Exceptions Overview

When an application encounters an unexpected situation (such as a missing file or input parameter) or a logical error (performing a division-by-zero operation, for example), by default the application will terminate and generate an error display like the one shown in Figure 3.1.

Figure 3.1. By default, .NET displays an error message and terminates the application when any error occurs.

Unhandled errors in an application can result in unexpected termination, lost data, and potentially even create security holes if input values are not properly restricted. The Common Language Runtime (CLR) represents all errors as exceptions. An exception is an instance of the Exception class (or a class that inherits from the Exception class). The Framework class library (FCL) includes a large number of standard exception classes that encapsulate information about common errors. Working with instances of these classes allows the developer to provide robust error-handling solutions.

The .NET Framework provides two general classes or exceptions that derive from the common Exception class:

• ApplicationException Exceptions thrown by the application.

• SystemException Exceptions thrown by the CLR.

Both of these child classes enjoy the same properties and are differentiated only in the source of the exception they represent. Table 3.1 details some of the more common properties of the Exception classes.

Table 3.1. Important Members of the Exception Class

Property	Description
HelpLink	The URL for a help file associated with this exception.
InnerException	An exception associated with the exception whose InnerException property you are accessing. This is helpful when a series of exceptions are involved. Each new exception can preserve the information about the previous one by storing it in this property.
Message	A message explaining the error and possibly offering ways to resolve it.
Source	The name of the application that caused the error.
StackTrace	Specifies where the error occurred. If debugging information is available, the stack trace includes the source file and program line number, along with other information.
TargetSite	The method that threw the exception.