Set default value for DateTime in optional parameter

Question

How can I set default value for DateTime in optional parameter?

public SomeClassInit(Guid docId, DateTime addedOn = DateTime.Now???)
{
    //Init codes here
}

This is not a duplicate Timespans work differently in c# than timespans. This is doable in c# with `DateTimeConstantAttribute` but timespans do not support this. — leat, May 04 '17 at 20:56

score 167 · Accepted Answer · answered Jun 13 '10 at 06:28

167

There is a workaround for this, taking advantage of nullable types and the fact that null is a compile-time constant. (It's a bit of a hack though, and I'd suggest avoiding it unless you really can't.)

public void SomeClassInit(Guid docId, DateTime? addedOn = null)
{
    if (!addedOn.HasValue)
        addedOn = DateTime.Now;

    //Init codes here
}

In general, I'd prefer the standard overloading approach suggested in the other answers:

public SomeClassInit(Guid docId)
{
    SomeClassInit(docId, DateTime.Now);
}

public SomeClassInit(Guid docId, DateTime addedOn)
{
    //Init codes here
}

answered Jun 13 '10 at 06:28

LukeH

263,068
57
365
409

1

I don't really understand why optional params must be compile-time constants. The compiler could easily create the 2 methods you wrote from the one with the optional param...I usually prefer having one method with several optional params instead of several methods, it makes the code smaller and thus easier to read. – user276648 May 22 '12 at 08:48
8

You can use coalesce here: `addedOn = addedOn ?? DateTime.Now` – Zaid Masud Sep 19 '12 at 20:36
Nullables can cause casting problems deep in your code, so I am not a fan. I am becoming increasing disappointed in C#. I mean, even VB.net has literal dates, such as: Optional addedOn As DateTime = #12:00:00 PM# – Brain2000 Mar 03 '14 at 17:40
2

VS wouldnt compile with this code. Instead i used DateTime date = default(DateTime), and then checked to see if date==default(DateTime) – Dan Hastings Sep 07 '16 at 10:18

score 59 · Answer 2 · edited May 23 '17 at 12:34

59

I guess that you did not really want addedOn = DateTime.Now because that would suggest you never get any result as everything would be added before 'Now'. :)

A default DateTime can be set like this:

public void SomeClassInit(Guid docId, DateTime addedOn = default(DateTime))

Update
If you deal with SQL Server, do not forget that it doesn't accept default(DateTime) what is 1/1/0001. SQL Server's minimal DateTime is 1/1/1753 (explanation). SQL's DateTime2 accepts 1/1/0001, though.

edited May 23 '17 at 12:34

Community

1
1

answered Jun 20 '12 at 02:34

Rick Glimmer

761
6
4

1

Dont understand why this isnt marked as the answer, it allows for much easier working with datetime, since datetime? != datetime – Peter Jul 11 '17 at 08:57
This is the correct answer. – A Person Oct 12 '17 at 10:11
I'm just going to voice what others have said, this IS the correct answer. The others are basically just cheating the system. – CyberClaw Jan 17 '18 at 15:42
I suppose this is gong to return a default value for datetime. what if someone wants the current datetime ? – Ankit Veer Singh Mar 07 '23 at 12:36

score 29 · Answer 3 · answered Jul 25 '11 at 08:15

29

I'd slightly modify LukeH's solution as:

public void SomeClassInit(Guid docId, DateTime? addedOn = null)
{
    DateTime TargetDateTimeProperty = addedOn ?? DateTime.Now;
}

which is shorter and more readable, it seems.

answered Jul 25 '11 at 08:15

Alexey Khoroshikh

425
4
10

It is not more readable but it allows me to access the .Year/.Month filelds. – Zac Jul 26 '19 at 09:48

score 7 · Answer 4 · answered Jun 13 '10 at 04:52

Don't use an optional parameter:

public SomeClassInit(Guid docId, DateTime addedOn)
{
    SomeClassInitCore(docId, addedOn);
}

public SomeClassInit(Guid docId)
{
    SomeClassInitCore(docId, null);
}

private SomeClassInitCore(Guid docId, DateTime? addedOn)
{
    // set default value
    if (addedOn.IsNull) addedOn = DateTime.Now;

    //Init codes here
}

Pure.Krome · Answer 5 · 2010-06-13T05:07:40.493

6

.NET 4.0 does have optional parameters. (google is also your friend, here.)

EDIT (because of Anthony Pegram correct, comment)...

~~And yes, that is how you would do it.~~

But DateTime.Now (static property, on that class) is not know until run-time. As such, you can't use that as an optional value.

.NET 3.5 doesn't ... so then you would have to do what JS Bangs said...

public SomeClassInit(Guid docId) 
{ 
    return SomeClassInit(docId, DateTime.Now);
}

public SomeClassInit(Guid docId, DateTime addedOn = DateTime.Now???) 
{ 
    //Init codes here 
}

or even the null checking/null value parameter from munificent's answer.

Cheers Anthony.

edited Jun 13 '10 at 05:07

answered Jun 13 '10 at 04:53

Pure.Krome

84,693
113
396
647

The default value must be a compile time constant, so this is not going to work even in 4.0. – Anthony Pegram Jun 13 '10 at 04:58
Confirmed Anothony. I'll re-edit my post. Cheers. – Pure.Krome Jun 13 '10 at 05:03
1

This Shoud Be The Accepted Answer (TSBTAA). This is the only answer that actually and clearly explains why it's not possible to use common `DateTime` "values" for optional argument. – adamency Jan 04 '23 at 01:46

score 2 · Answer 6 · answered Jun 13 '10 at 04:50

2

C# doesn't have optional parameters in this sense. If you want to make addedOn optional, you should write an overload that doesn't require that parameter, and passes DateTime.Now to the two-argument version.

answered Jun 13 '10 at 04:50

JSBձոգչ

40,684
18
101
169

1

er.. sure abot that JS Bangs? for framework version .NET 3.5 and below, you are correct. otherwise, 4.0 and above, it is supported. – Pure.Krome Jun 13 '10 at 04:54
1

@Pure.Krome, default values for optional parameters must be compile time constants. So this is *not* supported. (This answer could be worded better, however.) – Anthony Pegram Jun 13 '10 at 04:57
1

i too misunderstood the wording. soz JS. – Pure.Krome Jun 13 '10 at 05:09

Set default value for DateTime in optional parameter

6 Answers6

Linked