Bird’s Bits

Imports System.Console
Module Module1                       

    Sub Main()
        Dim iBBValue As Nullable(Of Integer)= Nothing                       

        Write("iBBValue = ")
        WriteLine(iBBValue)
        'The following line will cause a run time error
        'WriteLine(iBBValue.Value)                       

        If Not iBBValue.HasValue Then
            WriteLine("This variable has no value")
        Else
            Write("This variable has the value: ")
            WriteLine(iBBValue.Value)
        End If                       

        iBBValue = 2007
        If Not iBBValue.HasValue Then
            WriteLine("This variable has no value")
        Else
            Write("This variable has the value: ")
            WriteLine(iBBValue.Value)
        End If                       

        Write("Press any key to exit")
        ReadKey()                       

    End Sub                       

End Module

User Defined Types (Structures)

• Structures are very similar to classes, except they are value types, so they are created on the stack. Classes are reference types and are created on the heap.
• Structures have implicitly defined parameterless constructors, so you can only define a constructor that takes parameters. Here’s a VB example:

    Structure BBColor
          Dim Red As Short
          Dim Green As Short
          Dim Blue As Short              
  
          'You can't override the implicit paramterless  constructor.
          'This code will cause a design time error
          'Sub New()
          '    Red = 255
          '    Green = 255
          '    Blue = 255
          'End Sub              
  
          'You can provide a constructor with parameters
          Sub New(ByVal R As Short, ByVal G As Short, ByVal B As Short)
              Red = R
              Green = G
              Blue = B
          End Sub              
  
      End Structure              
  
      Sub Main()
          'Both of these declarations are valid and use the implicit contstructor
          Dim SomeColor As New BBColor
          Dim AnotherColor As BBColor              
  
          'This declaration uses the user defined constructor
          Dim YetAnotherColor As New BBColor(255, 0, 0)            
  
          'Now do something with your BBColor objects...
      End Sub

Structures are more efficient to process than classes. You should consider creating a structure if your new type:

• Represents a single “logical” value
  (although it may have multiple value fields like a point or a vector)
• Has an instance size less than 16 bytes
• Will not be changed after creation
• Will not be cast to a reference type

VB Example of declaring and using a structure:

Imports System.Console
Module Module1                            

    Structure Car
        Public Make As String
        Public Model As String
        Public Year As Integer                            

        'Constructor
        Public Sub New(ByVal _Make As String, ByVal _Model As String, ByVal _Year As Integer)
            Make = _Make
            Model = _Model
            Year = _Year
        End Sub                            

        Public Overloads Overrides Function ToString() As String
            Return Year.ToString + " " + Make + " " + Model
        End Function                            

    End Structure                            

    Sub Main()                            

        Dim BBCar As New Car("Honda", "Civic", 1998)
        WriteLine(BBCar)
        Write("Hit any key to exit")
        ReadKey()                            

    End Sub                            

End Module

Enumerations

• Enumerations are a set of symbols with fixed values.
• The symbols are usually related in some way.
• The value of enumeration members may optionally be initialized.
• Enumerations can optionally be declared as one of the following types: Byte, Integer, Long, SByte, Short, UInteger, ULong, or UShort.

VB Example of using enumerations:

   Enum CoffeeType
        Decaf
        Espresso
        Americano
    End Enum      

    Enum WeekDay
        Sunday = 1
        Monday = 2
        Tuesday = 3
        Wednesday = 4
        Thursday = 5
        Friday = 6
        Saturday = 7
    End Enum      

    Enum MaxAge As Short
        infants = 1
        toddlers = 3
        children = 12
        teenagers = 18
    End Enum      

    Sub Main()
        Dim FavoriteCoffee As CoffeeType
        FavoriteCoffee = CoffeeType.Americano
        System.Console.WriteLine(FavoriteCoffee.ToString)
        'Output: Americano     

        Dim WorkDay As WeekDay = 2
        System.Console.Write("I work on ")
        System.Console.WriteLine(WorkDay.ToString)
        'Output: I work on Monday     

        Dim DayOff As WeekDay = WeekDay.Sunday
        System.Console.Write("I'm off on ")
        System.Console.WriteLine(DayOff.ToString)
        'Output: I'm off on Sunday     

        Dim OurKidsAge As MaxAge = MaxAge.Teenagers
        System.Console.Write("Our kids are all ", OurKidsAge.ToString)
        System.Console.WriteLine(OurKidsAge.ToString)
        'Output: Our kids are all teenagers
        System.Console.Write("Our oldest kid is ", OurKidsAge)
        System.Console.WriteLine(OurKidsAge)
        'Output: Our oldest kid is 18     

    End Sub

That’s it for this lesson. I’m looking forward to your questions and comments!