What C# data types can be nullable types?

All value types (except Nullable<T> itself) can be used in nullable types – i.e. all types that derive from System.ValueType (that also includes enums!).

The reason for this is that Nullable is declared something like this:

struct Nullable<T> where T : struct, new() { … }

A type is said to be nullable if it can be assigned a value or can be assigned null, 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 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 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.

From Help Docs http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx