# Boxing Nullable Types (C# Programming Guide)

**Visual Studio 2008**

Objects based on nullable types are only boxed if the object is non-null. If HasValue is false, the object reference is assigned to null instead of boxing. For example:

bool? b = null; object o = b; // Now o is null.

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<T> 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, just like non-nullable boxed types, they can be unboxed into nullable types, as in the following example:

bool? b2 = (bool?)bBoxed; int? i2 = (int?)iBoxed;

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 and boxing behavior, see Nullable Sample.