A nullable type is a special version of the value type that is represented by a structure. In addition to the values defined by the underlying type, a nullable type can also store the value null. Thus, a nullable type has the same range and characteristics as its underlying type. It simply adds the ability to represent a value which indicates that a variable of that type is unassigned. Nullable types are objects of System.
Nullable, where T must be a non- nullable value type.