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Boxing Nullable Types (C# Programming Guide)

Objects based on nullable types are only boxed if 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;

The behavior of nullable types when boxed provides two advantages:

  1. 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.
  2. 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.