ZHANG Yu, Intelligence Engineering Lab at College of Computer Science and Technology in Jllin University 1 Programming in C#.NET.

ZHANG Yu, Intelligence Engineering Lab at College of Computer Science and Technology in Jllin University

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Programming in C#.NET


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Outline

1. Errors and Exceptions

2. Looking Into Errors And Exception Handling

Lecture 8 – C# Basics VIII


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Errors and ExceptionsOverview

• Looking at the exception classes• Using try – catch – finally to capture

exceptions• Creating user-defined exceptions


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Looking Into Errors And Exception Handling

• Exception Classes


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Base class exception classes


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Base class exception classes

• System.SystemException • System.ApplicationException • StackOverflowException • EndOfStreamException • OverflowException


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Catching Exceptions

• try blocks - the code might encounter some serious error conditions.

• catch blocks - the code that deals with the various error conditions.

• finally blocks - the code that cleans up any resources or takes any other action that you will normally want done at the end of a try or catch block.


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Catching Exceptions

1. The execution flow first enters the try block.2. If no errors occur in the try block, execution

proceeds normally through the block, and when the end of the try block is reached, the flow of execution jumps to the finally block if one is present (Step 5). However, if an error does occur within the try block, execution jumps to a catch block (next step).

3. The error condition is handled in the catch block.4. At the end of the catch block, execution

automatically transfers to the finally block if one is present.

5. The finally block is executed.


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Catching Exceptions

try{// code for normal execution}catch {// error handling}finally{// clean up}


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Catching Exceptions

• You can omit the finally block because it is optional.

• You can also supply as many catch blocks as you want to handle specific types of errors.

• You can omit the catch blocks altogether, in which case the syntax serves not to identify exceptions, but as a way of guaranteeing that code in the finally block will be executed when execution leaves the try block. This is useful if the try block contains several exit points.


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Catching Exceptions

throw new OverflowException();

catch (OverflowException ex){// exception handling here}


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Catching Exceptions

try {// code for normal execution

if (Overflow == true)throw new OverflowException();

// more processing

if (OutOfBounds == true)throw new IndexOutOfRangeException();

// otherwise continue normal execution}catch (OverflowException ex){// error handling for the overflow error condition}catch (IndexOutOfRangeException ex){// error handling for the index out of range error condition}finally{// clean up}


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Implementing multiple catch blocks

using System;

namespace Wrox.ProCSharp.AdvancedCSharp{public class MainEntryPoint{public static void Main(){string userInput;while (true){try{Console.Write("Input a number between 0 and 5 " +"(or just hit return to exit)> ");userInput = Console.ReadLine();if (userInput == "")break;int index = Convert.ToInt32(userInput);if (index < 0 || index > 5)throw new IndexOutOfRangeException("You typed in " + userInput);Console.WriteLine("Your number was " + index);


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}catch (IndexOutOfRangeException ex){Console.WriteLine("Exception: " +"Number should be between 0 and 5. " + ex.Message);}catch (Exception ex){Console.WriteLine("An exception was thrown. Message was: " + ex.Message);}catch{Console.WriteLine("Some other exception has occurred");}finally{Console.WriteLine("Thank you");}}}}}


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if (index < 0 || index > 5) throw new IndexOutOfRangeException("You typed in " + userInput);


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catch (IndexOutOfRangeException ex){ Console.WriteLine( "Exception: Number should be between 0 and 5." + ex.Message);

}


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catch (Exception ex){ Console.WriteLine("An exception was thrown. Message was: " + ex.Message);

}


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catch{ Console.WriteLine("Some other exception has occurred");

}


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Input a number between 0 and 5 (or just hit return to exit)> 4Your number was 4Thank youInput a number between 0 and 5 (or just hit return to exit)> 0Your number was 0Thank youInput a number between 0 and 5 (or just hit return to exit)>

10Exception: Number should be between 0 and 5. You typed in 10Thank youInput a number between 0 and 5 (or just hit return to exit)>

helloAn exception was thrown. Message was: Input string was not in

a correct format.Thank youInput a number between 0 and 5 (or just hit return to exit)>Thank you


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Catching exceptions from other code

• IndexOutOfRangeException, thrown by your own code.

• FormatException, thrown from inside one of the base classes.


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System.Exception properties

Data This provides you with the ability to add key/value statements to the excep-

tion that can be used to supply extra information about the exception. This is a new property in the .NET Framework 2.0.

HelpLink This is a link to a help file that provides more information about the

exception. InnerException If this exception was thrown inside a catch block, then InnerException

contains the exception object that sent the code into that catch block. Message This is text that describes the error condition. Source This is the name of the application or object that caused the exception. StackTrace This provides details of the method calls on the stack (to help track down the

method that threw the exception). TargetSite This is a .NET reflection object that describes the method that threw the

exception.


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System.Exception properties

if (ErrorCondition == true){Exception myException = new ClassmyException("Help!!!!");

myException.Source = "My Application Name";

myException.HelpLink = "MyHelpFile.txt";myException.Data["ErrorDate"] = DateTime.Now;

myException.Data.Add("AdditionalInfo", "Contact Bill from the Blue Team");

throw myException;}


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What happens if an exception isn't handled?

• FormatException • IndexOutOfRangeExceptionThe answer is that the .NET runtime would catch

it.


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Nested try blocks

try {

// Point Atry{

// Point B}catch{

// Point C}finally{

// clean up} // Point D

}catch{

// error handling}finally{

// clean up}


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Nested try blocks

• Important It is perfectly legitimate to throw exceptions from catch and finally blocks.


