There are cases where DateTime values can be blank. Is there a way to set a DateTime variable to Null in C#? If not, what's the correct way to handle these cases?
Alternatively , if you don't want to (or can't) use nullable DateTime type (noted DateTime? or Nullable<DateTime>) you still can use a "sentinel" value instead which you assume, by programmation to be equivalent to null.
All the answers here are good but ctheriault, I think, leads you in the best direction.
Give some thought to the logic/business reason for what is happening in your application. Firstly, I think you are probably talking about some sort of input widget (a text box?) which the user may decline to fill in, this is not a date time - it is a string
which you are trying to interpret as a date time. What does it mean to not supply this information?
Assuming that the information is not mandatory (if it is mandatory then you need to keep asking until you get a valid date, that's easy) then I think the best solution is to use, not a sentinel value, but a flag to indicate that no date was supplied. For
example, a bool called DOBGiven would only be true if the user supplied you with date of birth (or whatever) information. You then know that the DateTime called DOB should only be used when DOBGiven is true. When DOBGiven is false then DOB can be anything
(I would prefer DateTime.Today over DateTime.MinValue because often a database will not be able to store MinValue).
Sure, your code ends up being a little more complex but that is because you actually have a more complex situation. When you have mandatory fields, life is much simpler. If the field is not mandatory then there are actually two pieces of information "Was
a value given?" and "If a value was supplied what was it" (in some cases there may be a third piece of information "If in invalid response was made, what was that response" (eg they may have typed "Jan 1980")).
You code may be more complex but it should be easier to understand. Your code will have statements like if (DOBGiven) {// do something} rather than if (DOB == DateTime.MinValue) or if (DOB.HasValue) which do not show your intent.
Budhi Hin Tanu Janike, Sumirau Pavan Kumar
Bal budhi Vidya dehu mohe,Harahu Kalesa Vikar
[My Site: WCF, My Site and Silverlight site @ Silverlight version].
by far the easiest/simpliest way is using nullable types.....
e.g.
DateTime? MyDate = new DateTime();
Normally datetime data types cannot be null but by putting the "?" in it makes it a nullable type and the variable can have a value of null (i.e. no value).
My opinion is that the nullable approach is the best for this situation.
Just a comment on Paul Linton's reply:
DateTime DOB; // may contain a valid date, or just anything
bool DOBGiven; // true if DOB contains a valid date
Of course this works well, but what if you somewhere in your code forget to check the DOBGiven and use DOB when it doesn't contain a valid date? If using nullable types you get the same advantage, but without the risk of using an invalid date.
DateTime? DOB_nullable; DOB_nullable.HasValue // corresponds to DOBGiven DOB_nullable.Value // corresponds to DOB
I must say I can't see why
if (DOB_nullable.HasValue) { // do something with DOB_nullable.Value }
"does not show your intent" compared to
if (DOBGiven) { // do something with DOB }
Marked as answer by Tulsim83 on Jun 17, 2009 06:43 PM
One more question..... How would you handle this nullable date in a .aspx page? So for ex, if I had the following code:
<
asp:Label
ID="Label2"
runat="server"
Text="Date: "></asp:Label><%#(DateTime?)Eval("Date")%>
-- This works fine as is.
Problem happens when I try to do the following:
<asp:Label ID="Label2" runat="server" Text="Date: "></asp:Label><%#((DateTime?)Eval("Date")).ToString("d")%> -- This gives an exception in cases where the Date value was null.
What's the best way to handle this case in the .aspx page?
Also, thanks to everyone who responded to this post. Your replies were very helpful
It is true that you can't get an invalid date with the nullable type, you would just get an exception if you used .Value without checking .HasValue (and it is false). Because the compiler won't let me use a DateTime unless it has been initialised I would
probably end up with a screen that shows a DOB as 1/1/1 and using the nullable type I would have an exception and no screen at all, take your pick. Either way, it is a bug which should be caught by you TDD methods.
I prefer DOBGiven over DOB_nullable.HasValue because of the level of abstraction. To me, the HasValue property is a detail of the CLR (or is it the language?) whereas DOBGiven is more intention revealing. It's a case of the low-level technical details
versus the business case that is being addressed.
Tulsim83
Member
16 Points
26 Posts
How to set DateTime variable to Null in C#?
Jun 11, 2009 07:23 PM|LINK
Hello,
There are cases where DateTime values can be blank. Is there a way to set a DateTime variable to Null in C#? If not, what's the correct way to handle these cases?
Thank you.
Tulsi
bullpit
All-Star
21838 Points
4822 Posts
Re: How to set DateTime variable to Null in C#?
Jun 11, 2009 08:20 PM|LINK
If you are using .NET 2.0, you can have nullable DateTime.
DateTime
? date = null;Max
Let Me Google That For You!
rrmacman
Contributor
2176 Points
475 Posts
Re: How to set DateTime variable to Null in C#?
Jun 11, 2009 08:24 PM|LINK
You can't set the DateTime variable to null but you can define it as a Nullable<T> type like "DateTime?"
Nullable<T> types have properies such as Value and HasValue.
with this example you can do something like so.
DateTime? datetime;
if (datetime.HasValue)
{
//Do Something with datetime.Value which returns a DateTime type.
}
you can set DateTime? value to null instead of DateTime.MinValue or DateTime.MaxValue as you would a regular DateTime type.
