Lecture 7 - Debugging and Break Mode (VB 2008)

8/14/2019 Lecture 7 - Debugging and Break Mode (VB 2008)

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Lecture 7: Errors and Debugging


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Introduction

As developers, part of our job is to find and remove errors Such errors generally fall into 3 major categories:

Syntax Errors Runtime Errors Logic Errors (also called bugs)

Finding errors is called Debugging. This job can be quite difficult.

In lecture 7, we discuss these error types And introduce some powerful debugging tools provided by our .NET IDE :

Setting Breakpoints The Breakpoint Window


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Syntax Errors

Occur when the programmer uses incorrect VB.NET syntax. As a result, the Compilercannot understand the statement.

This is the simplest and most common type of error.

Typical examples : mis-spelling a declared variable, or VB .NET keyword trying to use an undeclared variable (see Example, below). providing incorrect parameters in a Method Call, etc

The VB .NET IDE helps us spot and fix syntax errors:

1. The error is underlined in blue. (see Example below)

2. Placing the cursor over the error causes a ToolTip to appear Which (usually) clearly explains the problem.

Here, we have mistakenly used the Private Keyword to declare a local variable.


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Syntax Errors (Auto-Correct)

3. Visual Studio.NET 2008 provides an AutoCorrection option : A convenient Drop-Down Dialog which suggests corrections

Click to view Intellisenses suggestions, if any for correcting the error. Along with a preview of the corrected code.

If you like, you may select one of the suggestions (just click the blue text) (here, we may correct by clicking Replace Private with Dim


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IntelliSense

This IntelliSense feature provided by the IDE

Also provides features to help prevent (not just correct) Syntax Errors.1.A drop-down menu to help recall Members/Methods of Classes, etc. This occurs when the dot operator(.) is used

Just select the Name from the Menu, and press Tab or Enter.

Here, my example shows the Methods of Class File As shown by Intellisense, the Method, File.OpenText(path as String)

makes and passes a StreamReader Object to read the file at path Thus, it is equivalent to the StreamReader() constructor we looked at in class.

This helps you to remember the available Members / Methods for a Class.


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IntelliSense (cont.) IntelliSense also provides help in calling Methods:

2.A Tool-Tip Parameter list for called Methods Which appears when you type the parentheses for a Method call.

Listed information includes: Parameter Number and Types, and Return Type

General description of parameters

A Drop Down list of selectable choices for the parameter (not all may be valid)

This helps remember and set the parameters for available Methods.

Note: Foroverloaded Methods, the menu lists available Method versions Allowing you to choose which Method version to use.


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Runtime Errors

Runtime errors occur during Program Execution Due to unexpected behavior while running

Causing the Program to Fail during operation; Providing the user with little explanation or choice.

Typical examples : Hard-disk error Loss of connection error (Internet)

Database or Server Error, etc

The .NET platform provides Error-Handling Blocks for Developers Developers may use these to Catch expected Errors...

And then Try to deal with them in a more graceful manner. Such a set of Error-Handlers is called an Error-Handling Logic.

Examples: Bypass the portion of code that failed. Allow the user to retry the failed operation (write, connection, etc.)

We will discuss Runtime Errors in the next Lecture (Error Handling). For now, we will focus on Debuggable Errors.


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The Exception Assistant

As shown below, VB Studio 2008 provides an Exception Assistant To assist us in handling errors that occur during run-time (= Exceptions).

For instance, the code below contains an error

Here, the String, strData has not been initialized.

In VB 2008, Strings are required to be assigned a value Before the actual String object is created in memory

And handed back to the named object reference (here, strData)

So, strData is just an empty object reference to a non-existent object(String).

Thus, when we run this code, and Button1 is pushed: Our code will attempt to reference the Text of a non-existent String; A Null Reference Exception (error) will be thrown

Lets take a look


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The Exception Assistant (cont.) As shown, our code runs to the line containing the error (yellow):

And halts upon occurrence of an unhandled exception. However, instead of crashing, we are provided with help:

An Exception Assistant Dialog Box.

This Dialog Box provides information about the exception: Type of exception (here, a NullReferenceException). Troubleshooting tips, to help us fix the error in the code.

Specific details about the exception. (click View Detail to see details)

Here, we have ( ): Object reference not set to an instance of the object,


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Logic Errors Occur when the developer does not fully understand his code.

In this case, the program may yield incorrect results or behavior Even though all of the Syntax is correct (Hard to Spot!).

Typical examples : An Infinite Loop (a loop that never stops looping; see example below)

Improper Branching (e.g., incorrect If-Then logic)

A comparison operation which does not yield the expected result.

The .NET platform provides good Debugging Tools for Developers

1. A Debug ToolBarfor access to these Debugging Tools

2. The ability to set and use Breakpoints A place in code where program execution is set to halt.

Either automatically or conditionally.

3. Several Debugging Windows to monitor code behavior during Runtime: The Breakpoint Window

The Command Window The Watch and Locals Windows


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Example: Debugging Demo Lets use ourTextEditor Project to demonstrate Debugging


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Debugging Demo (cont.)


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Setting Breakpoints It is common to need to debug a project at a particular place in

code For such cases it is useful to set a Breakpoint:

A place in code where execution is set to halt automatically.

This allows us to run normally up to ourpoint of interest and then stop... So we may step through the code (line-by-line) from there, to find the error.

Breakpoints are usually added while writing code But can also be added while a running program is waiting for user input.

Note: Debugging tools are available with the BuildConfiguration set to Debug But not Release.

When a breakpoint is encountered: Execution will stop automatically (default), or conditionally (if set)

A condition for halting may be set using the Breakpoints Window. This halted state is referred to as Break Mode.

Program execution will stop at the line just before the breakpoint.

(actually, at the very beginning of the breakpoint line) From here, we will generally step through the code using the Debug ToolBar.


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Breakpoint Ex (Setting Breakpoints)


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The Breakpoints Window


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Using the Toolbar Debugging Icons Several useful icons are available on the Standard Toolbar

The Step Into Icon: Allows you to step through your code line-by-line

Beginning from the current line.

The Step OverIcon: Allows you to step through a Function or Subroutine in 1 step

Note: The code still executes.

The Step Out Icon: Allows you to step to the end of the current Function/Subroutine in 1 step.

Note: Code still executes

The Run To CursorIcon: Allows you to set the cursor anywhere

after the current line And then execute all code up to that point.

To add this icon: Right-click an empty area on the Toolbar; Choose Customize in the Context Menu In the Customize Dialog:

Choose Commands > Debug Drag Run To Cursor onto the Toolbar.


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Using Step Into and Step Out


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Using the Breakpoint Hit Count

Lets continue working with our Debug Project


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Breakpoint Hit Count (cont.)


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Using a Breakpoint Condition


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Forward

This lecture, we discussed program errors Syntax, Logic, and Runtime Errors

And focused on Syntax and Logic Errors (bugs)

We also learned to use some .NET debugging tools for catching bugs: Setting Breakpoints

The Breakpoints Window

Next lecture, we continue our with some additional debugging tools: The Command and Autos Windows The Watch Window The Locals Window

Followed by a discussion of Run-time errors and Error Handling Using the TryCatchFinally structure.

Lecture 7 - Debugging and Break Mode (VB 2008)

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