Video: High-Quality Code - Debugging (Bulgarian)

Topics covered:

• Introduction to Debugging
• Visual Studio Debugger
• Breakpoints
• Data Inspection
• Threads and Stacks
• Finding a Defect

Video (in Bulgarian)

Presentation Content

What is Debugging?

• The process of locating and fixing or bypassing bugs (errors) in computer program code
• To debug a program:
  • Start with a problem and stable source state
  • Isolate the source of the problem
  • Fix it and send the fix to the source control
    • One change at a time
  • Create regression test
• Debugging tools (called debuggers) help identify coding errors

Debugging vs. Testing

• Testing
  • A means of initial detection of errors
• Debugging
  • A means of diagnosing and correcting the root causes of errors that have already been detected

Importance of Debugging

• Perfect code is an illusion
  • There is no non-trivial software without bugs
    • programmers.stackexchange.com/q/41248/163921
  • There are factors that are out of our control
• Debugging can viewed as one big decision tree
  • Individual nodes represent theories
  • Leaf nodes represent possible root causes
• You should be able to debug code that is written years ago
• $60 Billion per year in economic losses due to software defects (in USA only)

How to Avoid Bugs

• Reduce number of code lines
  • Write less and write smarter. Keep it simple.
• Reduce complexity / Good code separation
  • Reduce number of possible code paths
• Understand the size of the input & output spaces (be aware of the corner cases)
• Unit test (TDD) with almost 100% test coverage
• Always assume your code is not working
  • Prove it otherwise!
  • Always check the code you write
• Experience matters / Read, learn, code

Visual Studio Debugger

• Visual Studio IDE gives us a lot of tools to debug your application
  • Adding breakpoints
  • Visualize the program flow
  • Control the flow of execution
  • Data tips
  • Watch variables
  • Debugging multithreaded programs
  • and many more…

How To Debug a Process

• Starting a process under the Visual Studio debugger
• Attaching to an already running process
  • Without a solution loaded you can still debug
  • Useful when solution isn’t readily available
  • Debug menu -> Attach to Process

Debugging a Solution

• Debug menu, Start Debugging item
  • F5 is a shortcut
• Easier access to the source code and symbols since its loaded in the solution
• Certain differences exist in comparison to debugging an already running process
  • Hosting for ASP.NET application
    • VS uses a replacement of the real IIS

Debug Windows

• Debug Windows are the means to introspect on the state of a process
• Opens a new window with the selected information in it
• Window categories
  • Data inspection
  • Threading
• Accessible from menu
  • Debug -> Windows

Debugging Toolbar

• Convenient shortcut to common debugging tasks
  • Step into
  • Step over
  • Continue
  • Break
  • Breakpoints
• Customizable to fit your needs
  • Add and/or remove buttons

Controlling Execution

• By default, an app will run uninterrupted (and stop on exception or breakpoint)
• Debugging is all about looking at the state of the process
• Controlling execution allows:
  • Pausing execution
  • Resuming execution
  • Stepping through the application in smaller chunks
  • In the case of IntelliTrace (recording steps), allows backward and forward stepping

IntelliTrace

• IntelliTrace operates in the background, records what you are doing during debugging
• You can easily get a past state of your application from the IntelliTrace
• You can navigate your code with any part and see what’s happened
  • To navigate, just click any of the events that you want to explore

Options and Settings

• Visual Studio offers quite a few knobs and tweaks in the debugging experience
• Options and settings is available via Debug -> Options and Settings
• Examples of Options and Settings
  • Enable just my code (ignore other code)
  • Enable .NET framework source stepping
  • Source server support
  • Symbols (line numbers, variable names)
  • Much more…

Breakpoints

• Ability to stop execution based on certain criteria is key when debugging
  • When a function is hit
  • When data changes
  • When a specific thread hits a function
  • much more
• Visual Studio debugger has a huge feature set when it comes to breakpoints

Visual Studio Breakpoints

• Stops execution at a specific instruction (line of code)
  • Can be set using Debug->Toggle breakpoint
    • F9 shortcut
    • Clicking on the left most side of the source code window
• By default, the breakpoint will hit every time execution reaches the line of the code
• Additional capabilities: condition, hit count, value changed, when hit, filters

Managing Breakpoints

• Managed in the breakpoint window
• Adding breakpoints
• Removing or disabling breakpoints
• Labeling or grouping breakpoints
• Export/import breakpoints

Breakpoint Filters

• Allows you to excerpt even more control of when a breakpoint hits
• Examples of customization
  • Machine name
  • Process ID
  • Process name
  • Thread ID
  • Thread name
• Multiple can be combined using &, ||, !

