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

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.

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.

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.