When are two Datetime variables equal?

I try to compare Datetime.Now with a Datetime variable I set, using the Datetime.CompareTo() method. I use a timer to compare these every se开发者_JS百科cond and display the result, but as the current time approaches the time I set, the result changes from 1 to -1, but never 0, which means these two are never equal. I'm suspecting the Datetime structure contains milliseconds?

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You're suspecting correctly. It goes further than milliseconds though. The maximum resolution is the "tick", which is equal to 100 nanoseconds.

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As other have mentioned here, the resolution is 100ns.

The easiest approach would be to take your DateTime and subtract DateTime.Now. You then end up with a TimeSpan. If the TimeSpan's TotalSeconds property is 0, the difference between them is less than a second.

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You are correct in your suspicion. The DateTime struct smallest unit is the "Tick" which is measured in units of 100ns. (One tick is 100ns)

What you more likely want to do is check if everything down to the seconds is equal and you can do that like this by first comparing the Date property and then compare the hour, minute and second properties individually

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DateTime comparison is more exact than comparing with seconds. In your scenario, you can define an "error range", e.g. if the gap between two DateTime is less than 1 second, they are considered to be the same(in your program).

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Try this... (but change the test date, of course)

        DateTime d1 = new DateTime(2011, 12, 27, 4, 37, 17);
        DateTime d2 = DateTime.Now;

        if (d1.Subtract(d2).Seconds <= 1)
        {
            //consider these DateTimes equal... continue
        }

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I prefer to compare Datetime (as well as double) not with exact values but with value ranges, because it is quite unlikely that you have the exact value.

    DateTime d1  = new DateTime(2011, 12, 27, 4, 37, 17);
    DateTime d2  = DateTime.Now;

    if ((d2 >= d1) && (d2 <= d1.AddMinutes(1)))
       ....

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    'simulate comparison of two datetimes
    d1 = DateTime.Now
    Threading.Thread.Sleep(250)
    d2 = DateTime.Now

    'see if two dates are within a second of each other
    Dim ts As Double = ((d2 - d1).TotalSeconds)
    If ts < 1 Then
        'equal
        Debug.WriteLine("EQ " & ts.ToString("n4"))
    Else
        Debug.WriteLine("neq " & ts.ToString("n4"))
    End If