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Nested try blocks

• To modify the type of exception thrown• To enable different types of exception to be

handled in different places in your code


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User-Defined Exception Classes

• The user might type the name of a file that doesn't exist. This will be caught as a FileNotFound exception.

• The file might not be in the correct format. There are two possible problems here. First, the first line of the file might not be an integer. Second, there might not be as many names in the file as the first line of the file indicates. In both cases, you want to trap this oddity as a custom exception that has been written specially for this purpose, ColdCallFileFormatException.


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4George WashingtonZbigniew HarlequinJohn AdamsThomas Jefferson


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• The user might type the name of a file that doesn't exist. FileNotFound exception.

• The file might not be in the correct format. 1. the first line of the file might not be an integer. 2. there might not be as many names in the file as the first

line of the file indicates.

ColdCallFileFormatException.


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Catching the user-defined exceptions

using System;using System.IO;

namespace Wrox.ProCSharp.AdvancedCSharp{class MainEntryPoint{static void Main(){string fileName;Console.Write("Please type in the name of

the file " +"containing the names of the people to be

cold-called > ");


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fileName = Console.ReadLine();ColdCallFileReader peopleToRing = new

ColdCallFileReader();

try{peopleToRing.Open(fileName);for (int i=0 ; i<peopleToRing.NPeopleToRing; i++){peopleToRing.ProcessNextPerson();}Console.WriteLine("All callers processed

correctly");}


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catch(FileNotFoundException ex){Console.WriteLine("The file {0} does not exist",

fileName);}catch(ColdCallFileFormatException ex){Console.WriteLine("The file {0} appears to have been corrupted",

fileName);Console.WriteLine("Details of problem are: {0}",

ex.Message);if (ex.InnerException != null)Console.WriteLine("Inner exception was: {0}",

ex.InnerException.Message);


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}catch(Exception ex){Console.WriteLine("Exception occurred:\n"

+ ex.Message);}finally{peopleToRing.Dispose();}Console.ReadLine();}}


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Throwing the user-defined exceptions

class ColdCallFileReader :IDisposable

{FileStream fs;StreamReader sr;uint nPeopleToRing;bool isDisposed = false;bool isOpen = false;


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public void Open(string fileName){if (isDisposed)throw new ObjectDisposedException("peopleToRing");fs = new FileStream(fileName, FileMode.Open);sr = new StreamReader(fs);try {string firstLine = sr.ReadLine();nPeopleToRing = uint.Parse(firstLine);isOpen = true;}catch (FormatException ex){throw new ColdCallFileFormatException("First line isn\'t}}


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public void ProcessNextPerson(){if (isDisposed)throw new

ObjectDisposedException("peopleToRing");if (!isOpen)throw new UnexpectedException("Attempted to access cold call file that is not

open");try{string name;name = sr.ReadLine();if (name == null)throw new ColdCallFileFormatException("Not enough

names");if (name[0] == 'Z')


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{throw new LandLineSpyFoundException(name);}Console.WriteLine(name);}catch(LandLineSpyFoundException ex){Console.WriteLine(ex.Message);}

finally{}}


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public uint NPeopleToRing{get{if (isDisposed)throw new

ObjectDisposedException("peopleToRing");if (!isOpen)throw new UnexpectedException("Attempted to access cold call file that is not

open");return nPeopleToRing;}}


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public void Dispose(){if (isDisposed)return;

isDisposed = true;isOpen = false;if (fs != null){fs.Close();fs = null;}}


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Defining the exception classes

class LandLineSpyFoundException : ApplicationException

{public LandLineSpyFoundException(string spyName): base("LandLine spy found, with name " +

spyName){}

public LandLineSpyFoundException(string spyName, Exception innerException): base("LandLine spy found with name " + spyName,

innerException){}}


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class ColdCallFileFormatException : ApplicationException

{public ColdCallFileFormatException(string

message): base(message){}

public ColdCallFileFormatException(string message, Exception innerException): base(message, innerException){}}


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class UnexpectedException : ApplicationException

{public UnexpectedException(string message): base(message){}

public UnexpectedException(string message, Exception innerException)

: base(message, innerException){}}


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49George WashingtonZbigniew HarlequinJohn AdamsThomas Jefferson


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MortimerColdCallMortimerColdCallPlease type in the name of the file

containing the names of the people to be cold-

called > people.txtGeorge WashingtonLandLine spy found, with name

Zbigniew HarlequinJohn AdamsThomas JeffersonAll callers processed correctly


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called > people2.txtGeorge WashingtonLandLine spy found, with name Zbigniew

HarlequinJohn AdamsThomas JeffersonThe file people2.txt appears to have been

corruptedDetails of the problem are: Not enough

names


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called > people3.txtThe file people3.txt does not exist


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Summary

• This chapter took a close look at the rich mechanism This chapter took a close look at the rich mechanism C# has for dealing with error conditions through C# has for dealing with error conditions through exceptions. exceptions.

• You are not limited to the generic error codes that You are not limited to the generic error codes that could be output from your code, but you have the could be output from your code, but you have the ability to go in and uniquely handle the most granular ability to go in and uniquely handle the most granular of error conditions. of error conditions.

• Sometimes these error conditions are provided to you Sometimes these error conditions are provided to you through the .NET Framework itself, but at other through the .NET Framework itself, but at other times, you might want to go in and code your own times, you might want to go in and code your own error conditions as was shown in this chapter. error conditions as was shown in this chapter.

• In either case, you have a lot of ways of protecting the In either case, you have a lot of ways of protecting the workflow of your applications from unnecessary and workflow of your applications from unnecessary and dangerous faults.dangerous faults.

ZHANG Yu, Intelligence Engineering Lab at College of Computer Science and Technology in Jllin University 1 Programming in C#.NET.

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