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. By default, all reference types, such as string, are nullable, but all value types, such as int, are not.

In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. For example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned null.

The Nullable class provides complementary support for the Nullable`1 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.

Boxing and Unboxing

When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable`1 object, not the Nullable`1 object itself. That is, if the Nullable`1.HasValue property is true, the contents of the Nullable`1.Value property is boxed. If the HasValue property is false, null is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable`1 structure initialized to the underlying value.