Coding Defensively

1
Coding Defensively

• Coding Defensively
• Chapter 4 - Coder to Developer by Mike
  Gunderloy
• Instructor Dr.James
  Fawcett
• Presented by
• Priyaa Nachimuthu
• priyaa_at_syr.edu

2
Agenda

• Assertions
• Exceptions
• Comments

3
Defensive Coding

• Reusable code
• Easy maintenance
• Paves way to locate problems easily
• Allows modifications

4
Assertions

• Problem indicators
• Design time debugging
• Assert a boolean condition
• Microsoft .Net runtime

5
Using Assertions

• Add a reference to System. Diagnostics
• namespace
• Use Debug. Assert method or
• Trace. Assert method
• // This method should never be called
  
• // without a source Url
• Debug. Assert (((d.SourceURL ! string.
  Empty) (d.SourceUrl ! null)),Empty
  SourceUrl,Cant download a file unless a Source
  Url is supplied)

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Assertion Failed Alert
7
Mechanics of .Net Assertions
8
Debug Configuration
9
Release Configuration
10
Listener classes

• TraceListener abstract class in System.
  Diagnostics namespace
• DefaultTraceListener An object of this class is
  automatically added to Listeners collection of
  Trace and Debug classes. Writes output to output
  window or message boxes.
• TextWriterTraceListener writes messages to any
  class that derives from the Stream class.
• EventlogTraceListener writes messages to
  Windows event log.
• Inherit from TraceListener - custom behavior

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Guidelines for good assertions

• Hide assertions from end users
• Use Debug. Assert
• Assertions shouldn't have side effects
• // Make sure path is ok
• Debug. Assert ((newpath
  Path.Combine(foldername,filename))!
• string.Empty,Bad path to download )
• Dont verify compiler operation
• Int intSizes new int3
• Debug.Assert(intSizes.GetUpperBound(0)
  2,Failed to initialize
• array )

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Guidelines for good assertions

• Reasonable input data
• Checks in the method that uses the value
• Check assumptions after complex code
• execution

13
Exceptions

• Exceptions are for exceptional situations
• Run-time checking
• public void GetDownload (
  Download d )
• string filename
  string.Empty
• string foldername
  string.Empty
• WebClient wc new
  WebClient()
• try
• // code to do
  download
• catch ( Exception e )
• // bubble to any
  exception up to caller with custom info
• throw new
  DownloadException(Unable to download,d,e)

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Exception Mechanics

• try Exception handling block
• catch start of the actual exception
• handling
• finally start of the code that will always
  
• run. Used for cleanup
• throw generates an exception

15
Custom Exceptions

• There is no existing exception class that
• correctly represents the exceptional
• condition.
• To pass additional information to the
• parent code.

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Guidelines for good exceptions

• Exceptions are for exceptional situations
• Use exception constructors with string value for
  additional information
• While passing error information from lower level
  code to higher level code use one of the
  constructors that accepts an Exception object.
• Dont create custom exceptions when the built
  ones will suffice.

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Comments or Self-Documenting Code

• Code should be its own documentation, comments
  are superfluous.
• Well written comments make code easier to read.
• Writing good comments is no excuse to writing bad
  code.

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Noise Comments

• Comments that make code longer with no
• value addition
• // DefaultDownloadFolder property
• public string DefaultDownloadFolder
• // public getter
• get
• return _D
• // public setter
• set
• _D value

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Noise Comments

• Formal procedure header
• // ProcedureGetDownload
• // Module DownloadEngine.cs
• // Author Mike
• // Creation Date 21 October 2003
• //
• // Inputs
• // d - A Download variable with download file
  information
• // Outputs
• // None
• //
• // Revision History
• // 05 Nov 2003 Initial Creation
• // 07 Nov 2003 Download code implemented
• //
• Public void GetDownload(Download d)

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Placeholder comments

• // HACK Brute- force search, can replace this if
  this becomes a perf issue
• // TODO Add code to make sure user has access to
  folder
• // POSTPONE This is where we want to create
  custom properties
• Tools -gt Options -gt Environment -gt Task List

21
Visual Studio.NET Task List
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Summary and Intent Comments

• Intent comment
• // GetDownload uses the information in a download
  object
• // to get the specified file from the internet to
  a local hard
• // drive. Any download errors are wrapped in a
  custom
• // exception and returned to the caller.
• Public void GetDownload(Download d)
• // Method body goes here
• Summary comment
• public void GetDownload(Download d)
• try

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Defensive Coding Checklist

• Use assertions to catch design time errors
• Use exceptions to handle runtime errors
• Use exceptions only for exceptional situations
• Avoid peppering your code with noise comments
• Use comments as placeholders for future work
• Use comments to summarize and document the intent
  of the code
• When you change code, make sure to keep the
  comments up-to-date.