Loren Halvorson's Blog

If your only tool is a hammer...

Forehead slapper on DateTime.AddDays()

I should have read the documentation for DateTime.AddDays() a little more carefully. This one caused a minor bug for me this week:

DateTime dt = DateTime.Now;
dt.AddDays(1);

What's in dt after the call to AddDays()? Well I assumed it would be this time tomorrow. However, from the .NET documentation for AddDays() I finally noticed: "This method does not change the value of this DateTime. Instead, a new DateTime is returned whose value is the result of this operation." Doh!

I should have written

DateTime dt = DateTime.Now;
dt = dt.AddDays(1);

That's what I get for not reading the docs, and instead assuming from the function's name that I know what it does (one of the downsides of Intellisense is it almost makes finding functions too easy). Although I'm not sure what would I have called the function AddDays() so its behavior would be more clear. Maybe something like: DateTime.ReturnNewDateTimeWithThisNumberOfDaysAddedToIt()...nah maybe not.

 

Posted: Sep 20 2004, 09:58 PM by lorenh | with 14 comment(s)

Comments

Wes said:

I have been bitten by this bug before as well. When I was first learning .Net I did that kind of thing with strings as well. Doh!!
# September 20, 2004 11:51 PM

Edgar Sánchez said:

DateTime dt = DateTime.Now.AddDays(1);

I know current programming politeness asks you to use two lines, but I can't help sticking to my old C programming days... ;-)
# September 21, 2004 12:17 AM

Russ C. said:

I would agree with Edgar, but it seems actually too clever for the optimizer and it generates worse code ...
The problem is , that the shorted method seems to be too clever for the CLR

If you use the longer method, the optimizer generates it correctly into this

[mscorlib]System.DateTime LongDate() cil managed
{
// Code size 25 (0x19)
.maxstack 2
.locals init (valuetype [mscorlib]System.DateTime V_0) <-- Notice just 1 instance of DateTime
IL_0000: call valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::get_Now()
IL_0005: stloc.0
IL_0006: ldloca.s V_0
IL_0008: ldc.r8 1.
IL_0011: call instance valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::AddDays(float64)
IL_0016: stloc.0
IL_0017: ldloc.0
IL_0018: ret
} // end of method Class1::LongDate


The shorter C# method creates two instances of DateTime !

[mscorlib]System.DateTime ShortDate() cil managed
{
// Code size 25 (0x19)
.maxstack 2
.locals init (valuetype [mscorlib]System.DateTime V_0, valuetype [mscorlib]System.DateTime V_1)
IL_0000: call valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::get_Now()
IL_0005: stloc.1
IL_0006: ldloca.s V_1
IL_0008: ldc.r8 1.
IL_0011: call instance valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::AddDays(float64)
IL_0016: stloc.0
IL_0017: ldloc.0
IL_0018: ret
} // end of method Class1::ShortDate
# September 21, 2004 6:47 AM

Joe said:

Yo tuve el mismo problema y creo que es una pulga 'bug' de .Net ya que deberia hacerlo bien.

Pero bueno utilice DateTime dt = DateTime.Now.AddDays(-7); para obtener la fecha de hace 7 dias y me funcionó por lo menos.
# October 7, 2004 5:42 PM

Joe (Mi correo) burford2k@gmail.com said:

Yo tuve el mismo problema y creo que es una pulga 'bug' de .Net ya que deberia hacerlo bien.

Pero bueno utilice DateTime dt = DateTime.Now.AddDays(-7); para obtener la fecha de hace 7 dias y me funcionó por lo menos.
# October 7, 2004 5:43 PM

Jill said:

I'm still learning .NET, and I was trying to do a for loop to loop through a few dates.  In my incremental I was doing i.AddDays(1) and for the life of me I couldn't figure out why it wouldn't increment until I came across this post.  Thanks for putting it out there!

for (DateTime i = startdate; i <= enddate; i=i.AddDays(1))

{

}

# March 27, 2007 9:23 AM

__tom said:

I was slapping myself on the forehead trying to figure the exact same bug out.  Thank you for posting!

# June 24, 2008 11:58 AM

HC said:

hallelujah, hallelujah!!  thanks for posting : )

# July 27, 2008 10:06 AM

asp.net Rock said:

thank you very much

# August 26, 2008 9:08 AM

Jose Omar Guevara said:

Gracias por el comentario, acabo de resolver mi problema

# January 5, 2009 12:44 PM

Mitch Milam said:

Thanks Loren.  It's amazing that a blog post can still be valuable almost 5 years later.

Mitch

# September 16, 2009 12:26 PM

Suenthen said:

I should email my friend about this.

# December 15, 2009 7:02 PM

Auto-Populating Drop Down List-Sherzy.com said:

Pingback from  Auto-Populating Drop Down List-Sherzy.com

# February 19, 2010 2:26 PM

Acinnina said:

и всё эе: благодарю...  а82ч

# February 24, 2010 12:01 PM