Why is AddRange faster than using a foreach loop?

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Potentially, AddRange can check where the value passed to it implements IList or IList<T>. If it does, it can find out how many values are in the range, and thus how much space it needs to allocate… whereas the foreach loop may need to reallocate several times.

Additionally, even after allocation, List<T> can use IList<T>.CopyTo to perform a bulk copy into the underlying array (for ranges which implement IList<T>, of course.)

I suspect you’ll find that if you try your test again but using Enumerable.Range(0, 100000) for fillData instead of a List<T>, the two will take about the same time.

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