Home > Articles > Programming > Windows Programming

Structured Exception Handling in Visual Studio .NET

You have to handle exceptions in your programs, or the application will crash. (Users take exception to that.) Learn how to use Visual Studio's tools to throw and catch, elegantly and easily.

Structured Exception Handling in Visual Studio.Net

By Peter Aitken
June 17, 2004

For programmers, exceptions—that's just a fancy name for run-time errors—are a fact of life. Good programming practices can minimize exceptions, but it is impossible to prevent them altogether. Many of the causes, such as network errors or incorrectly set file permissions, are beyond the programmer's control.

You can, however, handle exceptions in your program. No, let me rephrase that—you must handle exceptions in your program! Fortunately, the .Net framework and languages include powerful exception handling tools. I'll use Visual Basic, in this article, but the same principles and techniques apply to C# as well.

An exception is a problem that occurs up while the program is executing. In .Net and object-oriented terminology, a program is said to throw an exception. Many situations can cause exceptions, including trying to open a non-existent file for reading, attempting to instantiate a class that is not available, or writing to a removable media drive when no media is inserted. If an exception is thrown that the program does not handle, a message is displayed to the user, and the program usually ends (although with some exceptions, continuing is also an option). This is a polite way of saying that the program crashes, and believe me: that's no way for a programmer to impress clients!

The .Net framework implements structured exception handling. The term structured means that the code that has the potential to cause an exception. Plus, the code to handle the exception is organized in structured blocks. You use the Try...Catch...End Try statement for this purpose. The syntax is as follows:

' Executable statements that may cause an exception are placed here.
Catch [optional filters]
' Statements that are to be executed when an exception occurs go here.
' The optional filters permit the code to respond only to specific
' types of exceptions. You can place multiple Catch blocks in a
' Try statement.
' Code placed here is executed after the code in the Try block
' or in the Catch block is executed. This block is optional.
End Try

Structured exception handling operates on the level of procedures. Each procedure in your program should contain exception handling code designed to catch the types of exceptions that the procedure might generate. If a procedure generates an unhandled exception, it is passed up the call stack to the caller. In other words, if procedure A calls procedure B, and procedure B generates an unhandled exception, it is passed up to procedure A for handling.

The ability to filter the Catch statement is an essential part of .Net's structured exception handling. If there is no filter—or, to be more precise, there is a filter that matches all exceptions—the code in the Catch block is executed for any and all exceptions that are thrown. Such a Catch block would be written like this:

Catch ex As Exception

Within the Catch block, the object ex, an instance of the System.Exception class, will contain information about the exception. You can use any legal variable name here, although ex or e is traditional. To filter for specific exceptions, you would write the Catch statement like this:

Catch ex As ExceptionType

replacing ExceptionType with the name of the specific exception class to be caught. The .Net framework defines a hierarchy of several dozen exception classes, all derived from System.Exception, with each individual exception class encompassing related exceptions. For example, all file-related exceptions are subsumed under IoException, but for more specific control you can catch file loading exceptions with FileLoadException and FileNotFoundException. You'll find a complete diagram of the exception hierarchy in the Visual Studio documentation.

The filtering capability lets you write Catch blocks to deal with specific exceptions. This makes perfect sense, because the message to the user and the program's response will differ, depending on the type of exception. Simply create a separate Catch block for each condition. It is also common practice to include a final Catch block to deal with any exceptions that fall through the preceding Catch filters. Here's an example:

Catch ex As FileNotFoundException

' Code here to respond to a "File Not Found" exception.

Catch ex As FileLoadException

' Code here to respond to a "File Load" exception.

Catch ex As Exception

' Code here to respond to any exceptions not caught by previous Catch statements.

The Catch keyword, by itself, will catch any exception, just like Catch ex As Exception. The disadvantage is that you will not have an Exception object in that block to provide information about the exception and its cause (as will be explained below).

In addition to filtering exceptions based on the exception type, you can perform additional filtering using the When clause in the Catch statement. The syntax is:

Catch ex As ExceptionType When Expression

Expression is any expression that evaluates to True or False. The Catch block is executed only if the exception that was thrown matches ExceptionType and Expression is True. This lets you execute one of two or more Catch blocks for the same exception, depending on program conditions. Here's an example:

' Code that might cause an OverflowException
Catch ex As OverflowException When Newuser = True
' Display error info for new users.
Catch ex As OverflowException When Newuser = False
' Display error info for experienced users.

What can your program do to respond to an exception? Of course, the details depend on your program and the specific exception, but you need some information to work with. All exception classes have a base set of properties that provide some information about the exception, as follows:

Property Description
HelpLink Specifies the Uniform Resource Name (URN) or Uniform Resource Locater (URL) of the help file associated with this exception.
Message A short description of the exception.
Source The name of the application or object that caused the exception
StackTrace Identifies the stack trace–the sequence of procedure calls leading up to the location in code where the exception occurred.

Some derived exception classes have additional properties specific to the associated exception. For example, the FileLoadException class, which is thrown when there is an error loading a file, has the FileName property; it provides the name of the file that you were attempting to load. You'll find details on these specific exception members in the .Net framework documentation.

What about the optional Finally block? It is always executed, no matter what happens in the Try and Catch sections. Use it for actions that have to be performed regardless of whether an exception was thrown, such as closing files. The Finally block is executed even if the thrown exception was not caught by any of the Catch statements.

The .Net framework also lets you throw your own exceptions. Why would you want to do this? You might want to test your exception handling code before compiling and distributing the program. Another reason is to create and use custom exceptions, so your program can make use of VB.Net's exception handling mechanism to handle program-specific error conditions (such as the user entering incorrect data).

Also, you may want to throw an exception inside a Catch block, when your code's response to a caught exception is to throw a new exception or to re-throw the original exception. This is sometimes desirable when handling exceptions in a component. You may want the code in the component to take some action in response to the exception while, at the same time, passing the exception back up the call stack for additional handling.

To throw an exception, use the Throw statement. The syntax is:

Throw ex

where ex is an object of type Exception. For example:

Throw New FileNotFoundException
  ("The file you requested does not exist.")

If used within a Catch block, you can use Throw with no argument to re-throw the exception that was caught originally.

Finally, Visual Basic.Net still supports unstructured exception handling. This is the older style of error handling that was available in Visual Basic version 6 and earlier, using the On Error Goto, Resume, and related statements. It is supported primarily for compatibility with legacy Visual Basic code; I see no reason to use it in new code. You can use both structured and unstructured exception handling in a program, but each procedure is limited to one or the other.

Exception handling is an integral part of every well-written program. There's no excuse for letting unhandled exceptions bedevil your end-users. I recommend that you built it in from the start. It's a lot easier than trying to go back to add it later.

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.


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.


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.


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.


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


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


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.


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.


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