Recently, I wrote code for adding seconds to a DateTime instance something like this:
DateTime time = DateTime.Now;
In the first round of unit test, I realize that it is not working correctly. A quick reference to MSDN revealed the fact:
“This method does not change the value of this DateTime. Instead, it returns a new DateTime whose value is the result of this operation.
The fractional part of value is the fractional part of a second. For example, 4.5 is equivalent to 4 seconds, 500 milliseconds, and 0 ticks.
The value parameter is rounded to the nearest millisecond.”
This means AddSeconds was creating a new DateTime instance rather than adding seconds to current instance. The correct code is something like this:
time = time.AddSeconds(100);
This is very similar to addition operator.
time = time + new TimeSpan(0, 0, 100);
time += new TimeSpan(0, 0, 100);
This is actually true for all Add methods (Add, AddSeconds, AdddMinutes, AddHours, AddMilliseconds, …) in the DateTime class.
I think my initial confusion was caused by my frequent usage of Add methods in collection classes.
Hope this will save some of your debugging effort.