try-catch-finally order of execution

Question

I'm having a problem understanding the order of execution for the try-catch-finally. All the example I've seen (like in:http://stackoverflow.com/questions/4191027/order-of-execution-of-try-catch-and-finally-block) have a very simple "catch" part which print to console. but what happen if I use a "throw" statement in the catch?

The simplest code I could think of which capture the problem:

public class TestClass
{
    void Foo(int num)
    {
        int answer = 100;
        try
        {
            answer = 100 / num;
        }
        catch (Exception e)
        {
            //Probably num is 0
            answer = 200;
            throw;
        }
        finally
        {
            Console.WriteLine("The answer is: " + answer);
        }
    }
}

If num == 2, then the output will be:

The answer is: 50

But what would be printed for num == 0?

The answer is: 100
The answer is: 200
No printing at all...

or is it just a "undefined behavior"?

Answer 1

If an exception occurs inside the try block, the code inside the catch block is executed. If you have several catch blocks, the one that matches the caught exception best is excuted.

class A : System.Exception {}
class B : A {}

void Test()
{
    try
    {
        throw new B();
    }
    catch (A a)
    {
        //as B is derived from A, this catch block will be invoked.
    }
    catch (Exception e)
    {
    }
}

The finally block is executed after all. It doesn't matter whether or not an exception occured.

[EDIT] To clarify the order a bit more (thanks to the comments)

void Test()
{
    Debug.WriteLine("1");
    try
    {
        Debug.WriteLine("2");
        throw new Exception();
        Debug.WriteLine("3");
    }
    catch
    {
        Debug.WriteLine("4");
        throw;
        Debug.WriteLine("5");
    }
    finally
    {
        Debug.WriteLine("6");
    }
    Debug.WriteLine("7");
}

What will be printed is:

1 2 4 6

3 is not printed because an exception si throw before it. The same for 5. 7 is not printed because of the throw in the catch block.

[/EDIT]

So answering your question: The answer is: 200

Answer 2

By popular demand from a comment to another answer:

it will execute the throw immediately. So any code in the catch block after the throw will not be executed. However, it will execute the finally before passing the thrown exception up to the appropriate parent.

Original answer

Your code will execute the try block. If this succeeds fine it will run the finally block. It will then end.

If the try block throws an exception somewhere then it will immediately stop executing at that point and start executing the catch block (assuming that exception type is caught). Once the catch block has finished executing it will execute the finally block. Then in this case it will propogate the thrown error up the stack.

Edited to add: yes, it will print 200 as the answer becuase the final is the final part to run, after your catch block has reset the answer.

Answer 3

The easiest way to test is to try it. It should print - the answer is 200, and then error out.

Finally will always be called (except for some exceptions that cannot be caught, e.g. stack overflow). You should take care to try and not throw an exception from within your finally block...

Here your flow will be:

exception caused
caught
answer variable set
exception thrown
finally block executed
exception propogated up the stack

Answer 4

It is difficult to explain, but generally when the scope (which is the stackframe in your case) is exited the finally clause will execute.