vb.net - 如何在 VB.net 中制作对象数组？

Question

我无法调试这个。

我的数组制作程序错了吗？

它显示的错误是

System.NullReferenceException：'对象引用未设置为对象的实例'

Module Module1

Class Toy
    Private Name, ID As String
    Private Price As Single
    Private MinimumAge As Integer
    Dim Count As Integer

    Public Sub New()

    End Sub

    Public Sub SetName(ByVal N As String)
        Name = N
    End Sub

    Public Sub SetID(ByVal I As String)
        'VALIDATION CHECK FOR ID !
        While Len(I) < 4
            Console.WriteLine("Kindly Enter the ID with Lenght of Max 4 Characters")
        End While
        ID = I
    End Sub

    Public Sub SetPrice(ByVal SP As Single)
        Price = SP
    End Sub

    Public Sub SetUserAge(ByVal SM As Integer)
        'VALIDATION CHECK FOR AGE
        While SM < 0 Or SM < 18
            Console.WriteLine("Minimum age is 18")
        End While
        MinimumAge = SM
    End Sub

    Public Function GetName()
        Return Name(Count)
    End Function

    Public Function GetID()
        Return ID
    End Function

    Public Function GetPrice()
        Return Price
    End Function

    Public Function GetAge()
        Return MinimumAge
    End Function

End Class

Class ComputerGame
    Inherits Toy
    Private Category, Consol As String

    Public Sub New()
    End Sub

    Public Sub SetConsole(ByVal C As String)
        While C <> "PS4" Or C <> "XBOX"
            Console.WriteLine("Invalid console entered")
        End While
        Consol = C
    End Sub

    Public Sub SetCategory(ByVal SC As String)
        Category = SC
    End Sub

    Public Function GetConsole()
        Return Consol
    End Function

    Public Function GetCategory()
        Return Category
    End Function

End Class

Class Vehicle
    Inherits Toy
    Private Type As String
    Private Length, Height, Weight As Single

    Public Sub New()
    End Sub

    Public Sub SetType(ByVal TY As String)
        While TY <> "Car" Or TY <> "Bus" Or TY <> "Truck"
            Console.WriteLine("vehicle is not in list !")
        End While
        Type = TY
    End Sub

    Public Sub SetLength(ByVal SL As Single)
        Length = SL
    End Sub

    Public Sub SetHeight(ByVal SH As Single)
        Height = SH
    End Sub

    Public Sub SetWeight(ByVal SW As Integer)
        Weight = SW
    End Sub

    Public Function GetT()
        Return Type
    End Function

    Public Function GetLength()
        Return Length
    End Function

    Public Function GetHeight()
        Return Height
    End Function

    Public Function GetWeight()
        Return Weight
    End Function

End Class

Sub Main()
    Dim ThisGame(6) As Toy
    ThisGame(6) = New Toy
    ThisGame(1).SetID("23456a")
    ThisGame(2).SetID("236789b")
    Console.WriteLine(ThisGame(1).GetID)

    Console.ReadKey()

End Sub

End Module

标签： vb.netoop

Answer 1

这就是您的 Toy 类使用属性的 long 方法时的样子。请注意，您有公共属性和一个私有字段来存储值（有时称为支持字段）每个属性都有一个 Get 和一个 Set 过程，您可以在其中添加验证代码。

Public Class Toy

    Private _Name As String
    Public Property Name As String
        Get
            Return _Name
        End Get
        Set(value As String)
            _Name = value
        End Set
    End Property

    Private _ID As String
    Public Property ID As String
        Get
            Return _ID
        End Get
        Set(value As String)
            If value.Length < 4 Then
                Console.WriteLine("Kindly Enter the ID with Lenght of Max 4 Characters")
                Exit Property
            End If
            _ID = value
        End Set
    End Property

    Private _Price As Single
    Public Property Price As Single
        Get
            Return _Price
        End Get
        Set(value As Single)
            _Price = value
        End Set
    End Property

    Private _MinimumAge As Integer
    Public Property MinimumAge As Integer
        Get
            Return _MinimumAge
        End Get
        Set(value As Integer)
            If value < 0 Or value < 18 Then
                Console.WriteLine("Minimum age is 18")
                Exit Property
            End If
            _MinimumAge = value
        End Set
    End Property

    Private _Count As Integer
    Public Property Count As Integer
        Get
            Return _Count
        End Get
        Set(value As Integer)
            _Count = value
        End Set
    End Property

End Class

当前版本具有自动属性，其中编译器提供 getter、setter 和 backer 字段。请注意，您仍然可以在需要添加代码的地方编写自己的 Get 和 Set。

Public Class Toy
    Public Property Name As String
    Public Property Price As Single
    Public Property Count As Integer

    Private _ID As String
    Public Property ID As String
        Get
            Return _ID
        End Get
        Set(value As String)
            If value.Length < 4 Then
                Console.WriteLine("Kindly Enter the ID with Lenght of Max 4 Characters")
                Exit Property
            End If
            _ID = value
        End Set
    End Property

    Private _MinimumAge As Integer
    Public Property MinimumAge As Integer
        Get
            Return _MinimumAge
        End Get
        Set(value As Integer)
            If value < 0 Or value < 18 Then
                Console.WriteLine("Minimum age is 18")
                Exit Property
            End If
            _MinimumAge = value
        End Set
    End Property

End Class

请阅读有关您的阵列的在线评论。

Sub Main()
        Dim ThisGame(6) As Toy 'You are defining and array with 7 elements of Toy
        'Each element is Nothing at this point and you will get a null exception error if you
        'try to access an element.
        ThisGame(6) = New Toy 'Here you have created an instance of your class for the 7th element
        'The other six elements are still nothing so the next 3 lines will get 
        'NullReferenceException: 'Object reference not set to an instance of an object.' 
        'ThisGame(1).ID = "23456a"
        'ThisGame(2).ID = "236789b"
        ThisGame(1) = New Toy
        ThisGame(1).ID = "23456a"
        ThisGame(2) = New Toy
        ThisGame(2).ID = "236789b"
        'Remember the first element in you array is ThisGame(0)
        Console.WriteLine(ThisGame(1).ID)

        Console.ReadKey()

    End Sub