We've seen plenty of questions about when and why to use try
/catch
and try
/catch
/finally
. And I know there's definitely a use case for try
/finally
(especially since it is the way the using
statement is implemented).
We've also seen questions about the overhead of try/catch and exceptions.
The question I linked to, however, doesn't talk about the overhead of having JUST try-finally.
Assuming there are no exceptions from anything that happens in the try
block, what's the overhead of making sure that the finally
statements get executed on leaving the try
block (sometimes by returning from the function)?
Again, I'm asking ONLY about try
/finally
, no catch
, no throwing of exceptions.
Thanks!
EDIT: Okay, I'm going to try to show my use case a little better.
Which should I use, DoWithTryFinally
or DoWithoutTryFinally
?
public bool DoWithTryFinally()
{
this.IsBusy = true;
try
{
if (DoLongCheckThatWillNotThrowException())
{
this.DebugLogSuccess();
return true;
}
else
{
this.ErrorLogFailure();
return false;
}
}
finally
{
this.IsBusy = false;
}
}
public bool DoWithoutTryFinally()
{
this.IsBusy = true;
if (DoLongCheckThatWillNotThrowException())
{
this.DebugLogSuccess();
this.IsBusy = false;
return true;
}
else
{
this.ErrorLogFailure();
this.IsBusy = false;
return false;
}
}
This case is overly simplistic because there are only two return points, but imagine if there were four… or ten… or a hundred.
At some point I would want to use try
/finally
for the following reasons:
- Keep to DRY principles (especially as the number of exit points gets higher)
- If it turns out that I'm wrong about my inner function not throwing an exception, then I want to make sure
this.Working
is set tofalse
.
So hypothetically, given performance concerns, maintainability, and DRY principles, for what number of exit points (especially if I can assume that all inner exceptions are caught) do I want to incur whatever performance penalty is associated with try
/finally
?
EDIT #2: I changed the name of this.Working
to this.IsBusy
. Sorry, forgot to mention this is multithreaded (though only one thread will ever actually call the method); other threads will be polling to see if the object is doing its work. The return value is merely success or failure for if the work went as expected.
Best Answer
Why not look at what you actually get?
Here is a simple chunk of code in C#:
And here is the resulting IL in the debug build:
and here's the assembly generated by the JIT when running in debug:
Now, if I comment out the try and finally and the return, I get nearly identical assembly from the JIT. The differences you'll see are a jump into the finally block and some code to figure out where to go after the finally is executed. So you're talking about TINY differences. In release, the jump into the finally will get optimized out - braces are nop instructions, so this would become a jump to the next instruction, which is also a nop - that's an easy peephole optimization. The pop eax and then jmp eax is similarly cheap.
So you're talking very, very tiny costs for try/finally. There are very few problem domains where this matters. If you're doing something like memcpy and put a try/finally around each byte being copied and then proceed to copy hundreds of MB of data, I could see that being an issue, but in most usage? Negligible.