I usally cast datetime and null values to (DateTime?) when assigning it the variable.
ie.
DateTime? date = (DateTime?)DateTime.Now
or
DateTime? Date = (DateTime?)null;
ctheriault
Participant
1559 Points
280 Posts
Re: How to set DateTime variable to Null in C#?
Jun 11, 2009 11:18 PM|LINK
Alternatively , if you don't want to (or can't) use nullable DateTime type (noted DateTime? or Nullable<DateTime>) you still can use a "sentinel" value instead which you assume, by programmation to be equivalent to null.
For example:
Don't forget to mark this posting as an "Answer" if it is helpful to you
Paul Linton
Star
13403 Points
2531 Posts
Re: How to set DateTime variable to Null in C#?
Jun 12, 2009 12:27 AM|LINK
All the answers here are good but ctheriault, I think, leads you in the best direction.
Give some thought to the logic/business reason for what is happening in your application. Firstly, I think you are probably talking about some sort of input widget (a text box?) which the user may decline to fill in, this is not a date time - it is a string which you are trying to interpret as a date time. What does it mean to not supply this information?
Assuming that the information is not mandatory (if it is mandatory then you need to keep asking until you get a valid date, that's easy) then I think the best solution is to use, not a sentinel value, but a flag to indicate that no date was supplied. For example, a bool called DOBGiven would only be true if the user supplied you with date of birth (or whatever) information. You then know that the DateTime called DOB should only be used when DOBGiven is true. When DOBGiven is false then DOB can be anything (I would prefer DateTime.Today over DateTime.MinValue because often a database will not be able to store MinValue).
Sure, your code ends up being a little more complex but that is because you actually have a more complex situation. When you have mandatory fields, life is much simpler. If the field is not mandatory then there are actually two pieces of information "Was a value given?" and "If a value was supplied what was it" (in some cases there may be a third piece of information "If in invalid response was made, what was that response" (eg they may have typed "Jan 1980")).
You code may be more complex but it should be easier to understand. Your code will have statements like if (DOBGiven) {// do something} rather than if (DOB == DateTime.MinValue) or if (DOB.HasValue) which do not show your intent.
Have a nice day
Rimbik
Participant
1359 Points
382 Posts
Re: How to set DateTime variable to Null in C#?
Jun 12, 2009 06:10 AM|LINK
Nullable<DateTime> aDate;
aDate = GetFromDatabase(); //
if you get Date then it must be in
aDate.Value
else
aDate.Vaue will be null;
-----
aDate = null;
aDate = DateTime.Today();
aDate.Value == DateTime.Today()
Soumen, India
Bal budhi Vidya dehu mohe,Harahu Kalesa Vikar
[My Site: WCF, My Site
and Silverlight site @ Silverlight version].
richiej
Participant
1771 Points
354 Posts
Re: How to set DateTime variable to Null in C#?
Jun 12, 2009 08:26 AM|LINK
by far the easiest/simpliest way is using nullable types.....
e.g.
DateTime? MyDate = new DateTime();
Normally datetime data types cannot be null but by putting the "?" in it makes it a nullable type and the variable can have a value of null (i.e. no value).
try it....
anders__f
Participant
1810 Points
327 Posts
Re: How to set DateTime variable to Null in C#?
Jun 12, 2009 05:32 PM|LINK
My opinion is that the nullable approach is the best for this situation.
Just a comment on Paul Linton's reply:
Of course this works well, but what if you somewhere in your code forget to check the DOBGiven and use DOB when it doesn't contain a valid date? If using nullable types you get the same advantage, but without the risk of using an invalid date.
I must say I can't see why
"does not show your intent" compared to
Tulsim83
Member
16 Points
26 Posts
Re: How to set DateTime variable to Null in C#?
Jun 16, 2009 08:59 PM|LINK
Thanks for the reply. This was very helpful.
One more question..... How would you handle this nullable date in a .aspx page? So for ex, if I had the following code:
<
asp:Label ID="Label2" runat="server" Text="Date: "></asp:Label><%#(DateTime?)Eval("Date")%> -- This works fine as is.Problem happens when I try to do the following:
<asp:Label ID="Label2" runat="server" Text="Date: "></asp:Label><%#((DateTime?)Eval("Date")).ToString("d")%> -- This gives an exception in cases where the Date value was null.
What's the best way to handle this case in the .aspx page?
Also, thanks to everyone who responded to this post. Your replies were very helpful
Tulsi
Paul Linton
Star
13403 Points
2531 Posts
Re: How to set DateTime variable to Null in C#?
Jun 17, 2009 12:09 AM|LINK
Hi anders,
It is true that you can't get an invalid date with the nullable type, you would just get an exception if you used .Value without checking .HasValue (and it is false). Because the compiler won't let me use a DateTime unless it has been initialised I would probably end up with a screen that shows a DOB as 1/1/1 and using the nullable type I would have an exception and no screen at all, take your pick. Either way, it is a bug which should be caught by you TDD methods.
I prefer DOBGiven over DOB_nullable.HasValue because of the level of abstraction. To me, the HasValue property is a detail of the CLR (or is it the language?) whereas DOBGiven is more intention revealing. It's a case of the low-level technical details versus the business case that is being addressed.
Nice to have the discussion
Cheers Paul