问题
I just noticed what seems like a ridiculous flaw with DateTime comparison.
DateTime d = DateTime.Now;
DateTime dUtc = d.ToUniversalTime();
d == dUtc; // false
d.Equals(dUtc); //false
DateTime.Compare(d, dUtc) == 0; // false
It appears that all comparison operations on DateTimes fail to do any type of smart conversion if one is DateTimeKind.Local and one is DateTimeKind.UTC. Is the a better way to reliably compare DateTimes aside from always converting both involved in the comparison to utc time?
回答1:
Edited, my original answer was partially incorrect:
When you call .Equal or .Compare, internally the value .InternalTicks is compared. This field is unequal, because it has been adjusted a couple of hours to represent the time in the universal time. You should see it this way: the DateTime object represents a time in an unnamed timezone, but not a universal time plus timezone. The timezone is either Local (the timezone of your system) or UTC. You might consider this a lack of the DateTime class.
When converting to another timezone, the time is — and should be — adjusted. This is probably why Microsoft chose to use a method as opposed to a property, to emphasize that an action is taken when converting to UTC.
Originally I wrote here that the structs are compared and the flag for System.DateTime.Kind is different. This is not true: it is the amount of ticks that differs:
t1.Ticks == t2.Ticks; // false
t1.Ticks.Equals(t2.Ticks); // false
To safely compare two dates, you could convert them to the same kind. If you convert any date to universal time before comparing you'll get the results you're after:
DateTime t1 = DateTime.Now;
DateTime t2 = t1;
DateTime.Compare(t1.ToUniversalTime(), t2.ToUniversalTime()); // 0
DateTime.Equals(t1.ToUniversalTime(), t2.ToUniversalTime()); // true
The moral: never compare DateTime naively
回答2:
To deal with this, I created my own DateTime object (let's call it SmartDateTime) that contains the DateTime and the TimeZone. I override all operators like == and Compare and convert to UTC before doing the comparison using the original DateTime operators.
来源：https://stackoverflow.com/questions/6930489/safely-comparing-local-and-universal-datetimes