language agnostic - Why Re-throw Exceptions?

Question

I've seen the following code many times:

try
{
    ... // some code
}
catch (Exception ex)
{
    ... // Do something
    throw new CustomException(ex);

    // or
    // throw;

    // or
    // throw ex;
}

Can you please explain the purpose of re-throwing an exception? Is it following a pattern/best practice in exception handling? (I've read somewhere that it's called "Caller Inform" pattern?)

Answer 1

Rethrowing the same exception is useful if you want to, say, log the exception, but not handle it.

Throwing a new exception that wraps the caught exception is good for abstraction. e.g., your library uses a third-party library that throws an exception that the clients of your library shouldn't know about. In that case, you wrap it into an exception type more native to your library, and throw that instead.