public static double StandardDeviation(IEnumerable‹double› sequence)
{
    (int Count, double Sum, double SumOfSquares) computation = ComputeSumsAnSumOfSquares(sequence);

    var variance = computation.SumOfSquares - computation.Sum * computation.Sum / computation.Count;
    return Math.Sqrt(variance / computation.Count);
}

private static (int Count, double Sum, double SumOfSquares) ComputeSumsAnSumOfSquares(IEnumerable‹double› sequence)
{
    var computation = (count: 0, sum: 0.0, sumOfSquares: 0.0);

    foreach (var item in sequence)
    {
        computation.count++;
        computation.sum += item;
        computation.sumOfSquares += item * item;
    }

    return computation;
}