Why does .NET use banker's rounding as default?

Question

According to the documentation, the decimal.Round method uses a round-to-even algorithm which is not common for most applications. So I always end up writing a custom functi

Answer 1

Probably because it's a better algorithm. Over the course of many roundings performed, you will average out that all .5's end up rounding equally up and down. This gives better estimations of actual results if you are for instance, adding a bunch of rounded numbers. I would say that even though it isn't what some may expect, it's probably the more correct thing to do.

Answer 2

While I cannot answer the question of "Why did Microsoft's designers choose this as the default?", I just want to point out that an extra function is unnecessary.

Math.Round allows you to specify a MidpointRounding:

• ToEven - When a number is halfway between two others, it is rounded toward the nearest even number.
• AwayFromZero - When a number is halfway between two others, it is rounded toward the nearest number that is away from zero.

Answer 3

Decimals are mostly used for money; banker’s rounding is common when working with money. Or you could say.

It is mostly bankers that need the decimal type; therefore it does “banker’s rounding”

Bankers rounding have the advantage that on average you will get the same result if you:

• round a set of “invoice lines” before adding them up,
• or add them up then round the total

Rounding before adding up saved a lot of work in the days before computers.

(In the UK when we went decimal banks would not deal with half pence, but for many years there was still a half pence coin and shop often had prices ending in half pence – so lots of rounding)

Answer 4

The other answers with reasons why the Banker's algorithm (aka round half to even) is a good choice are quite correct. It does not suffer from negative or positive bias as much as the round half away from zero method over most reasonable distributions.

But the question was why .NET use Banker's actual rounding as default - and the answer is that Microsoft has followed the IEEE 754 standard. This is also mentioned in MSDN for Math.Round under Remarks.

Also note that .NET supports the alternative method specified by IEEE by providing the MidpointRounding enumeration. They could of course have provided more alternatives to solving ties, but they choose to just fulfill the IEEE standard.

Answer 5

Use another overload of Round function like this:

decimal.Round(2.5m, 0,MidpointRounding.AwayFromZero)

It will output 3. And if you use

decimal.Round(2.5m, 0,MidpointRounding.ToEven)

you will get banker's rounding.