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.