What is the point of finally in a try catch/except finally statement

exception-handling finally
Ali picture Ali · Mar 13, 2012 · Viewed 9.2k times · Source

I have used try-catch/except-finally variants in many languages for years, today someone asked me what is the point of finally and I couldn't answer.

Basically why would you put a statement in finally instead of just putting it after the whole try-catch block? Or in other words is there a difference between the following blocks of code:

try{ //a}
catch {//b}
finally {//c}


try{//a}
catch{//b}
//c

EDIT:
PEOPLE, I know what finally does, I have been using it for ages, but my question is in the above example putting //c in finally seems redundant, doesn't it?

Answer

supercat picture supercat · Mar 13, 2012

The purpose of a finally block is to ensure that code gets run in three circumstances which would not very cleanly be handled using "catch" blocks alone:

  1. If code within the try block exits via return
  2. If code within a catch block either rethrows the caught exception, or--accidentally or intentionally--ends up throwing a new one.
  3. If the code within the try block encounters an exception which for which the try has no catch.

One could copy the finally code before every return or throw, and wrap catch blocks within their own try/catch to allow for the possibility of an accidental exception occurring, but it's far easier to forgo all that and simply use a finally block.

BTW, one thing I wish language designers would include would be an exception argument to the finally block, to deal with the case where one needs to clean up after an exception but still wants it to percolate up the call stack (e.g. one could wrap the code for a constructor in such a construct, and Dispose the object under construction if the constructor was going to exit with an exception).

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