The example is obvious and does nothing useful。 If you attempted to call the Object。 ToString（） method on value； an exception would be generated。 So； what the code has created is a variable value that references nothing and is nothing。 This works because the example uses a reference type。 Let’s see what happens when you assign a value to a value type that we equate to no real value。 Dim value As Integer = 0 If value = 0 Then Console。WriteLine（〃Yup a Nothing？？？〃） End If This time； value is not of the type Object； but is of the type Integer。 And value is assigned a value of 0 to indicate no real value。 Now many of you will be saying； “Hey； wait a minute— 0 Nothing。” Yet； in the programming languages C＋＋ and C； Nothing and 0 have the same contextual meaning； so it’s not so outlandish。 Visual Basic has a programming construct called the nullable type。 The nullable type solves the problem of knowing whether or not a value type has been assigned。 This is needed when inter acting with relational databases。 For example； there is no way to indicate that an integer value is not assigned； since declaring a value type means declaring a stack variable； and it will have a default value。 In other words； there is no chance a value type will have a null reference。 A nullable type in Visual Basic is used to define a value…type variable that might have a null state。 In the value type example； the integer value was assigned a value of 0； but a value of Nothing would have been more appropriate。 However； value types cannot be assigned a value of Nothing； they must be assigned some valid value。 To further demonstrate the restrictions of a value type； the following is a Structure declaration。 …………………………………………………………Page 449…………………………………………………………… CH AP T E R 1 6 ■ L E A R N I N G A B OU T O TH E R V IS U AL B A SI C T E C HN IQ U E S 427 Structure ExampleStructure Public value As Integer End Structure Here’s how we might use it： Dim ex As ExampleStructure = Nothing The structure ExampleStructure is defined using the Structure keyword； which means that ExampleStructure is a value type。 The next line； where the variable ex is assigned a value of Nothing； will result in an action where nothing happens other than resetting the variable ex to 0。 And as was illustrated earlier； 0 is not Nothing。 The inability to assign Nothing is odd； because structures behave similarly to reference types in many ways。 Instantiating a default value of a structure is not always the right answer； since the data members might not have a default value。 The nullable type makes it possible for value types （for example； structures） to be nothing。 In Visual Basic； you define a nullable type by using the generics class Nullable or the nullable type notation； as follows： Dim ex As Nullable（Of ExampleStructure） = Nothing or Dim ex As ExampleStructure？ = Nothing Using nullable types； it is possible to assign a Nothing value to a value type。 To assign a value to a nullable type； you need to explicitly assign the variable； as in the following example。 ex = New ExampleStructure（） With ｛。value = 10｝ And then to reference the structure—in particular； the property value—use the following code。 Console。WriteLine（〃（〃 & ex。Value。value & 〃）〃） The additional reference of Value is the link between the reference type Nullable and the value type ExampleStructure。 ■Note Nullable types are not intended to be used in conjunction with reference types。 If you attempt to use a nullable type with a reference type； you will get a Visual Basic piler error。 Nullable types are designed for use by value types； and there is no other mystical or magical reason to use a nullable type。 Partial Classes and Methods By default； whenever you create a type； you must declare the plete type in one file and as one entity。 For example； the following class would need to be declared in the context of a single file。 …………………………………………………………Page 450…………………………………………………………… 428 CH AP T E R 1 6 ■ L E A R N I N G A B OU T O TH E R V IS U AL B A SI C TE C H N IQ U E S Class Example Const BaseValue as Integer = 10 Public Sub AddNumbers（ByVal value1 As Integer； _ ByVal value2 As Integer； _ ByRef response As Integer） response = BaseValue ＋ value1 ＋ value2 End Sub End Class The Example class has a single method AddNumbers； and the method implementation adds two numbers to a base value （BaseValue）。 Everything is in a single file。 But suppose you wanted to automatically generate Example； and have it generate the data member BaseValue； but not the method AddNumbers。 To do that； you would need to split the class into two separate classes； where one class subclasses the other： Class BaseGenerated Protected Const BaseValue As Integer = 10 End Class Class Example ： Inherits BaseGenerated Public Sub AddNumbers（ByVal value1 As Integer； _ ByVal value2 As Integer； _ ByRef response As Integer） response = BaseValue ＋ value1 ＋ value2 End Sub End