DateTime精度？

Question

(ignoring the optimized compiler flag )

can it be that this code will enter the block on some systems ?

if (Datetime.Now!=Datetime.Now)
{
 ...
}

I mean , how does it evaluate the values here ? (is it by order) ?

Is there any situations where the condition might be true ?

again , ignore the optimized flag.

Answer 1

DateTime has a precision of 100ns. But on typical implementations, DateTime.Now only changes every few milliseconds.

Datetime.Now != Datetime.Now can be true, but it's extremely unlikely to happen. This is a typical race conditions of the kind you often see in multi-threaded code. ie you should not rely on DateTime.Now not changing, but rather store a copy in a local variable.

Answer 2

DateTime.Now calls internally :

public static DateTime Now
{
    get
    {
        return DateTime.UtcNow.ToLocalTime();
    }
}

which calls internally:

public static DateTime UtcNow
{
    get
    {
        long systemTimeAsFileTime = DateTime.GetSystemTimeAsFileTime();
        return new DateTime((ulong)(systemTimeAsFileTime + 504911232000000000L | 4611686018427387904L));
    }
}

where GetSystemTimeAsFile is WindowsAPI function that return system clock information. The accurasy depends on system, so.

If you have a delay, for some reason between different gets ( DateTime.Now ) it may produce different enough result that equality comparer fails. But I, personally, never met this kind of condition in my experience.