What are some best practices for creating my own custom exception?

Question

In a follow-up to a previous question regarding exceptions, what are best practices for creating a custom exception in .NET?

More specifically should you inherit from System.Exception, System.ApplicationException or some other base exception?

Answer 1

Inherit from System.Exception. System.ApplicationException is useless and the design guidelines say "Do not throw or derive from System.ApplicationException."

See http://blogs.msdn.com/kcwalina/archive/2006/06/23/644822.aspx

Answer 2

In the C# IDE, type 'exception' and hit TAB. This will expand to get you started in writing a new exception type. There are comments withs links to some discussion of exception practices.

Personally, I'm a big fan of creating lots of small classes, at that extends to exception types. For example, in writing the Foo class, I can choose between:

1. throw new Exception("Bar happened in Foo");
2. throw new FooException("Bar happened");
3. throw new FooBarException();

where

class FooException : Exception 
{
    public FooException(string message) ... 
}

and

class FooBarException : FooException 
{
    public FooBarException() 
        : base ("Bar happened") 
    {
    }
}

I prefer the 3rd option, because I see it as being an OO solution.

Answer 3

There is a code snippet for it. Use that. Plus, check your code analysis afterwards; the snippet leaves out one of the constructors you should implement.

Answer 4

I think the single most important thing to remember when dealing with exceptions at any level (making custom, throwing, catching) is that exceptions are only for exceptional conditions.

Answer 5

The base exception from where all other exceptions inherit from is System.Exception, and that is what you should inherit, unless of course you have a use for things like, say, default messages of a more specific exception.