What are best practices for arithmetic overflow checking in C#?

Question

There are some good articles on the web / stackoverflow about the usage of the 'checked' keyword vs. the compiler option 'Check for arithmetic overflow/underflow', like those:

http://www.codethinked.com/c-trivia-what-no-overflow

Why doesn't C# use arithmetic overflow checking by default?

Best way to handle Integer overflow in C#?

Still I don't know which one I should use. Per default I would prefer to go with the compiler option to be always safe, to not clutter my code with the unchecked keyword, not to forget it in some places and finally it is not very commonly used, so probably unknown to many developers.

But then how bad is the actual performance hit I take? I guess MS set the default not to do the overflow checking for a good reason. Does the compiler option only concern my code or also every consumed libraries and the framework itself?

I'm also interested in knowing the rationale behind unchecked being the default. It's often claimed that it's for performance reasons, but I've seen conflicting reports about the actual performance impact. Some say it's significant, others say it's imperceptible. — Magnus Grindal Bakken
– Magnus Grindal Bakken, Commented Oct 7, 2013 at 13:02
Turn checking on, and worry about speed when and where a Profiler tells you to. — Henk Holterman
– Henk Holterman, Commented Oct 7, 2013 at 13:14

vlabatut · Accepted Answer · 2014-01-31 09:21:33Z

I had the same question. I prefer piece of code to be checked by default in my company's code, because overflow side effects can cost a lot and be hard to diagnose. Discovering the real reason of those side effects can be every valuable.

The question is, what do we lose in terms of performance?

Here is a very simple bench :

static void Main(string[] args)
{
    long c = 0;
    var sw = new Stopwatch();
    sw.Start();
    unchecked
    {
        for (long i = 0; i < 500000000; i++) c += 1;
    }
    sw.Stop();
    Console.WriteLine("Unchecked: " + sw.ElapsedMilliseconds);

    c = 0;
    sw.Restart();
    checked
    {
        for (long i = 0; i < 500000000; i++) c += 1;
    }
    sw.Stop();
    Console.WriteLine("Checked: " + sw.ElapsedMilliseconds);
}

In the generated IL, I see that the checked and unchecked keyword determines whether the add.ovf or add instruction will be used. (both for Debug and Release configs)

IL_001c:  ldloc.2
IL_001d:  ldc.i4.1
IL_001e:  conv.i8
IL_001f:  add


IL_0066:  ldloc.2
IL_0067:  ldc.i4.1
IL_0068:  conv.i8
IL_0069:  add.ovf

Results (x64 host)

Debug

Unchecked: 2371
Checked: 2437

Release

Unchecked: 2088
Checked: 2266

Other results by replacing long(s) by an int(s) (x64 host)

Debug

Unchecked: 1665
Checked: 1568

Release

Unchecked: 189
Checked: 566

The performance hit is there, it looks like it is more important to choose the right variable type that to go checked or unchecked. Anyway it doesn't change my opinion. I'll turn on the "Check for arithmetic overflow/underflow" in all our projects! (Advanced Build Settings).

When in need of performance, I'll simply use an unchecked block.

FYI, performance hit looks similar for other types such as decimal. Or with multiplications...

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What are best practices for arithmetic overflow checking in C#?

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