Does the c# compiler optimizes Count properties?

Question

List<int> list = ...

for(int i = 0; i < list.Count; ++i)
{
           ...
}

So does the compiler know the list.Count does not have to be called each iteration?

Answer 1

Are you sure about that?

List<int> list = new List<int> { 0 };

for (int i = 0; i < list.Count; ++i)
{
    if (i < 100)
    {
        list.Add(i + 1);
    }
}

If the compiler cached the Count property above, the contents of list would be 0 and 1. If it did not, the contents would be the integers from 0 to 100.

Now, that might seem like a contrived example to you; but what about this one?

List<int> list = new List<int>();

int i = 0;
while (list.Count <= 100)
{
    list.Add(i++);
}

It may seem as if these two code snippets are completely different, but that's only because of the way we tend to think about for loops versus while loops. In either case, the value of a variable is checked on every iteration. And in either case, that value very well could change.

Typically it's not safe to assume the compiler optimizes something when the behavior between "optimized" and "non-optimized" versions of the same code is actually different.