CHAPTER 11 EXCEPTIONS

The Exception Classes

There are many different types of exceptions that can occur in a program. The BCL defines a number of exception classes, each representing a specific type. When one occurs, the CLR does the following:

•It creates an exception object for the type.

•It looks for an appropriate catch clause to handle it.

All exception classes are ultimately derived from the System.Exception class. Figure 11-2 shows a portion of the exception inheritance hierarchy.

Figure 11-2. Structure of the exception hierarchy

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An exception object contains read-only properties with information about the exception that caused it. Table 11-1 shows some of these properties.

Table 11-1. Selected Properties of an Exception Object

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The catch Clause

The catch clause handles exceptions. There are three forms, allowing different levels of processing. Figure 11-3 shows the forms.

Figure 11-3. The three forms of the catch clause

The general catch clause can accept any exception but can’t determine the type of exception that caused it. This allows only general processing and cleanup for whatever exception might occur.

The specific catch clause form takes the name of an exception class as a parameter. It matches exceptions of the specified class or exception classes derived from it.

The specific catch clause with object form gives you the most information about the exception. It matches exceptions of the specified class, or exception classes derived from it. It gives you a reference to the exception object created by the CLR, by assigning it to the exception variable. You can access the exception variable’s properties within the block of the catch clause to get specific information about the exception raised.

For example, the following code handles exceptions of type IndexOutOfRangeException. When one occurs, a reference to the actual exception object is passed into the code with parameter name e. The three WriteLine statements each read a string field from the exception object.

Exception type Exception variable

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Examples Using Specific catch Clauses

Going back to our divide-by-zero example, the following code modifies the previous catch clause to specifically handle exceptions of the DivideByZeroException class. While in the previous example, the catch clause would handle any exception raised in the try block, the current example will only handle those of the DivideByZeroException class.

catch ( DivideByZeroException )

{

...

Console.WriteLine("Handling an exception.");

}

You could further modify the catch clause to use an exception variable. This allows you to access the exception object inside the catch block.

Exceptions 1\Program.cs:line 14

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