Visual Basic 2005: A Developer's Notebook

Make Simple Data Types Nullable

With the new support for generics that's found in the .NET Framework, a number of new features become possible. One of these features—generic strongly typed collections—was demonstrated in the previous lab, "Build Typesafe Generic Classes." Now you'll see another way that generics can solve common problems, this time by using the new nullable data types.

Note

Do you need to represent data that may or may not be present? VB . NET's new nullable types fill the gap.

How do I do that?

A null value (identified in Visual Basic by the keyword Nothing), is a special flag that indicates no data is present. Most developers are familiar with null object references, which indicate that the object has been defined but not created. For example, in the following code, the FileStream contains a null reference because it hasn't been instantiated with the New keyword:

Dim fs As FileStream
If fs Is Nothing
    ' This is always true because the FileStream hasn't
    ' been created yet.
    Console.WriteLine("Object contains a null reference.")
End If

Core data types like integers and strings can't contain null values. Numeric variables are automatically initialized to 0. Boolean variables are False. String variables are set to an empty string (''") automatically. In fact, even if you explicitly set a simple data type variable to Nothing in your code, it will automatically revert to the empty value (0, False, or ""), as the following code demonstrates:

Dim j As Integer = Nothing If ...