What is nullable type

An important point to note about nullable types (which are always value types) is that the 'null' they use is not the same as the 'null' used with reference types.

For example this line:

  object obj = null;

means that the object variable, obj, doesn't currently refer to an object. So 'null' represents the null reference which is the memory address 0x0000 on a 32-bit system.

In contrast, this line:

  int? i = null;

means that the Nullable variable, i, doesn't currently have a value i.e. its HasValue property is false.  So 'null' is just syntactic sugar for this state of affairs and is nothing to do with memory addresses.