Debugging

1
Debugging

• Tonga Institute of Higher Education

2
Fixing Errors

• Bug An error in a program
• Debug - To find and remove errors (bugs) from a
  software program.
• When an error occurs, try to understand the error
• Try to fix the error yourself before asking for
  help.
• If you ask for help, be able to describe what's
  going on
• The more experience you have with errors, the
  better you will be able to fix them in the future

3
Compile Errors, Runtime Errors and Logical Errors

• Compile Errors Errors that are found during the
  compilation of a program
• Your program is unable to compile
• Runtime Errors Errors that cause an abnormal
  termination of a program
• Your program crashes
• Logical Errors Errors that are not found during
  the compilation of a program and do not cause an
  abnormal termination of the program
• These are the most difficult to find and often
  results in very big problems.
• You set an employees pay check to be 500,000
  instead of 50,000

4
Common Compile Errors

• Parenthesis must occur in matching pairs
• Error Message
• ')' expected.
• A function must specify a return type
• Error Message
• Option Strict On requires all function and
  property declarations to have an 'As' clause.

5
Common Compile Errors

• A function must return the correct type
• Error Message
• Option Strict On disallows implicit conversions
  from ltdatatypegt' to ltdatatypegt'.
• A subroutine must not try to return something
• Error Message
• 'Return' statement in a Sub or a Set cannot
  return a value.
• Creating an overloaded method with the same
  signature
• Error Message
• Method ltMethodNamegt' has multiple definitions
  with identical signatures.

6
Common Runtime Errors

• Not provided a way for Loops to end
• The program will run forever
• A function must have a return value specified.
• Null is returned
• Dividing a number by 0
• Trying to access a resource that is not available

7
Task List

• Use the task list to quickly take you to the
  source of your error
• This only works for compile errors

Double Click on this to go to the error
8
Break Points - 1

• Helps keep track of the break points in your
  program
• When the program finds a line with a break point,
  it will pause
• Some lines of code cannot have breakpoints.
  (Example Comments)
• You can break on conditions. Right click on the
  break point for more options.
• Example Stop when a variable x 15

Click here to set, disable and remove breakpoints
Red highlight means a break point is set
9
Break Points - 2

• You can do many things while paused
• Examine variables
• Step into a method
• Step over a method
• Step out of a method
• Go back to the break point
• Continue running the program

Step over a function
Step out of a function
Step into a function
Go back to break point
Continue running the program
10
Watch Points, Locals and Autos

• Watch Points
• Displays information about variables you select
  to examine
• You must pick the information you need
• There are 2 ways to add a watch point
• Right click on a variable when the program is
  paused to add a watch point
• Type in the variable name
• Select the variable in the watch window and press
  the Delete button to remove it
• When a variable changes, it turns red in the
  watch window
• Locals / Autos
• Displays information about different variables

11
Call Stack

• Sometimes, we can become confused when we have
  many events and methods.
• A call stack helps us remember how we arrived at
  a location
• It lists the methods that were called to reach
  this point in code
• We can change the information we see by right
  clicking in the call stack

We paused here
Shows how we got here
12
Output

• Shows output from your program
• Useful for outputting lots of data when debugging
  a program
• The output will not be seen by the user when the
  program is run
• Use this code to print text in the output window
• Debug.Writeline(Output!)