How does Math.Round(2.5) behave?

In C#, the result of Math.Round(2.5) is 2.

It is supposed to be 3, isn't it? Why is it 2 instead in C#?

Answer:

The integer nearest a. If the fractional component of a is halfway between two integers, one of which is even and the other odd, then the even number is returned.

 and so 2.5, being halfway between 2 and 3, is rounded down to the even number (2). this is called Banker's Rounding (or round-to-even), and is a commonly-used rounding standard.

The behavior of this method follows IEEE Standard 754, section 4. This kind of rounding is sometimes called rounding to nearest, or banker's rounding.

You can specify the behavior of Math.Round using an overload:

Math.Round(2.5, 0, MidpointRounding.AwayFromZero); // gives 3

Math.Round(2.5, 0, MidpointRounding.ToEven); // gives 2