Debugging applications, using propertiesJim Warren – jim@cs.auckland.ac.nz

COMPSCI 280 S2 2015Enterprise Software Development

Learning Objectives

COMPSCI2802

To be able to take advantage of the features of an IDE such as VS in debugging your applications Including setting break points and watch windows and

working effectively in break mode

To understand the value of, and be able to apply Properties to improve the software quality of classes that you define

Additional information on debugging in VS at: http://msdn.microsoft.com/en-us/library/k0k771bt.aspx

Given where you’re up to with Assignment 2, I thought debugging might be a popular topic!

Three Types of Errors in Coding

COMPSCI2803

Syntax errors Syntax Error (compile error) is a programming mistake that

violates the rules of the language. VS points out syntax error while you type program statements VS prompts if you attempt to run a program before you fix all

the syntax errors. Run-time errors

A run-time error is a mistake that causes a program to stop unexpectedly during execution

Logic errors A logic error is where the program runs, but a programming

mistake makes the program code produce the wrong results. Debugging efforts are particularly focused on tracking down

the logic errors, but also help with understanding sources of run-time errors

Debugging

COMPSCI2804

The Visual Studio debugger is a powerful tool that allows you to observe the run-time behavior of your program and locate logic errors.

With the debugger, you can break, or suspend, execution of your program to examine your code, evaluate and edit variables in your program

The debugger works with several files in order to cross-reference memory to the source files used to build the assembly A program database (PDB) file holds debugging and project

state information that allows incremental linking of a debug configuration of your program.

To debug your code, the debugger needs PDB files containing symbols for your application

Debugger Settings

COMPSCI2805

You can specify various settings for debugger behavior, including how variables are displayed, whether certain warnings are presented, how breakpoints are set, and how breaking affects running programs.

To set debugger options Choose Tools -> Options. Expand Debugging

Debugger Roadmap

COMPSCI2806

Code Stepping Step Into Step Over Step Out

Breakpoints Insert Breakpoint Break at Function

Break Mode Run to Cursor Call Stack

Viewing Data in the Debugger DataTips Variable Windows

Locals Autos Watch Quick Watch

Immediate Window Hit Counts

Code Stepping

7

One of the most common debugging procedures is stepping: executing code one line at a time.

Choose Debug -> Step Into/Over/Out Step Into/Over

Instruct the debugger to execute the next line of code Note:

Step Into executes only the call itself for a function call, then halts at the first line of code inside the function.

Step Over executes the entire function, then halts at the first line outside the function.

Step Out resumes execution of your code until the function returns, then breaks at the return point in the calling function.

Stepping controls

Stop debugging

Breakpoints

COMPSCI2808

Breakpoints tell the debugger that an application should break – i.e. pause execution, at a certain point.

When a break occurs, your program and the debugger are said to be in break mode

To set a simple breakpoint Click on a line of executable code in the source window Choose Debug->Toggle Breakpoint, or Right-click, choose Breakpoint->Insert Breakpoint

To start/stop debugging Choose Debug->Start/Stop Debugging

To stop debugging and restart Choose Debug->Restart.

Note: Restart stops the current debugging session and restarts the project from the beginning.

Breakpoints (con’t)

COMPSCI2809

To insert a function breakpoint In a source window, click on the name of a function. Choose Debug->New Breakpoint->Break at Function.

To set a breakpoint on a function call In the Call Stack window, right-click the function call Choose Insert Breakpoint

To delete a breakpoint In the Breakpoints window, right-click on a breakpoint, and choose

Delete from the shortcut To disable/enable a breakpoint

Right-click on a line containing the breakpoint Choose

Disable/EnableBreakpoint fromthe shortcutmenu

Break Mode

COMPSCI28010

Breaks execution: When execution reaches a breakpoint Or when an unhandled exception occurs Break execution manually at any time

Choose Debug->Break All Break at the location where the cursor is set.

In a source window, right-click on a line of executable code, and Right-click, choose Run to Cursor

Break at a specified function Choose Debug->Windows->Call Stack In the Call

Stack window, right-click thefunction nameand chooseRun To Cursorfrom the shortcut menu.

Viewing Data in the Debugger

COMPSCI28011

The Visual Studio debugger provides a variety of tools for inspecting and modifying the state of your program.

Most of these tools function only in break mode. DataTips Variable Windows

Locals Autos Watch

QuickWatch dialog box Immediate Window Hit Counts

Data Tips

COMPSCI28012

Conditions DataTips work only in break mode, and Only with variables that are within the current scope of

execution. To display a DataTip

Enter break mode In a source window, place the

mouse pointer over any variable inthe current scope.

Expanding Information With enhanced DataTips, you can expand an array, structure,

or object to view its members. To expand a variable to see its elements

Hover the mouse cursor over the + sign that precedes the variable name.

In the case of variable assigned a LINQ query, this may cause the query to be executed

Variable Windows

COMPSCI28013

The debugger provides a number of variable windows for displaying, evaluating, and editing variables and expressions.

Each variable window includes a grid with three columns: Name, Value, and Type. The Name column contains variable names or expressions. The Value and Type column display the value and data type

of the variable or expression. Variable windows

Locals Autos Watch

Locals Windows

COMPSCI28014

Locals Window It displays variables local to the current context or scope. Usually, this means the procedure or function you are

currently executing. Note: the debugger populates this window automatically.

To open the Locals window Enter break mode Choose Debug->Windows->Locals

To expand a variable to see its elements Click with the mouse on the triangle arrow that precedes the

variable name.

Autos Windows

COMPSCI28015

Autos Window It displays variables used in the current line of

code and the preceding line of code. Again, the debugger populates this window

automatically. To open the Autos window

Enter break mode Choose Debug->Windows->Autos

Watch Windows

COMPSCI28016

Watch window It is where you can add variables whose value you want

to watch. Note you can undock the window from its default position

You can add variables (e.g. “j”) or any valid expressions (e.g. “(j+i)*2”) You can also interactively edit/change the value of a variable

in a row of a watch window You can open up to four watch windows, numbered

Watch 1, Watch 2, Watch 3, and Watch 4. To open the watch window

Enter break mode Select the variable you want to add from the Code

editor,right click on it, and click the Add Watch command

Or choose Debug->Windows->Watch 1

QuickWatch dialog box

Handout15COMPSCI28017

The QuickWatch dialog box is similar in concept to the Watch window, but QuickWatch can display only one variable or expression at a time.

Useful when you want to take a quick look at a variable or expression without bringing up the Watch window.

QuickWatch is a modal dialog box, you have to close it before you can continue debugging

To open the QuickWatch window Enter break mode Select the variable you want to add from the Code editor,

right click on it, and click the Quick Watch command to open the QuickWatch Dialog box

To add a QuickWatch expression to the Watch window Click the Add Watch button Enter the expression and press Enter.

To make it really ‘quick’ use theCtrl+Alt+Q shortcut

Immediate Window

COMPSCI28018

The Immediate window is used to debug and evaluate expressions, execute statements, print variable values, and so forth. It allows you to enter expressions to be evaluated or

executed by the development language during debugging. To display the Immediate window

Enter break mode Choose Debug->Windows->Immediate. Type the following in the Immediate window in a break mode

To modify a variable using the Immediate window Enter break the Immediate window Type “age=15” and press Enter Watch the changes in the Watch 1 window

?age<Enter>?Message<Enter>?age+3<Enter>

age=15

Hit counts

COMPSCI28019

Can further tailor the behaviour for watching a line of code

Right-click a break point and select Hit Count… Can set the break point to only be invoked when the

line of code has been reached a certain number of time (or on a multiple of n times)

Fields & Properties Fields

Fields store the data a class needs to fulfill its design Fields should (generally) be declared as Private.

Properties Properties are retrieved and set like fields, but are implemented

using property Get and property Set methods, which provide more control on how values are set or returned. It helps isolate your data and allows you to validate values before they

are assigned or retrieved. Property has Public access The value keyword is used to define the value being assigned by the set

method. Property can be read-only (with the Get portion only) or write-only (with

the Set portion only)

COMPSCI 28020

class MyPoint { private int _x; private int _y;

}

Attributes represent the internal “state” of a given instance of this

class.

public int X { get { return _x; } set { if (value > 0) _x = value; }}

public int Y { get { return _y; }}

Read-only

property

Fields Vs Property

COMPSCI 28021

Use property when: You need to control when and how a value is set or retrieved. The property has a well-defined set of values that need to be

validated. Setting the value causes some perceptible change in the

object's state, such as an IsVisible property. Setting the property causes changes to other internal

variables or to the values of other properties. A set of steps must be performed before the property can be

set or retrieved. Use field when:

The value is of a self-validating type. Any value in the range supported by the data type is valid.

This is true of many properties of type Single or Double. The property is a String data type, and there is no constraint

on the size or value of the string.

Auto-implemented properties Auto-implemented properties make property

declaration more concise when no additional logic is required in the property accessor methods Compiler creates a private, anonymous backing field

that can only be accessed through the property's get and set accessors

COMPSCI28022

public class emp { public decimal salary { get; set; } }

public class emp2 : emp { public new decimal salary { get {return base.salary;} set { if (value >= 0) base.salary = value; else Console.WriteLine("Cannot accept negative salary!"); } } }

Auto-implemented accessors for anonymous private variable (the ‘backing field’)

A child class can override the parent property and selectively reuse the parent (‘base’) class get and set methods

Conclusion… and where we’re up to IDEs offer a range of features for debugging

Breakpoints Tooltip and window based review of variables Windows to watch variables Ability to manipulate variables in the running program in the

Immediate window Properties provide a logical layer over fields

You should be pretty darn focused on Assignment 2 Next lecture we flesh out C# language features

around exception handling

COMPSCI 28023