ByVal在VB.NET中无法正常工作

Question

I have a code in VB.NET like this:

' This code will sort array data
Public Sub SelectionSort(ByVal array as ArrayList)
   For i as Integer = 0 To array.Count -1
      Dim index = GetIndexMinData(array, i)
      Dim temp = array(i)
      array(i) = array(index)
      array(index) = temp
   Next
End Sub

Public Function GetIndexMinData(ByVal array As ArrayList, ByVal start As Integer) As Integer
    Dim index As Integer
    Dim check As Integer = maxVal
    For i As Integer = start To Array.Count - 1
        If array(i) <= check Then
            index = i
            check = array(i)
        End If
    Next
    Return index
End Function

' This code will sort array data
Public Sub SelectionSortNewList(ByVal array As ArrayList)
    Dim temp As New ArrayList

    ' Process selection and sort new list
    For i As Integer = 0 To array.Count - 1
        Dim index = GetIndexMinData(array, 0)
        temp.Add(array(index))
        array.RemoveAt(index)
    Next
End Sub

Private Sub btnProcess_Click(sender As System.Object, e As System.EventArgs) Handles btnProcess.Click
    Dim data as new ArrayList
    data.Add(3)
    data.Add(5)
    data.Add(1)
    SelectionSort(data)
    SelectionSortNewList(data)
End Sub

When I run this code, in the btnProcess event click, variable "data" is array = {3,5,1}. By SelectionSort(data) procedure, variable data is changed. Item in variable data have sorted by that procedure, so when SelectionSortNewList(data) is run, array "data" have sorted became {1,3,5}. Why does it happen?

Although I have used "Byval parameter" in SelectionSort and SelectionSortNewList, I don't want variable data to be changed when it pass to SelectionSort.

Answer 1

Even if you use a ByVal on an object, the properties of the object can be modified.

The instance of the object cannot be modified but not its properties.

Example:

Public Class Cars

    Private _Make As String
    Public Property Make() As String
        Get
            Return _Make
        End Get
        Set(ByVal value As String)
            _Make = value
        End Set
    End Property

End Class

If I pass the class as ByVal;

Private sub Test(ByVal MyCar as Car)

MyCar.Make = "BMW"

End Sub

The value of the property will change as you're pointing to the same object and modifying its property.