对于ORM对象为空

Question

Based on reading Nullable Types , I understand that it's proper to use Nullable for a primitive type that may be null in a database.

Is it necessary to use the ? (Nullable) type for objects?

Example:

public DateTime? DateCreated {get; set; }

or

 public DateTime DateCreated {get; set; }

Answer 1

Your example is correct. You will need DateTime? to have the ORM (say NH) treat it as a nullable type. For objects you dont need to put the question mark.

Also, on a side-note, the question mark T? is an alias for Nullable<T> . The compiler will translate the T? to Nullable<T> . You can check this on the debugger if you want a proof :)

Answer 2

In your example, Nullable<T> is valid as DateTime is not a reference type. However, for objects (and types derived from object), you cannot use Nullable<T> as the compiler will throw an error; see: C# nullable string error