arrays - 如何检查空数组

Question

我在 VB.net 工作，我的课程如下：

Public Class vertex
    Public wasVisited As Boolean
    Public name, type As String
    Public x_pos, y_pos As Double

    Public Sub New(ByVal x_pos As Double, ByVal y_pos As Double, ByVal name As Integer, ByVal type As String)
        Me.x_pos = x_pos
        Me.y_pos = y_pos
        Me.name = name
        Me.type = type
        wasVisited = False
    End Sub
End Class

我有一些名为“图”的其他类的对象，在图类的构造函数中，我正在调用顶点类的构造函数。

我有顶点类数组： Public vertices() As vertex

并且 redim vertices(2000): 出于某种原因再次调整数组大小。

现在，当我循环数组以检查空值时，它会引发错误：

你调用的对象是空的。（因为值包含“无”）

即使我这样检查，

If (vertices(i).name) Is Nothing Then
            Exit For
        End If

如何检查数组的空元素？

score 1 · Accepted Answer

vertices()redim 操作之前的大小是多少？如果它小于 2000，则添加的元素将Nothing在数组放大之后立即添加，因此当您尝试访问超出初始数组大小的 i 值的name属性时，您实际上是在尝试取消引用空对象引用。vertices(i)

您要么需要vertices(i) IsNot Nothing在测试其属性值之前检查这一点，要么确保为数组的每个元素分配一个new vertex对象。

If vertices(i) Is Nothing OrElse vertices(i).name Is Nothing Then
    Exit For
End If

这是关于类似问题的 vbforums 线程：http ://www.vbforums.com/showthread.php?546668-RESOLVED-Redim-array-of-objects

score 1 · Accepted Answer

由于您似乎希望您的收藏是动态的，因此 List(Of vertex) 会更好地为您服务。它与默认的 New() 构造函数一起，您可以添加、删除、排序、搜索，无论您需要什么。要检查您可以使用的任何空值If Vertices(i).name = "" then

Public Class vertex
    Public wasVisited As Boolean
    Public name, type As String
    Public x_pos, y_pos As Double
    Public Sub New()
        wasVisited = False
        name = ""
        type = ""
        x_pos = 0
        y_pos = 0
    End Sub

    Public Sub New(ByVal x_pos As Double, ByVal y_pos As Double, ByVal name As String, ByVal type As String)
        Me.x_pos = x_pos
        Me.y_pos = y_pos
        Me.name = name
        Me.type = type
        wasVisited = False
    End Sub
End Class

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    Dim Vertices As New List(Of vertex)
    For I = 0 To 99
        Vertices.Add(New vertex())
        Vertices(I).name = "Test" + I.ToString
    Next
End Sub

score 0 · Accepted Answer

你有没有尝试过：

If Not vertices Is Nothing AndAlso Not vertices(i) Is Nothing _
            AndAlso Not vertices(i).name Is Nothing Then

   Dim value as string= vertices(i).name

End If

arrays - 如何检查空数组

3 回答 3

Related

Reference