From Java To C# A Developers Guide

13.2 C# exception hierarchy

Like Java, all exception classes are subclasses of one special exception class. ^[8] In Java, it is java.lang.Exception; ^[9] in C# the grandfather of all exception classes is System.Exception.

^[8] C++ developers will find this a new idea. In C++, anything can be an exception ?there is no rule to say that exceptions must be instances of any special class.

^[9] Which is in turn a direct subclass of java.lang.Throwable. For Java, Throwable is a direct subclass of java.lang.Object. From Throwable, we have java.lang.Error and java.lang.Exception. C# does not differentiate between exceptions and errors.

Figure 13.4 shows some significant predefined exception classes in C#.

Table 13.1 briefly describes some of the predefined exceptions. It should be evident from their names what each exception does.

Table 13.1. Some common exceptions and their descriptions

Exception class	Comments
ApplicationException	Thrown by a user program (not by the CLR) when a non-fatal error occurs. If you are writing your own exception classes, derive them from this class. Although this exception extends System.Exception, it does not add new functionality ?rather, its main purpose is to differentiate user-written application exceptions from those thrown by the CLR.
ArgumentException	Thrown when a method is invoked and at least one of the passed arguments does not meet the parameter specification of the called method. Instances of this exception should carry a meaningful error message describing the invalid argument, as well as the expected range of values for the argument.
ArgumentNullException	Thrown when a method is invoked and at least one of the passed arguments is a null reference which should not be null.
ArithmeticException	Base class for DivideByZeroException NotFiniteNumberException, and OverflowException.
DirectoryNotFoundException	Thrown when part of a file or directory cannot be found.
FileNotFoundException	Thrown on an attempt to access a file that does not exist.
IndexOutOfRangeException	Thrown when an attempt is made to access an element of an array with an index outside the bounds of the array. Similar to java.lang.ArrayIndexOutOfBoundsException.
InvalidCastException	Thrown for invalid casting or explicit conversion. Similar to java.lang.ClassCastException.
IOException	Base class for exceptions thrown while accessing information using streams, files, and directories. Subclasses of IOException include DirectoryNotFoundException, EndOfStreamException, FileNotFoundException, FileLoadException, and PathTooLongException. I/O exceptions are in the System.IO namespace.
NullReferenceException	Thrown when there is an attempt to dereference a null object reference. Similar to the infamous java.lang.NullPointerException.
OutOfMemoryException	Thrown when there is not enough memory to continue the execution of a program. Similar to java.lang.OutOfMemoryError. [1]
SystemException	Thrown by the CLR when errors occur that are non-fatal and recoverable by user programs. Although this exception extends System.Exception, it does not add new functionality ?rather, its main purpose is to differentiate exceptions thrown by the CLR and user-written application exceptions

^[1] Unlike Java, C# does not differentiate between errors and exceptions, hence the name OutOfMemoryException.