Professional .NET 2.0 Generics

4.8. System.Nullable<T>

With the introduction of version 2.0 of the .NET Framework, developers are finally provided with a solution to the age-old problem of dealing with nullable types. The basic issue here is that not all data types provide a mechanism for determining if they have a "null" value. Clearly, with objects, there's a well-defined means of making this determination. However, with an int data type, there's no predefined value that could be used to determine if that int has been assigned a value. To resolve this, Visual Studio 2005 is introducing a new Nullable type that provides a uniform way of determining if a value is null.

Although nullable types are not exactly a generics concept, they are implemented using generics. A type is made nullable using the built-in Nullable generic class (which is in the System namespace). This generic class will be used to keep track of when its underlying type is assigned a value. Consider this example:

[VB code]
Public Class MyTest
    Public Shared Sub NullableTest(ByVal intVal1 As Nullable(Of Int32), _
                                   ByVal intVal2 As Int32)
        If (intVal1.HasValue() = True) Then
            Console.WriteLine(intVal1)
        Else
            Console.WriteLine("Value1 is NULL")
        End If

        If (intVal2 > 0) Then
            Console.WriteLine(intVal2)
         Else

Console.WriteLine("Value2 is Null?")
         End If
    End Sub
End Class

[C# code] using System; public class MyTest { public static void NullableTest(Nullable<int> intVal1, int intVal2) { if (intVal1.HasValue == true) Console.WriteLine(intVal1); else Console.WriteLine("Value1 ...