System.DateTime? vs System.DateTime

Question

I was writing to some code where I needed to read the date value from a Calendar control in my page (Ajax toolkit: calendar extender).

The code below:

Answer 1

var Endtime = DateTime.Now();
DateTime startTime = item.REPORT_TIME.HasValue ? item.REPORT_TIME.Value : Endtime;

Means: the type of item.item.REPORT_TIME is system.DateTime?
howerver the type of startTime is system.DateTime; so the code can changed it like

`var Endtime=DateTime.Now; var startTime=item.REPORT_TIME.HasValue?Convert.ToDateTime(item.REPORT_TIME.HasValue):Endtime

`

Answer 2

DateTime? is the same as Nullable<DateTime> That is: an instance of DateTime? can contain 'NULL', whereas an instance of DateTime does not. (This is true for all value - types since .NET 2.0. A Value type cannot contain NULL , but, as from .NET 2.0, nullable value types are supported via the Nullable<T> construct (or the ? shorthand).

You can get the value of DateTime? and put it in DateTime by doing this:

DateTime? someNullableDate;
DateTime myDate;

if( someNullableDate.HasValue )
   myDate = someNullableDate.Value;

Another, conciser way to get the value of a Nullable, is by using the null-coalescing operator:

DateTime myDate = someNullableDate?? default(DateTime);

Answer 3

You can use the ?? operator to work with nullables in a very concise way in C#. See my comment on Travis's answer for a shorter way of expressing the "if not null then use it else use some default value" concept. They both do the same thing.

Answer 4

The Calendar control returns a Nullable<DateTime> (shorthand in C# is DateTime?) in its SelectedDate Property, since DateTime is a struct. The null value allows the Control to have a "no date selected" state. Thus, you'll need to check if the nullable has a value before you can use it.

var nullable = myCalendarExtender.SelectedDate;
var newSelectedDate = (nullable.HasValue) ? nullable.Value : SomeDefaultValue;

EDIT: Even more concise, thanks to Josh's comment:

var newSelectedDate = myCalendarExtender.SelectedDate ?? SomeDefaultValue;

I like it!

Answer 5

A much better solution and IMO the best feature of all with the nullable class

DateTime newSelectedDate = myCalendarExtender.SelectedDate.GetValueOrDefault();

Answer 6

? means that the type is nullable. For details, see e.g. MSDN

Nullable is a compiler-supported wrapper around value types that allows value types to become null.

To access the DateTime value, you need to do the following:

DateTime? dateOrNull = myCalendarExtender.SelectedDate;
if (dateOrNull != null)
{
    DateTime newSelectedDate = dateOrNull.Value;
}