n桁の有効数字に丸める

private void button_Click(object sender, EventArgs e)
{
    Debug.WriteLine(calc_significant_figures(1040000.00M, 2, 0)); // -> 1000000
    Debug.WriteLine(calc_significant_figures(1050000.00M, 2, 0)); // -> 1100000
    Debug.WriteLine(calc_significant_figures(10.0000400M, 6, 0)); // -> 10.0000
    Debug.WriteLine(calc_significant_figures(10.0000500M, 6, 0)); // -> 10.0001

    Debug.WriteLine(calc_significant_figures(1050000.00M, 2, 1)); // -> 1000000
    Debug.WriteLine(calc_significant_figures(10.0000500M, 6, 1)); // -> 10.0000

    Debug.WriteLine(calc_significant_figures(1040000.00M, 2, 2)); // -> 1100000
    Debug.WriteLine(calc_significant_figures(10.0000400M, 6, 2)); // -> 10.0001
}

static decimal calc_significant_figures(decimal value, int digits, int type)
{
    if (value == 0M)
    {
        return 0M;
    }

    var integer_part = (int)Math.Log10(Math.Abs((int)value)) + 1;

    //var decimal_part = decimal.GetBits(value)[3] >> 16 & 0x0FF;

    switch (type)
    {
        case 0:
            {
                var scale = (decimal)Math.Pow(10, integer_part);
                var result = scale * Math.Round(value / scale, digits, MidpointRounding.AwayFromZero);

                //return Math.Round(result, decimal_part);

                var decimals = digits - integer_part;
                return Math.Round(result, (decimals < 0) ? 0 : decimals);
            }
        case 1:
            {
                var scale = (decimal)Math.Pow(10, integer_part - digits);
                return scale * Math.Floor(value / scale);
            }
        case 2:
            {
                var scale = (decimal)Math.Pow(10, integer_part - digits);
                return scale * Math.Ceiling(value / scale);
            }
    }

    throw new Exception("error: calc_significant_figures");
}

有効数字 @wikipedia