What does "DateTime?" mean in C#?

Question

I am reading a .NET book, and in one of the code examples there is a class definition with this field:

private DateTime? startdate 

What does DateTime? mean?

Answer 1

Since DateTime is a struct, not a class, you get a DateTime object, not a reference, when you declare a field or variable of that type.

And, in the same way as an int cannot be null, so this DateTime object can never be null, because it's not a reference.

Adding the question mark turns it into a nullable type, which means that either it is a DateTime object, or it is null.

DateTime? is syntactic sugar for Nullable<DateTime>, where Nullable is itself a struct.

Answer 2

It's a nullable DateTime. ? after a primitive type/structure indicates that it is the nullable version.

DateTime is a structure that can never be null. From MSDN:

The DateTime value type represents dates and times with values ranging from 12:00:00 midnight, January 1, 0001 Anno Domini, or A.D. (also known as Common Era, or C.E.) through 11:59:59 P.M., December 31, 9999 A.D. (C.E.)

DateTime? can be null however.