What is the difference between an error and an exception in .NET?

Question

Could you please explain to me what the difference is between an error and an exception?

Answer 1

An exception is a class that takes advantage of language semantics. As others have stated, exceptions interrupt execution up the stack until caught. An exception can be used to convey an error, but more generally is used to convey that something exceptional has occurred.

Errors, on the other hand, can be exceptional or not.

There are several kinds of errors:

• User error - this should be handled without an exception
• Syntax error - this shouldn't compile in statically typed languages (in dynamic languages, they're a little harder to discover)
• Runtime error - this will either result in an exception, or silently fail (usually creating unexpected results)

Really, exceptions should be limited to handling runtime errors, since a user inputting bad data is not "exceptional." To handle user errors, you should take the following approaches:

• Prevent bad data from being input (front-end validation)
• Prevent bad data from being persisted (back-end validation)

Exceptions should be used as a "last line of defense" for user error. If you're writing a persistence layer, you can rely on exceptions to ensure that bad data that falls through validation does not get persisted. You should, however, fix any of these by putting a fix in the validation that prevents the error from occurring in the first place.

Answer 2

An exception is an object of a type deriving from the System.Exception class. It is used in a throw statement to transfer control to a catch clause in a try block somewhere further up the call stack.

An error is just some code or message that you're meant to interpret. The problem with error codes is that you can decide to ignore them:

MethodThatReturnsAnError();
SomeCodeThatShouldNotExecuteOnError();

That call will simply ignore the error code if one is returned. However:

MethodThatThrowsAnException();
SomeCodeThatShouldNotExecuteOnError();

This cannot be ignored, and will transfer control up the stack, past "SomeCodeThatShouldNotExecuteOnError();".