Data Inspection

• Debugging is all about data inspection
  • What are the local variables?
  • What is in memory?
  • What is the code flow?
  • In general - What is the state of the process right now and how did it get there?
• As such, the ease of data inspection is key to quick resolution of problems

Visual Studio Data Inspection

• Visual Studio offers great data inspection features
  • Watch windows
  • Autos and Locals
  • Memory and Registers
  • Data Tips
  • Immediate window

Watch Window

• Allows you to inspect various states of your application
• Several different kinds of “predefined” watch windows
  • Autos
  • Locals
• “Custom” watch windows also possible
  • Contains only variables that you choose to add
  • Right click on the variable and select “Add to Watch”

Autos and Locals

• Locals watch window contains the local variables for the specific stack frame
  • Debug -> Windows -> Locals
  • Displays: name of the variable, value and type
  • Allows drill down into objects by clicking on the + sign in the tree control
• Autos lets the debugger decide which variables to show in the window
  • Loosely based on the current and previous statement

Memory and Registers

• Memory window can be used to inspect process wide memory
  • Address field can be a raw pointer or an expression
  • Drag and drop a variable from the source window
  • Number of columns displayed can be configured
  • Data format can be configured
• Registers window can be used to inspect processor registers

Data Tips

• Provides information about variables
  • Variables must be within scope of current execution
• Place mouse pointer over any variable
  • Variables can be expanded by using the + sign
• Pinning the data tip causes it to always stay open
• Comments can be added to data tips
• Data tips support drag and drop
• Importing and exporting data tips

Immediate Window

• Useful when debugging due to the expansive expressions that can be executed
  • To output the value of a variable <name of variable>
  • To set values, use <name of variable>=<value>
  • To call a method, use <name of variable>.<method>(arguments)
  • Similar to regular code
  • Supports Intellisense

Threads

• Fundamental unit of code execution
• Commonly, more than one thread
  • .NET, always more than one thread
• Each thread has a memory area associated with it known as a stack used to
  • Store local variables
  • Store frame specific information
• Memory area employs last in first out semantics

Threads Window

• Contains an overview of thread activity in the process
• Includes basic information in a per thread basis
  • Thread ID’s
  • Category
  • Name
  • Location
  • Priority

Callstacks

• A threads stack is commonly referred to as a callstack
• Visual Studio shows the elements of a callstack
  • Local variables
  • Method frames

Finding a Defect

• Stabilize the error
• Locate the source of the error
  • Gather the data
  • Analyze the data and form hypothesis
  • Determine how to prove or disprove the hypothesis
  • Prove or disprove the hypothesis by 2c
• Fix the defect
• Test the fix
• Look for similar errors

Tips for Finding Defects

• Use all available data
• Refine the test cases
• Check unit tests
• Use available tools
• Reproduce the error several different ways
• Generate more data to generate more hypotheses
• Use the results of negative tests
• Brainstorm for possible hypotheses
• Narrow the suspicious region of the code
• Be suspicious of classes and routines that have had defects before
• Check code that’s changed recently
• Expand the suspicious region of the code
• Integrate incrementally
• Check for common defects
• Talk to someone else about the problem
• Take a break from the problem

Fixing a Defect

• Understand the problem before you fix it
• Understand the program, not just the problem
• Confirm the defect diagnosis
• Relax
• Save the original source code
• Fix the problem not the symptom
• Make one change at a time
• Add a unit test that expose the defect
• Look for similar defects

Psychological Considerations

• Your ego tells you that your code is good and doesn’t have a defect even when you’ve seen that it has one.
• How “Psychological Set” Contributes to Debugging Blindness