What data types is to be used

A type is said to be nullable if it can be assigned a value or can be assigned Nothing, which means the type has no value whatsoever. Consequently, a nullable type can express a value, or that no value exists. For example, a reference type such as String is nullable, whereas a value type such as Int32 is not. A value type cannot be nullable because it has enough capacity to express only the values appropriate for that type; it does not have the additional capacity required to express a value of null.

The Nullable(Of T) structure supports using only a value type as a nullable type because reference types are nullable by design.

The Nullable class provides complementary support for the Nullable(Of T) structure. The Nullable class supports obtaining the underlying type of a nullable type, and comparison and equality operations on pairs of nullable types whose underlying value type does not support generic comparison and equality operations.