Welford's method(variance and standard deviation)

Variance.cs

using System;

namespace variance
{
    public class Variance
    {
        private int _count = 1;
        private double _mean = 0.0;
        private double _variance = 0.0;

        // Caculate Standard deviation
        public double NewStdev(int newValue)
        {
            double oldMean = _mean;
            _mean = oldMean + ((newValue - oldMean) / _count);
            _variance = _variance + ((newValue - _mean) * (newValue - oldMean));

            return Math.Sqrt(_variance / (_count++ - 1));
        }
    }
}