Question

I’ve got an array of integers, and I’m looping through them:

for (int i = 0; i < data.Length; i++)
{
  // do a lot of stuff here using data[i]
}

If I do:

for (int i = 0; i < data.Length; i++)
{
  int value = data[i];
  // do a lot of stuff with value instead of data[i]
}

Is there any performance gain/loss?

From my understanding, C/C++ array elements are accessed directly, i.e. an n-element array of integers has a contiguous memory block of length n * sizeof(int), and the program access element i by doing something like *data[i] = *data[0] + (i * sizeof(int)). (Please excuse my abuse of notation, but you get what I mean.)

So this means C/C++ should have no performance gain/loss for referencing array variables.

What about C#?
C# has a bunch of extra overhead like data.Length, data.IsSynchronized, data.GetLowerBound(), data.GetEnumerator().

Clearly, a C# array is not the same as a C/C++ array.

So what’s the verdict? Should I store int value = data[i] and work with value, or is there no performance impact?

Answer 1

Yes, there is a performance loss due to the bounds check for every access to the array.

No, you most likely don’t need to worry about it.

Yes, you can should store the value and work with the value. No, this isn’t because of the performance issue, but rather because it makes the code more readable (IMHO).

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By the way, the JIT compiler might optimize out redundant checks, so it doesn’t mean you’ll actually get a check on every call. Either way, it’s probably not worth your time to worry about it; just use it, and if it turns out to be a bottleneck you can always go back and use unsafe blocks.