Cannot implicitly convert 'System.TimeSpan?' to 'System.TimeSpan'

Question

The below code works fine :

DateTime d1 = DateTime.Now;
DateTime d2 = DateTime.Now.AddDays(-1);
int d3 = (int)(d1 - d2).TotalDays;

But what if I define DateTime as DateTime? :

DateTime? d1 = DateTime.Now;
DateTime? d2 = DateTime.Now.AddDays(-1);
int d3 = (int)(d1 - d2).TotalDays;

underlined red with error

Cannot implicitly convert 'System.TimeSpan?' to 'System.TimeSpan'

Is it possible to get the difference of number of days between two datetimes that are defined as nullable?

Molybdic

int d3 = (int)(d1.Value - d2.Value).TotalDays; – Anthea

Answer 1

Well yes, but you need to use the Value property to "un-null" it:

int d3 = (int)(d1 - d2).Value.TotalDays;

However, you should consider the possibility that either d1 or d2 is null - which won't happen in your case, but could in other cases. You may want:

int? d3 = (int?) (d1 - d2)?.TotalDays;

That will give a result of null if either d1 or d2 is null. This is assuming you're using C# 6, of course - otherwise the ?. operator isn't available.

(You could use GetValueOrDefault() in the first case as suggested by user3185569, but that would silently use an empty TimeSpan if either value is null, which feels unlikely to be what you want.)

Nero

The use of GetValueOrDefault() is a reasonable optimisation of the code given and applicable to other cases where the nullable is known to not be null (most often when following a branch on testing precisely that), but like all optimisations, knowing when not to use it is more important than knowing the micro-opt it gives. – Ghostly

Answer 2

Yes, using GetValueOrDefault():

DateTime? d1 = DateTime.Now;
DateTime? d2 = DateTime.Now.AddDays(-1);
int d3 = (int)(d1 - d2).GetValueOrDefault().TotalDays;

d1 - d2 return Nullable TimeSpan which doesn't directly contains a property called TotalDays. But using GetValueOrDefault() you can return a TimeSpan object and get 0 Total Days if the value was NULL

If you do really expect Null Values, it is better to differentiate between 0 days (What the above approach returns) and and invalid operation (date - null), (null - date) and (null - null). Then you might need to use another approach:

int? d3 = (int) (d1 - d2)?.TotalDays;

Or if you're using a version prior to C# 6 :

int? d3 = d1.HasValue && d2.HasValue ? (int)(d1 - d2).Value.TotalDays : new int?();

Hardened

This is kind of neat code, but the OP would have to understand that null - null = 0 which probably isn't true. It would probably be better if null - null = null. – Icj

@Icj You're right. But what i understood from the OP is that the dates are valid but are just defined as Nullable. Otherwise, it makes perfect sense to differentiate between 0 days of difference and an invalid operation. – Hardened

Answer 3

use long and convert

long x
System.TimeSpan y= new TimeSpan(x);

Septicidal