All this is about cultures. If you have any other culture than “US English” (and also as good manners of development), you should use something like this:
var d = Convert.ToDecimal("1.2345", new CultureInfo("en-US"));
// (or 1,2345 with your local culture, for instance)
(obviously, you should replace the “en-US” with the culture of your number local culture)
the same way, if you want to do ToString()
d.ToString(new CultureInfo("en-US"));