Why datetime cannot compare?

Question

my C# unit test has the following statement:

Assert.AreEqual(logoutTime, log.First().Timestamp); 

Why it is failed with following information:

Assert.AreEqual failed. Expected:<4/28/2010 2:30:37 PM>. Actual:<4/28/2010 2:30:37 PM>. 

Are they not the same?

Update:

Use this if you only care to second:

Assert.AreEqual(logoutTime.ToString(), log.First().Timestamp.ToString());

Answer 1

Have you verified that the number of ticks/milliseconds are equal?

If you do DateTime.Now() twice back to back, they will appear to be the same number down to the minute and probably even down to the second, but they will often vary by ticks. If you want to check equality only to the minute, compare each DateTime only to that degree. For information on rounding DateTimes, see here

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A note about resolution:

The Now property is frequently used to measure performance. However, because of its low resolution, it is not suitable for use as a benchmarking tool. A better alternative is to use the Stopwatch class.

Answer 2

The Assert fail method is probably calling ToString() on the DateTime which returns a truncated, human-readable form of the date without the milliseconds component. This is why it appears they are equal when, in fact, the DateTime object has a precision of a 100-nanosecond unit (known as a Tick). That means it is highly unlikely two DateTime objects will have the exact same value. To compare you probably want to truncate the value, perhaps by formatting the date to the fidelity you require.