Reflection and Nullable

You might have already read
this: VS2005 made the last-minute DCR related to boxed Nullable<T>.
Runtime now treats Nullable<T> differently from other generic value types
when boxing:

Int32? x = null;

object y = x; 

// y is simply null, not a truly boxed "nullable<int> struct" (which then
tells it has no value)


Int32 ? x = 100;

object y = x; 

// y is boxed Int32 100 (same as "object y = 100"), no longer
a boxed "nullable<int> struct" with a field of value 100.

Reflection late-bind invocation always deals with "object"; value type instances
are always getting boxed first before invocation. One of such APIs: PropertyInfo.SetValue(Object
obj, Object value, Object[]
index). There was
one question in Microsoft Technical Forums: Exception is thrown when setting
DateTime value to a property with type Nullable<DateTime>. With this DCR change,
such late-bind calls can be written in a more natural way.

class C


   Int32? m_field;

   public Int32?


    get { return
m_field; }

    set { m_field =
value; }



PropertyInfo pi = typeof(C).GetProperty("Property");

C c = new C();

pi.SetValue(c, 100, null);        
// works

pi.SetValue(c, (Int32?)100, null);
// no longer have to write like this

After boxing, 100 and (Int32?)100 are the same thing: boxed Int32 object. There
is no more boxed object with type "Int32?"; late-invocation has to accept it and
set to Int32? property. Or think it this way, runtime can not tell where a boxed
Int32 object was originally from: either just Int32 instance or Int32? instance;
it is reasonable to allow setting a boxed Int32 object to field/property of either
Int32 or Int32? type (same for argument pass-in when doing method invocation)

pi.SetValue(c, null, null);
       // works

This actually worked before the DCR. And ,

Object ret = pi.GetValue(c, null);

ret will be either boxed Int32 object or simply null. There is no problem in casting
the result to Int32? type.

Here are some reflection behavior changes as a result of this DCR:

  • Probably the most important one is "typeof(T?).IsAssignableFrom(typeof(T)) == true".
    PropertyInfo.SetValue/GetValue use this first to ensure the pass-in value is assignable
    to Nullable<T> property.

  • Object.GetType() will never get back any type of Nullable<T>. Joe had an interesting

    post on this.

  • Activator.CreateInstance could never be expected to return null before; with this
    DCR, it will return null when creating instance of type Nullable<T> but not
    providing non-null T value. For example, Activator.CreateInstance(typeof(Int32?))
    returns null.