Question

Possible Duplicate:
Nullable type is not a nullable type?

In the following code:

DateTime? dt = DateTime.Now;
MessageBox.Show(dt.GetType().ToString());

the message box shows “System.DateTime”, instead of Nullable<DateTime>. The following also returns false (because the GetType is wrong):

if (dt.GetType().IsAssignableFrom(typeof(DateTime?))) 
 ...

(btw, using DateTime? or Nullable<DateTime> doesn’t make a difference)
In the watch window, you have the “Type” column that’s displaying the correct type (System.DateTime?).

In my code I have reference to dt as an object, so I need to get to the underlying type correctly. How can I do this?

Answer 1

Quoting MSDN (How to: Identify a Nullable Type):

Calling GetType on a Nullable type
causes a boxing operation to be
performed when the type is implicitly
converted to Object. Therefore GetType
always returns a Type object that
represents the underlying type, not
the Nullable type.

so basically your code is equal to:

    DateTime? dt = DateTime.Now;
    object box = (object)dt;
    Console.Write(box.GetType().ToString());

also, looking at “Boxing Nullable Types” on MSDN we read:

If the object is non-null — if
HasValue is true — then boxing
occurs, but only the underlying type
that the nullable object is based on
is boxed. Boxing a non-null nullable
value type boxes the value type
itself, not the System.Nullable(Of T)
that wraps the value type.

this clearly explain the “strange” behavior of Nullable<T>.GetType()