C# 0 The Complete Reference

X

 that defines a member called 

x

 and the 

Add( )

method, which adds the invoking object’s 

x

 to the 

x

 in the object passed as a parameter. In 

Main( )

, two 

X

 objects are created. The first, 

p

, is initialized. The second, 

q

, is not. Instead, it 

is explicitly assigned 

null

. Then 

p.Add( )

 is called with 

q

 as an argument. Because 

q

 does 

not refer to any object, a 

NullReferenceException

 is generated when the attempt is made to 

obtain the value of 

q.x

.

An interesting exception is 

Exception'>StackOverflowException

, which is thrown when the system 

stack is overrun. One situation in which this can happen is when a recursive method runs 

wild. Because the stack is exhausted, a 

StackOverflowException

 can’t be caught by your 

program. Instead, a stack overflow results in the abnormal termination of your program.

Deriving Exception Classes

Although C#’s built-in exceptions handle most common errors, C#’s exception handling 

mechanism is not limited to these errors. In fact, part of the power of C#’s approach to 

exceptions is its ability to handle exceptions that you create. You can use custom exceptions 

to handle errors in your own code. Creating an exception is easy. Just define a class derived 

from 

Exception

. Your derived classes don’t need to actually implement anything—it is their 

existence in the type system that allows you to use them as exceptions.

N

OTE

N

OTE

In the past, custom exceptions were derived from 

ApplicationException

 since this is the 

hierarchy that was originally reserved for application-related exceptions. However, Microsoft 

no longer recommends this. Instead, at the time of this writing, Microsoft recommends deriving 

custom exceptions from 

Exception

. For this reason, this approach is used here.

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