public double? diap(double left, double right, int channel, List<double> _wspom, Random random)
        {
            Class_Compute clC = new Class_Compute();
            double value = clC.opros(channel, _wspom, random);
            if ((value >= left) && (value <= right))
                return value;
            else
            {
                double? z = null; //использование null
                return z;
            }
        }

SqlParameter param = new SqlParameter();
param.Value = calcul.diap(left, right, channel, wspom, random);