DateTime.AddDays() not working as expected

Question

I have this simple program:

        DateTime aux = new DateTime(2012, 6, 12, 12, 24, 0);
        DateTime aux2 = new DateTime(2012, 6, 12, 13, 24, 0);
        aux2.AddDays(1);

       Console.WriteLine((aux2 - aux).TotalHours.ToString());

        Console.ReadLine();

I debugged this and found aux2.AddDays(1); doesn't seem to work, what am I missing here? it should return 25 but the answer is one.

What is the problem?

also AddHours doesn't work, I guess that the others aren't working too.

`DateTime` instances are immutable. You have to assign the result of `.AddDays` to another instance (or to itself). — mellamokb, Jul 20 '12 at 16:43

Jodrell · Accepted Answer · 2012-11-13T10:43:45.487

65

It does work but you don't do anything with the return value, try

aux2 = aux2.AddDays(1);

DateTimes share this facet of immutability with Strings.

EDIT

There is a little paragraph about it on MSDN

This method does not change the value of this DateTime. Instead, it returns a new DateTime whose value is the result of this operation.

edited Nov 13 '12 at 10:43

answered Jul 20 '12 at 16:43

Jodrell

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seriously :| , i am out of words, i thought that aux2 will change automatically and that this is a bug. – Sas Gabriel Jul 20 '12 at 16:46
yes, "Returns a new DateTime string ..."", tool tip on mouse over – Sas Gabriel Jul 20 '12 at 16:48
3

@SasGabriel, note that tooltip/docs in .NET <= 2.0 were misleading, stating that `DateTime.Add()` "Adds the specified number of days to the value of this instance." despite behaving exactly the same way. – Kuba Wyrostek Mar 03 '17 at 10:08
VS tells you "unnecessary assignment" when done this way. Obviously it is very necessary, someone should tell Microsoft. – Lucas925 Oct 07 '20 at 15:13
@Lucas925 are you using the variable? – Jodrell Oct 09 '20 at 08:44

Zbigniew · Answer 2 · 2012-07-21T08:35:40.580

5

DateTime.AddDays returns new DateTime that adds specified number of days. You need to assign it to your variable:

aux2 = aux2.AddDays(1);

edited Jul 21 '12 at 08:35

answered Jul 20 '12 at 16:43

Zbigniew

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score 0 · Answer 3 · answered Oct 09 '18 at 11:01

You are working with immutable functions.

The DateTime function is immutable, once you set variable equal to it, it cannot change, you can though set new variables equal to a working of the function. The AddDay function takes the variable you put into it, but it does not change the original variable, that remains immutable. So you need to set a new variable equall to the original variable + a day.

So all you really need to do is change

aux2.AddDays(1);

to

aux2 = aux2.Adddays(1);

and then the rest of your comparison functions should work

This behaviour is not associated with the immutability of the function. The immutability of the type or, in this case, the structure is what matters. — Jodrell, Oct 26 '21 at 08:42

DateTime.AddDays() not working as expected

3 Answers3

You are working with immutable functions.

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