures
that were called in order to get to the current statement. The window
may either be displayed by selecting "Call Stack"
from the "View" menu or by clicking the "Locals"
window's "Call Stack" button.
ERROR TRAPPING
The On Error statement is used to trap errors in
Visual Basic 6. VB.Net uses the Try, Catch methods
in order to eliminate the need for a GoTo statement. The
On Error command must be placed in the procedure at a position
before the error could arise. The "Exit Sub" statement
is used before the error handling part in order to avoid running
the error handling code when there is no error. The Error object
is built-in with global scope and contains the state of errors that
may have occurred. The Number property provides the error
number, and the Description property is a translation of
the error.
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Private Sub drvList_Change()
On Error GoTo driveError
dirList.Path = drvList.Drive
Exit Sub
driveError:
MsgBox Err.Description, vbExclamation,
"Drive Error"
End Sub |
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THE RESUME STATEMENT
The Resume statement is used to specify where to restart
the flow of execution. This can either be a label, or Next. If Next
is used, execution is continued from the statement following the
statement that caused the error.
The following example prompts the user what to do should
a drive error occur using the, Resume Next statement.
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Private Sub drvList_Change()
On Error GoTo driveError
retryDrive:
dirList.Path = drvList.Drive
Exit Sub
driveError:
Dim response As Integer, description
As Integer
description = vbExclamation +
vbRetryCancel
response = MsgBox(Err.Description,
description, "Drive Error")
If response = vbRetry Then
Resume
retryDrive
End If
End Sub |
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On Error Resume Next, skips any run-time errors that may occur.
On Error GoTo 0, turns error trapping off.
The following example ignores errors.
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Private Sub drvList_Change()
On Error Resume Next
dirList.Path = drvList.Drive
End Sub |
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RAISING AN ERROR
The Raise method of the Err object may be used to rethrow
the error. This is particularly useful with class modules, as
you can specify the source and a description of the problem.
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description = "Unable to process
the data provided"
Err.Raise Err.Number, "myObject", description |
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The following example handles all errors returned from
the InputBox, except incorrect data types (Error code 13).
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Dim age As Integer
On Error GoTo incorrectDataType
age = InputBox("Enter your name", "Age")
Me.Print "You are " & age & " years old"
Exit Sub
incorrectDataType:
If Err.Number = 13 Then
Err.Raise 13
Else
MsgBox Err.description, vbExclamation,
"Error"
End If |
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