CIS 338 Debugging Dr Ralph D Westfall May

CIS 338: Debugging Dr. Ralph D. Westfall May, 2011 Change font back to Tahoma from Ajax. Surreal. Freak (see Come On, I Thought I Knew That! for rationale)

Error Handling/Debugging syntax errors logical errors good coding practices to minimize errors testing your code debugging methods "bullet proofing"

Syntax Errors spelling or punctuation errors "select * where Sitty="Pomona" 'in SQL Msg. Box "Hello" 'use Msg. Box("Hello") missing parts of control structures n n For but no Next 'on tests If but no End If ' " " extra parts for control structures n extra End Sub at end

Syntax Errors - 2 using a method as a property Msg. Box = "Welcome" n don't use = with methods using a property as a method n lbl. Name. Caption("Name") properties are values, so need to use =

Syntax Errors - 3 writing to a "read only" property n changed values could destroy data or make it inaccessible lst. Specialty. Items. Count() = 10 not using New to create an object obj. People = Persons() 'should be obj. People = New Persons()

Syntax Errors - 4 type mismatch n e. g. can't do arithmetic with text 5 / "hello" Msg. Box(5/txt. Name. Text) 'text or empty n or with an undefined value Msg. Box(5/CInt(txt. Name. Text))'missing dot n won't show as an error right away if not using Option Strict On)

Logical Errors compiler usually) catches division by literal value of zero as a syntax error Dim z As Integer = 3 / 0 division by a variable that becomes zero is much harder to spot Dim x as Integer = 3 Dim y as Integer = 3 y = x (x - y) 'integer division 'divide decimals by 0 = infinity

Logical Errors - 2 code that doesn't do what you want it to do n example: trying to swap values of n. A, n. B n. A = n. B 'destroys value of n. A n. B = n. A 'n. B gets own previous value back

Logical Errors - 3 code that will never run (“dead code”) n example: comparison If n. Age < 3 Then Print "less than 3" Else. If n. Age >= 3 Then Print "3 or older" Else. If n. Age >= 55 Then Print "55 or older" End If 'will never run

Logical Errors - 4 changing value of the counter in a loop n n students make this mistake often on tests final exam tip: memorize loop syntax For i = 0 to 9 lst. Name. Item. Add(names(i)) i += 1 'NO!!!! Next

Good Coding Practices indent code inside loops and conditionals n "pretty printing" For n. I = 1 to 2 n. J = n. I^2 'exponentiation Next n. I 'show (optional) counter use comments n but only where they are needed

Good Coding Practices - 2 use Option Strict at start of code forms and other modules n n n forces you to declare all variables helps spot spelling mistakes stops implicit data type conversions should you use Option Strict when the code goes into "production? " n n why or why not? turn off debugging when ASP goes live

Good Coding Practices - 3 consistent object naming convention frm. Data. Entry 'a Windows form lst. Name 'a List. Box VB capitalizes function names for you n e. g. string function names s. Name. To. Upper() 'to Upper Case s. Address. Trim. Start() 'removes left side spaces

Good Coding Practices - 4 type checking: is data what you expect? If Is. Numeric(txt. Quantity. Text) Then n. Tot = n. Tot + CInt(txt. Quantity. Text) End If 'can check Is. DBNull( ) in databases type conversion: change data so it is usable n. Value = CInt(d. Item 1 + d. Item 2) 'convert to Integer

Good Coding Practices - 5 use subprocedures, functions, queries n for anything more than 3 -5 lines of same or similar code in more than one place advantages: n n n shorter programs easier to debug and maintain may be able to reuse them elsewhere

VB. NET Debug Settings Tools>Options>Projects>VB Defaults or project>Properties> Compile n n Option Explicit On requires that all variables be declared Option Strict On prevents implicit conversions e. g. , w from a String to a numeric data type w from a larger numeric type to a "narrower" type e. g. , Double to Integer

VB. NET Debug Settings -2 Tools>Options>Text Editor>Basic n Auto list members shows/inserts object properties and procedures w can help avoid typing errors n Parameter information – shows function parameters (order, data type) Options>Text Editor>Basic>VB Specific n Pretty listing (reformatting) code indents next line the same as previous one

After You Code Project must be tested n n alpha testing – in house beta testing – outside users how much (%) of budget for testing? n depends on intended use of project and consequences of a failure w minor inconvenience vs. someone dies

Regression Testing maintenance accounts for up to 80% of system costs, so it's important to test for errors in code that used to work but has been modified this is called "regression testing" n testing to see if the system has "gone backwards" from working as it should

Alpha Testing Approaches manual automated testing software n n Test. Complete QA Wizard Pro 2009

Manually Test Your Code test the different ways a user could go through the application data entry example: Add, Change, Delete and navigation buttons n n what happens if click a button when no data shows? what happens if do something twice e. g. , Delete, then Delete again without moving to different record?

Testing Your Code - 2 test possible data values example: zip codes n boundary values (included or not? ) w 00000 and 99999 n values in between boundaries w 00001 to 99998 (random samples) n out-of-bounds values w negative #s? 4 or 6 digit #s? w text #s? ("three" has 5 characters!)

