Question

If a DateTime instance has not been assigned yet, what is it’s value?

To look at a specific example: In the class below, would “UnassignedDateTime==null” return true?

And if so, surely it is massively illogical that such a reference could be null, but not assigned null?

class TestClass
{
    public DateTime AssignedDateTime {get; set;}
    public DateTime UnassignedDateTime {get; set;}

    public TestClass()
    {
        AssignedDateTime=DateTime.Now;
        //Not assigning other property
    }

}

I’ve already checked this answer to a similar question, but it’s about DateTime? which is nullable.. How to check if DateTime object was not assigned?

Answer 1

It will be default(DateTime) which by a design-decision happens to be DateTime.MinValue

default(T) is what types are initialized to when used as fields or array members.
default(int) == 0, default(bool) == false etc.
The default for all reference types is of course null.

It is legal to write int i = default(int); but that’s just a bit silly. In a generic method however, T x = default(T); can be very useful.

surely it is massively illogical that such a reference could be null, but not assigned null?

DateTime is a Value-type, (struct DateTime { ... }) so it cannot be null. Comparing it to null will always return false.

So if you want find out the assigned status you can compare it with default(DateTime) which is probably not a valid date in your domain. Otherwise you will have to use the nullable type DateTime?.