What is the real overhead of try/catch in C#?

So, I know that try/catch does add some overhead and therefore isn't a good way of controlling process flow, but where does this overhead come from and what is it's actual impact?

Solution 1:

Three points to make here:

• Firstly, there is little or NO performance penalty in actually having try-catch blocks in your code. This should not be a consideration when trying to avoid having them in your application. The performance hit only comes into play when an exception is thrown.

• When an exception is thrown in addition to the stack unwinding operations etc that take place which others have mentioned you should be aware that a whole bunch of runtime/reflection related stuff happens in order to populate the members of the exception class such as the stack trace object and the various type members etc.

• I believe that this is one of the reasons why the general advice if you are going to rethrow the exception is to just throw; rather than throw the exception again or construct a new one as in those cases all of that stack information is regathered whereas in the simple throw it is all preserved.

Solution 2:

I'm not an expert in language implementations (so take this with a grain of salt), but I think one of the biggest costs is unwinding the stack and storing it for the stack trace. I suspect this happens only when the exception is thrown (but I don't know), and if so, this would be decently sized hidden cost every time an exception is thrown... so it's not like you are just jumping from one place in the code to another, there is a lot going on.

I don't think it's a problem as long as you are using exceptions for EXCEPTIONAL behavior (so not your typical, expected path through the program).

Solution 3:

Are you asking about the overhead of using try/catch/finally when exceptions aren't thrown, or the overhead of using exceptions to control process flow? The latter is somewhat akin to using a stick of dynamite to light a toddler's birthday candle, and the associated overhead falls into the following areas:

• You can expect additional cache misses due to the thrown exception accessing resident data not normally in the cache.

• You can expect additional page faults due to the thrown exception accessing non-resident code and data not normally in your application's working set.

  • for example, throwing the exception will require the CLR to find the location of the finally and catch blocks based on the current IP and the return IP of every frame until the exception is handled plus the filter block.
  • additional construction cost and name resolution in order to create the frames for diagnostic purposes, including reading of metadata etc.

  • both of the above items typically access "cold" code and data, so hard page faults are probable if you have memory pressure at all:

    • the CLR tries to put code and data that is used infrequently far from data that is used frequently to improve locality, so this works against you because you're forcing the cold to be hot.
    • the cost of the hard page faults, if any, will dwarf everything else.
• Typical catch situations are often deep, therefore the above effects would tend to be magnified (increasing the likelihood of page faults).

As for the actual impact of the cost, this can vary a lot depending on what else is going on in your code at the time. Jon Skeet has a good summary here, with some useful links. I tend to agree with his statement that if you get to the point where exceptions are significantly hurting your performance, you have problems in terms of your use of exceptions beyond just the performance.

Solution 4:

In my experience the biggest overhead is in actually throwing an exception and handling it. I once worked on a project where code similar to the following was used to check if someone had a right to edit some object. This HasRight() method was used everywhere in the presentation layer, and was often called for 100s of objects.

bool HasRight(string rightName, DomainObject obj) {
  try {
    CheckRight(rightName, obj);
    return true;
  }
  catch (Exception ex) {
    return false;
  }
}

void CheckRight(string rightName, DomainObject obj) {
  if (!_user.Rights.Contains(rightName))
    throw new Exception();
}

When the test database got fuller with test data, this lead to a very visible slowdown while openening new forms etc.

So I refactored it to the following, which - according to later quick 'n dirty measurements - is about 2 orders of magnitude faster:

bool HasRight(string rightName, DomainObject obj) {
  return _user.Rights.Contains(rightName);
}

void CheckRight(string rightName, DomainObject obj) {
  if (!HasRight(rightName, obj))
    throw new Exception();
}

So in short, using exceptions in normal process flow is about two orders of magnitude slower then using similar process flow without exceptions.