Boxing Nullable Types (C# Programming Guide)
Objects based on nullable types are only boxed it the object is non-null. If HasValue is false, then, instead of boxing, the object reference is simply assigned to null. For example :
bool? b = null;
object o = b;
// Now o is null.
If the object is non-null -- if HasValue is true -- then boxing takes place, but only the underlying type that the nullable object is based upon is boxed. Boxing a non-null nullable value type boxes the value type itself, not the System.Nullable that wraps the value type. For example:
bool? b = false;
int? i = 44;
object bBoxed = b; // bBoxed contains a boxed bool.
object iBoxed = i; // iBoxed contains a boxed int.
The two boxed objects are identical to those created by boxing non-nullable types. And, like non-nullable boxed types, can be unboxed into nullable types, like this:
bool? b2 = (bool?)bBoxed;
int? i2 = (int?)iBoxed;
Remarks
The behavior of nullable types when boxed provides two advantages:
Nullable objects and their boxed counterpart can be tested for null:
bool? b = null; object boxedB = b; if (b == null) { // True. } if (boxedB == null) { // Also true. }Boxed nullable types fully support the functionality of the underlying type:
double? d = 44.4; object iBoxed = d; // Access IConvertible interface implemented by double. IConvertible ic = (IConvertible)iBoxed; int i = ic.ToInt32(null); string str = ic.ToString();
For more examples of nullable types, including boxing behavior, see Nullable Sample.