Saturday, December 12, 2009

Weekly Source Code: C# DateTime Developer Pitfall

Base upon my previous blog post that every language has some pitfalls I had a production issue and saw one of the pitfalls that C# have with the DateTime class which is part of the .Net framework.

I renamed this title since its more a pitfall of the developer not understanding the proper use of the DateTime class. The all point is that your telling the constructor to create a date, you don't tell it to work with increments.

In the next code you want to get the 1ste date of the following month:

DateTime currentDate = new DateTime(2009, 11, 05);
DateTime nextDate = new DateTime(currentDate.Year, currentDate.AddMonths(1).Month, 1);
Console.WriteLine("nextDate: " + nextDate.ToString("dd/MM/yyyy"));

So the answer is 01/12/2009 which is correct but what if you do the following:

DateTime currentDate = new DateTime(2009, 12, 05);
DateTime nextDate = new DateTime(currentDate.Year, currentDate.AddMonths(1).Month, 1);
Console.WriteLine("nextDate: " + nextDate.ToString("dd/MM/yyyy"));

Code is exactly the same, I just change the month to December since you want to get the day for the 1ste of Jan 2010 but the end result is: 01/01/2009. Ouch!!

Yes the DateTime object doesn't increment the year (2009 -> 2010).

To fix:

DateTime currentDate = new DateTime(2009, 12, 05);
DateTime nextDate = new DateTime(currentDate.Year, currentDate.Month, 1);
nextDate = nextDate.AddMonths(1);
Console.WriteLine("nextDate: " + nextDate.ToString("dd/MM/yyyy"));

You see that you can't increment in a DateTime.

2 comments:

Forumtroll said...

Excellent article covering the pitfall of datetime arithmetrics.

However, if I were to try adding 1 month to Dec 5th, 2009, Iæd do it via the DateTime.Add(TimeSpan) overload, such as:

DateTime date = new DateTime(2009, 12, 5);
TimeSpan span = new TimeSpan { Months = 1 }; // Thank god for initializers
DateTime future = date.Add(span);

This will result in the correct increment of the year. I always use TimeSpan to add to DateTime objects since I just don't want to bother with the finer arithmetrics of calendar manipulation. The TimeSpan method also covers any specific locale calendar quirks too.

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