100% solution for codility sample test

equilibriumIndex.cs

class Solution {
    public int solution(int[] A) {
        long totalSum = 0;
        long rightSum = 0, leftSum = 0;
        
        Array.ForEach(A, i => totalSum += i);

        for (int i = 0; i < A.Length; i++)
        {
            leftSum += i > 0 ? A[i-1] : 0;
            rightSum = totalSum - (leftSum + A[i]);

            if (rightSum == leftSum)
                return i;
        }

        return -1;
    }
}

This is what got me 100%, if you have ideas for improvements let me know :)

Array.ForEach and long types are being used to handle larger numbers (account for OverflowException)

You can read the task description and full results here:
https://codility.com/demo/results/demoSQ5HUA-N6D/

I'm not posting it here to avoid copyright infringement.