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 function to do the more natural round-half-up algorithm:

public static decimal RoundHalfUp(this decimal d, int decimals)
{
    if (decimals < 0)
    {
        throw new ArgumentException("The decimals must be non-negative", "decimals");
    }

    decimal multiplier = (decimal)Math.Pow(10, decimals);
    decimal number = d * multiplier;

    if (decimal.Truncate(number) < number)
    {
        number += 0.5m;
    }
    return decimal.Round(number) / multiplier;
}

Does anybody know the reason behind this framework design decision? Is there any built-in implementation of the round-half-up algorithm into the framework? Or maybe some unmanaged Windows API?

It could be misleading for beginners that simply write decimal.Round(2.5m, 0) expecting 3 as a result but getting 2 instead.

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

I would expect 72.5, but the result is 72.4

I wouldn't.

Are you saying you'd round 2.444444444444444449 up to three? (If you were rounding to zero decimal places)

Answer 4

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”