Home > Articles > Software Development & Management > Object Technology

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Exception Handling

Exceptions are C# objects that are used to handle unexpected errors in an application. The exception describes the error with messages, error codes, and stack traces. Even though exceptions occur at unexpected times, they always behave in a predictable manner. Because of the predictable behavior of a raised exception, it can be handled with exception handlers. The ability to plan for and handle exceptions is an essential ingredient in building reliable enterprise-class applications. The following sections explain how to find out whether an exception can be raised, handle raised exceptions, and create custom exceptions to meet all error-handling needs of an application.

Handling Exceptions with try/catch/finally Blocks

Before moving into exception-handling strategies and other important information, it is necessary to fully understand the mechanics and syntax of exception handling. Exceptions are handled by using try/catch/finally blocks. Each of these keywords decorates its own block to handle exceptions. A single try block encloses code that could possibly generate an exception. Following the try block are one or more catch blocks that are ordered from most specific exception to more general. The catch blocks are where raised exceptions are handled. A single finally block can follow the catch blocks and hold code that is guaranteed to be executed regardless of whether an exception is raised or not. A finally block is the appropriate place to put code that releases resources like database connections and file streams. Listing 3.11 implements a try/catch/finally block.

Listing 3.11 Demonstration of a try/catch/finally Block (TryCatchFinally.cs)

using System;
using System.IO;

class TryCatchFinally
{
  static void Main()
  {
   Stream stream = null;

   try
   {
     stream = new FileStream(
      "MyFile.txt",
      FileMode.OpenOrCreate,
      FileAccess.Read);

     stream.Write(new byte[] {1, 2, 3}, 0, 3);
   }
   catch (IOException ioe)
   {
     Console.WriteLine("Caught IOException: \n\n" + 
      ioe.ToString());
   }
   catch (Exception ex)
   {
     Console.WriteLine("Caught Exception: \n\n" + 
      ex.ToString());
   }
   finally
   {
     Console.WriteLine("\nExecuting Finally Block.");

     if (stream != null)
      stream.Close();
   }

   Console.ReadLine();
  }
}

The code in Listing 3.11 generates a System.NotSupportedException exception. The exception was generated because of a write attempt to a read-only stream. Although the IOException catch block didn't (and shouldn't) catch it, the Exception catch block did.

Because this is a stream operation, it would have been logical to assume that an IOException exception would have been generated for any problems, but this is not the case. This illustrates that you can't make any assumptions about the hierarchy of a particular exception. To be truly sure of what exceptions a given method will execute, you should look in the documentation. If you don't have documentation, and the source code is unavailable, and you have an assembly that hasn't been obfuscated, another option is to use the Borland Reflection tool, discussed in Chapter 1, to view the contents of an assembly to find out what exceptions a type will raise.

Some mixing and matching takes place, depending on the type of handling needed. For example, a try/catch block without a finally block may be implemented when there is no need to provide guaranteed code execution. Another possible scenario is that an algorithm doesn't want to catch the exception, but needs to guarantee that certain code will be executed, which would call for a try/finally block without any catch blocks.

Creating Custom Exception Types

When designing custom types, it may be necessary to throw exceptions. If an existing Exception derived type handles the condition, use the existing exception type. If you want to find out what exceptions are available in .NET, see the .NET framework help documentation. The System.Exception type has a link for derived classes on its overview page. However, there are times when existing exception types are not descriptive enough. In those cases, it is better to define a custom exception.

Custom exceptions can be derived from either the System.Exception class or another more specific exception class in the exception hierarchy. There are no set rules for defining which exception to derive from, but it should be one related to the condition the custom exception is trying to resolve. If nothing related exists or fits right, it is perfectly okay to derive from System.Exception. Listing 3.12 shows a custom exception that derives from System.Exception.

Listing 3.12 Building a Custom Exception Type (CustomException.cs)

using System;
using System.IO;

public class CustomException : Exception
{
  object extraInfo;

  public CustomException(string message, object extra) : base(message)
  {
   extraInfo = extra;
  }

  public override string ToString()
  {
   return "CustomException: " +
       extraInfo.ToString() + "\n" +
       base.Message     + "\n" +
       base.StackTrace;
  }
}

class CustomExceptionGenerator
{
  static void Main()
  {
   try
   {
     SomeMethod();
   }
   catch (CustomException ce)
   {
     Console.WriteLine(ce.ToString());
   }

   Console.ReadLine();
  }

  public static void SomeMethod()
  {
   //something went wrong
   throw new CustomException("Something went wrong", "ExtraInfo");
  }
}

The CustomException class in Listing 3.12 inherits System.Exception and defines its own member named extraInfo. When declaring the CustomException constructor, both a message and a value to initialize extraInfo are accepted. The message parameter is used to properly initialize the base class through the syntax: : base(message). CustomException overrides the ToString method to provide a custom message. A typical custom exception would have implemented a property for extraInfo also to provide access to this information to catch blocks handling this exception. Exceptions are raised with the throw statement, as shown in SomeMethod of Listing 3.12.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020