string number = "12.400";
double result;

var flag = Double.TryParse(number, NumberStyles.Float,
               CultureInfo.InvariantCulture.NumberFormat, out result);

decimal result;
Decimal.TryParse (...);

string number = "12.400";
int digitCount = number.Split('.')[1].Length;

double result;
var flag = Double.TryParse(number, NumberStyles.Float, CultureInfo.InvariantCulture.NumberFormat, out result);
string numberFormated = String.Format("{0:0.".PadRight(digitCount, "0") + "}", result);