指定小数位数时，Math.Round与MidpointRounding.ToEven的行为是否正确？

Question

I'm trying to use Math.Round however the results I'm getting are not what I expected. I'm finding that it only rounds to even when the following digit is a 5 and cannot have any other digits after that.

Assert.AreEqual<double>(4d, Math.Round(4.5));          // Pass
Assert.AreEqual<double>(5d, Math.Round(4.6));          // Pass
Assert.AreEqual<double>(6d, Math.Round(5.5));          // Pass
Assert.AreEqual<double>(4d, Math.Round(4.500001));     // Fail - 5
Assert.AreEqual<double>(4d, Math.Round(4.45));         // Pass
Assert.AreEqual<double>(4.4, Math.Round(4.45, 1));     // Pass
Assert.AreEqual<double>(4.4, Math.Round(4.450001, 1)); // Fail - 4.5 

The behaviour is also the same for Decimal.Round .

Assert.AreEqual<decimal>(4m, Decimal.Round(4.500001m));      // Fail - 5
Assert.AreEqual<decimal>(4.4m, Decimal.Round(4.450001m, 2)); // Fail - 4.5

Shouldn't rounding only take into account the digit directly following the decimal place you're rounding to, as described in MidpointRounding ?

A rounding operation takes an original number with an implicit or specified precision; examines the next digit, which is at that precision plus one; and returns the nearest number with the same precision as the original number.

Answer 1

The behaviour is correct. You are arguing that 4.50001, 4.51, 4.59, 4.599999999999 should all round down to 4. It is clear that figures over 4.5 are closer to 5 than they are to 4 and therefore should be rounded up to 5.