How to Debug VB .NET Code

1
How to Debug VB .NET Code
2
Types of Error

• Programming errors are generally broken down into
  three types
• Design-time,
• Runtime,
• and Logic errors.

3
Design-time error

• Also known as a syntax error. These occur when
  the environment you're programming in doesn't
  understand your code.
• These are easy to track down in VB.NET, because
  you get a blue wiggly line pointing them out.
• If you try to run the program, youll get a
  dialogue box popping up telling you that there
  were Build errors.

4
Runtime errors

• A lot harder to track down. As their name
  suggests, these errors occur when the program is
  running. They happen when your program tries to
  do something it shouldnt be doing. An example is
  trying to divide by zero.
• Runtime errors usually cause your program to
  crash. If and when that happens, you get the
  blame. After all, youre the programmer, and you
  should write code to trap runtime errors.

5
Logic errors

• Also occur when the program is running. They
  happen when your code doesnt quite behave the
  way you thought it would.
• A classic example is creating an infinite loop of
  the type Do While x is greater than 10. If x is
  always going to be greater than 10, then the loop
  has no way to exit, and just keeps going round
  and round.
• Logic errors tend not to crash your program. But
  they will ensure that it doesnt work properly.

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Logic errors

• There will be no wiggly lines and therefore no
  Build errors.
• Also, no runtime exception will crash the
  program.
• So how to find out where is the logical error?
• Here comes the debugger job.

7
Breakpoints

• Another weapon in your debugging armory is the
  Breakpoint.
• A breakpoint is like a note to VB.NET to stop
  your program at a particular place. You add one
  to your code by clicking in the margins.
• A brown circled then appears, indicating where
  the code will break.
• The following two images show how to add one

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Breakpoints

• Run your program again. You are immediately
  returned to the coding window.
• The place where you put the breakpoint will now
  have a yellow arrow on top of the brown circle.
• The brown highlighted line will now be yellow

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Breakpoints

• The yellow highlight indicates where in your code
  VB.NET is.
• To continue checking
• your code, press F10 on your keyboard (you
  can also press F11, but this will jump into any
  Subs or Functions youve set up.)
• The next line in your code will be highlighted

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Breakpoints

• The yellow arrow, and the yellow highlight, jump
  down one line. Press the F10 key again.
• Then hold you mouse of the letter variable.
• The value this variable currently holds will be
  displayed

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Breakpoints

• You can continue pressing the F10 key until
  youve spotted the error. Or you can use
• another debugging tool the Locals window.
• While your program is still in Debug mode (the
  yellow line will still be there, if it is), click
  Debug gt Windows gt Locals from the menu bar.
• You should see the following in the bottom left
  of your screen

Locals means Local variables. That is,
variables declared in this section of the code.
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Commenting your code

• In some cases you need to comment out a whole
  block of code.
• To do so, highlight it first. Then click the
  comment icon on the toolbar
• The icon next to it removes the comments from
  highlighted code.