Inaccuracy of decimal in .NET

Question

Yesterday during debugging something strange happened to me and I can't really explain it:

So maybe I am not seeing the obvious here or I misunderstood something about decimals in .NET but shouldn't the results be the same?

Answer 1

decimal is not a magical do all the maths for me type. It's still a floating point number - the main difference from float is that it's a decimal floating point number, rather than binary. So you can easily represent 0.3 as a decimal (it's impossible as a finite binary number), but you don't have infinite precision.

This makes it work much closer to a human doing the same calculations, but you still have to imagine someone doing each operation individually. It's specifically designed for financial calculations, where you don't do the kind of thing you do in Maths - you simply go step by step, rounding each result according to pretty specific rules.

In fact, for many cases, decimal might work much worse than float (or better, double). This is because decimal doesn't do any automatic rounding at all. Doing the same with double gives you 22 as expected, because it's automatically assumed that the difference doesn't matter - in decimal, it does - that's one of the important points about decimal. You can emulate this by inserting manual Math.Rounds, of course, but it doesn't make much sense.