Supports a value type that can be assigned Nothing like a reference type. This class cannot be inherited.
Assembly: mscorlib (in mscorlib.dll)
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 class provides complementary support for the Nullable(Of T) structure. The 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.
Use nullable types to represent things that exist or do not exist depending on the circumstance. For example, an optional attribute of an HTML tag might exist in one tag but not another, or a nullable column of a database table might exist in one row of the table but not another.
You can represent the attribute or column as a field in a class and you can define the field as a value type. The field can contain all the valid values for the attribute or column, but cannot accommodate an additional value that means the attribute or column does not exist. In this case, define the field to be a nullable type instead of a value type.
Boxing and Unboxing
When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable(Of T) object, not the Nullable(Of T) object itself. That is, if the HasValue property is true, the contents of the Value property is boxed. If the HasValue property is false, Nothing is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable(Of T) structure initialized to the underlying value.
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.