Rounding decimal value

Question

I am facing problem while rounding the decimal value in C# using Math.Round(a, 2);

When I'm rounding 1.275 by 2 decimal points, the result is 1.27. When I'm doing the same for 1.375, the result is 1.38.

Why is it not rounding 1.275 to 1.28?

Thanks

Answer 1

I cannot reproduce your problem:

Math.Round(1.275m, 2) => 1.28m
Math.Round(1.375m, 2) => 1.38m

I suspect that your claim that you use a decimal value is false, and that you use double value instead. double can't represent many decimal values exactly, so when you write 1.275, it's actually 1.27499... 1.375 is one of the few representable onces, so it's actually 1.375.

If your code cares about exact decimal representation, for example when you work on money, you must use decimal and not binary floating point such as double or float.

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But even if you use decimal representation, rounding behaves unexpectedly for many users:

Math.Round(1.265m, 2) => 1.26m
Math.Round(1.275m, 2) => 1.28m

By default Math.Round uses MidpointRounding.ToEven, also known as Banker's round. This avoids accumulating a bias from always rounding up at .5.

You can use an overload of Round that takes a rounding mode, and set it to AwayFromZero to get the behaviour you expect.

Math.Round(1.275m, 2, MidpointRounding.AwayFromZero) => 1.28m