How to safely convert from a double to a decimal in c#

Question

We are storing financial data in a SQL Server database using the decimal data type and we need 6-8 digits of precision in the decimal. When we get this value back through our data access layer into our C# server, it is coming back as the decimal data type.

Due to some design constraints that are beyond my control, this needs to be converted. Converting to a string isn't a problem. Converting to a double is as the MS documentation says "[converting from decimal to double] can produce round-off errors because a double-precision floating-point number has fewer significant digits than a decimal."

As the double (or string) we can round to 2 decimal places after any calculations are done, so what is the "right" way to do the decimal conversion to ensure that we don't lose any precision before the rounding?

Answer 1

The conversion won't produce errors within the first 8 digits. double has 15-16 digits of precision - less than the 28-29 of decimal, but enough for your purposes by the sounds of it.

You should definitely put in place some sort of plan to avoid using double in the future, however - it's an unsuitable datatype for financial calculations.

Answer 2

If you round to 2dp, IMO the "right" way would be store an integer that is the multiple - i.e. for 12.34 you store the integer 1234. No more double rounding woe.

If you must use double, this still works; all integers are guaranteed to be stored exactly in double - so still use the same trick.