decimal to double

Question

I have the following test code:

decimal test1 = 0.0500000000000000045656554454M;
double test2 = (double)test1;

This results in test2 showing as 0.05 when debugging. Why is it being rounded to 2 decimal places?

Thanks

Answer 1

The reason is that double can contain no more than 15-16 significant digits.

see double (C# Reference)

Answer 2

There are two reasons I can think of:

1. Due to the different representation of decimal and double. See this article for more information about floating point representation. It is possible that there are not enough bits for the whole number representation in the double.
2. Due to the way numbers are printed. It is possible that in your printing options, there are less than 18 numbers after the decimal point specified - in which case, you'll get the rounded result.

I would check for tweaking the printing options first to make sure that the problem isn't there first.

.. But know that the only solution for the first problem is stop using double :-)

Answer 3

You should take a look at this article about floating-point arithmetic and .NET. The rounding occurs due to a combination of how the number gets converted to a double-precision floating point value and how it is formatted when printed, since .NET defaults to 15 decimals for doubles, and your original number contains decimal past the 15th.

You could try test2.ToString("0.000000000000000000000000") to see if you might squeeze out any more information from the number, but I doubt it will.

Answer 4

here's a link to the documentation

Answer 5

The value from that conversion is actually 0.050000000000000009714451465470119728706777095794677734375, as shown by DoubleConverter. That's the exact value of the nearest double to the decimal you converted.

When you use the debugger or normal string formatting, you aren't usually shown the exact result.