Exercise: Create Test Plan data entry form n n n 4 Text. Boxes Add, Edit, and Delete buttons 4 Navigation buttons (First, etc. ) write down list of steps that you think would be adequate to test this form n n indicate reasons for each test identify things that could go wrong

Debugging Statements "trace" statements can help determine values before place where problem is Msg. Box(s. SQL) n 'view SQL string files with no user interface could use Console. Writeline instead to show values if use Autos Window, can see variable values when crashes occur n discussed in later slides

Debugging Class Modules create another VB project to just provide inputs to module and get outputs from it n n set up code ("driver") in a form to call class module's procedure(s) with possible test values similar to how you would test classes in Java

Other VB. NET Debugging as you write code, real time error checking spots syntax errors if running and execution is halted at a "break, " can see data values in current procedure by moving mouse pointer over variable names good tutorial on commonly used debugging capabilities

VB Debugging Environment debug menu items n Debug toolbar has some of same items breakpoints toggle on and off n click on editor's left border to stop (break) where want to start running line by line w e. g. , call to Demo. Step( ) in notes n can also Clear All Breakpoints with a selection on menu

VB Debug Capabilities - 2 running code one line at a time from breakpoint with Step icons n n Step Over – only steps into lines in this procedure (doesn't step in called ones) Step Into – also runs lines one by one in called procedures Step Out - finishes current procedure or block, then stops on line after it started

Debugging Example set a breakpoint in some VB code n e. g. , in a loop or at a subroutine call run program use different step keys n n to view path through code view changes in values in when mouse goes over a variable in code that has already ran

Debugging Example - 2 run program open Debug>Windows>Locals can expand items by clicking + use different step keys n view changes in values in Locals window as step through code

Run to Cursor click in code at statement to run to n right-click: >Run to Cursor, then Start the project or can click/drag yellow arrow on Breakpoint to line to execute next, then click on Step Into icon n n can skip executing lines with this note "tool tip" when mouse over arrow

Other Debug Windows while running, can use following n n n right-click variable> Add Watch – shows values for variables you identify Debug>Windows>Locals (slide 30) Debug>Windows>Autos – like Locals, but limited to current line and up to three lines before/after it (7 lines)

Debug Windows Example open Debug>Windows>Immediate while code is running can change the value of a variable n n n assign a value, calculate a new value, run a Msg. Box, etc. view effect in Locals window type in a line of executable code e. g. , [declared variable] += 1

Debug Windows Example - 2 open a code window and Start program highlight and right-click variable name n n Add Watch Step Into, view changes in Watch window how is a Watch removed? n right click variable in Watch window>Delete Watch

Debug Windows Example - 3 Debug>Windows>Call Stack n n n can click when program is running shows current subroutine, routine that called it, routine that called the previous routine, etc. 'see Notes indicates place where each called from Clear All Breakpoints on Debug menu n don't have to remember where you set individual breakpoints

"Bullet Proofing" Code prevent all possible errors from happening n n make it impossible for user to enter bad data or to trigger an invalid action include code in project that will "trap" errors that can't be prevented and take appropriate corrective action

Controlling Program Use example: state abbreviations n all have 2 characters e. g. , CA, OR, WA w 3 characters could crash database n only around 50 valid combinations (states) possible "design-time" solutions: n n set Text. Box Max. Length property = 2 use a drop-down List. Box or Combo. Box instead so user can't enter bad values

Controlling Program Use - 2 example: key field in a database should not be changed or input because database creates key value n n property Text. Box. Enabled = False shows data but user can't do anything with it property Text. Box. Read. Only = True shows data, allows user to copy (but not change) it

Controlling Program Use - 3 can use code to prevent inappropriate events example: repeating some buttons used to cause a crash e. g. , Delete n n when the first button is clicked, have code set btn[name]. Enabled = False for the button that causes the crash need to enable the button again (after user moves away from the deleted record) 'notes

Validating/Validated Events code can prevent bad inputs into Text. Boxes Validating – when control loses Focus n for preventing bad inputs Validated - after Validating has occurred n can use Validated to trigger different actions based on different input values, after bad inputs have been prevented

Validating Example Private Sub Text. Box 1_Validating(etc. 'runs when textbox loses focus '. Text property initially set to: "me" Msg. Box("Text. Box 1 is validating") If Text. Box 1. Text <> "Me" Then Msg. Box("Don't try to change Me!!!") Text. Box 1. Focus() 'returns to it e. Cancel = True 'event not Validated End If End Sub 'or can use regular expression conditions

Validated Example Private Sub Text. Box 1_Validated(etc. Msg. Box("Text. Box 1 has been validated") Msg. Box("Thanks 4 leaving Me alone : -)") End Sub

Exception Handling catch (trap) exceptions (errors) before they crash the program, and then: n n n end the program with all normal clean up or keep running but avoid code related to the error (limited functionality) or give a message to user to fix problem w e. g. , put a CD in the E: drive

Try … Catch Try block "traps" exceptions n more efficient than writing subroutines to handle the same types of problems Try block redirects ("throws") flow to Catch block(s) below, where programmer's code handles problem(s) n can also have a Finally block to deal with exceptions not handled in individual Catch block(s)

Types of Exceptions VB. NET has Exception classes that handle specific types of errors Exception classes can have more specific subclasses n Arithmetic. Exception 'general w Divide. By. Zero. Exception 'more specific n Argument. Exception w Argument. Null. Exception

Exception Handling Structure Try [code that could cause a crash] [Catch [object] as [type of]Exception ] [code for general [or specific] problem] 'there can be 0, 1 or several Catch blocks [Finally] (optional) [catch other exceptions, or do something else e. g. , Dispose of object resources] End Try

Exception Handling Example Option Strict On Private Sub etc. Dim int. Z As Integer = 33 Try Dim int. A As Integer = 3 int. Z = 5 (int. A - 3) 'integer division Catch ex As Arithmetic. Exception Msg. Box("Error: " & ex. To. String()) Finally 'not needed here, just for example Msg. Box(CStr(int. Z)) 'still original value End Try