由于 DateTime 是一个 code>，而不是一个类，你得到一个 DateTime 对象当您声明该类型的字段或变量时，引用。而且，与 int 不能 null 相同，所以这个 DateTime object never not null，因为它不是一个引用。

添加问号将它变成一个 可空类型 ，这意味着 它是一个 DateTime 对象，或它是 null Nullable 本身就是一个 struct 。

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?

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 can this DateTime object 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.