Variables that are based on value types directly contain values. Assigning one value type variable to another copies the contained value. This differs from the assignment of reference type variables, which copies a reference to the object but not the object itself. All value types are derived implicitly from the System.ValueType.Nullable is a value type with a HasValue flag that can be false to indicate that there is no value. It still has a value (when HasValue is false, Value is default(T)), but the HasValue flag tells you to ignore the value. It has nothing to do with null, except that the CLR automatically unboxes null boxed values to a Nullable with HasValue set to false.

A reference type is storeed as a reference (like a pointer) to an object instance. null means a reference that isn't pointing to an instance of an object. Value types are stored as the values themselves, without any references. Therefore, it doesn't make sense to have a null value type—the value type by definition contains a value.