问题
Why does this result in 0 and not 1?
Math.Round(0.5, 0, MidpointRounding.AwayFromZero)
Here's an example: http://ideone.com/ayMVO
回答1:
Normally, with issues like that, it's because the number cannot be represented exactly in IEEE754 - it's most likely it's being converted to 0.4999999999... or something like that which would round to zero.
However, 0.5 (1/2) is exactly representable in IEEE754 so that's not the case here.
It's possible that the compiler made a mistake in converting the text to a number but I would think that unlikely. In fact, when I compile and run the following in VC#2010:
using System;
namespace ConsoleApplication1 {
class Program {
static void Main (string[] args) {
Console.WriteLine (Math.Round (0.5, 0, MidpointRounding.AwayFromZero));
}
}
}
I get the output of 1 as expected.
So I don't think your question is quite complete. Now it may be that your actual situation doesn't use a hard-coded value of 0.5 but instead uses a variable which you believe to be 0.5.
My answer to that would be that, while it may be close to 0.5 (certainly close enough that a simple WriteLine of it might output 0.5), it's probably a touch below 0.5, which would cause my comments in the first paragraph above to once again kick in.
Based on your link, it appears that Mono 2.8 may have a bug. I'd suggesting seeking support from the developers (or just raising a bug report) here.
Actually, having tried it locally with gmcs 2.6.7, it's definitely a bug. That exact code compiled okay but generates 0 rather than 1. A bug report has been raised, and I've added my own information to it.
回答2:
This also returns 1.0 for me when I debugged it.
Math.Round(0.5, 0, MidpointRounding.AwayFromZero)
回答3:
0.5 is a double.
If you do
Math.Round(0.5M, 0, MidpointRounding.AwayFromZero)
It will work as expected
来源：https://stackoverflow.com/questions/9475448/why-does-0-5-rounded-with-midpointrounding-awayfromzero-result-in-0