ByRef没有通过IN vb.net

Question

The one calls form 2 as a dialog, and passed by ref a "pointer" to the base class(abstract). //Form 1 calling form two. And Passing a ref object

Dim CreateForm As New frmCreate(Robot)

//Second Forms Overloaded New

Public Sub New(ByRef Robot As cRobot)
    InitializeComponent()
    thisRobot = Robot
End Sub

Select Case (cbType.SelectedIndex)
        Case 0
            lblOther.Text = "Bullet Proof Value"
            Dim SecRobot = New cSecurityRobot
            SecRobot.Name = txtName.Text
            SecRobot.Temperature = nudTemp.Value
            SecRobot.Threshold = nudThreshold.Value
            SecRobot.BulletproofValue = nudOther.Value
            thisRobot = SecRobot
        Case 1
            lblOther.Text = "Special Moves"
            Dim SpRobot = New cSportsRobot
            SpRobot.Name = txtName.Text
            SpRobot.Temperature = nudTemp.Value
            SpRobot.Threshold = nudThreshold.Value
            SpRobot.SpecialMoves = nudOther.Value
            thisRobot = SpRobot
        Case 2
            lblOther.Text = "Domestic Skills"
            Dim SerRobot = New cServiceRobot
            lblOther.Text = "Domestic Skills"
            SerRobot.Name = txtName.Text
            SerRobot.Temperature = nudTemp.Value
            SerRobot.Threshold = nudThreshold.Value
            SerRobot.DomesticSkills = nudOther.Value
            thisRobot = SerRobot
        Case Else
            lblOther.Text = "Bullet Proof Value"
            Dim SecRobot = New cSecurityRobot
            SecRobot.Name = txtName.Text
            SecRobot.Temperature = nudTemp.Value
            SecRobot.Threshold = nudThreshold.Value
            SecRobot.BulletproofValue = nudOther.Value
            thisRobot = SecRobot
    End Select

Form 2 assigns some values and terminates, but there is always a NULL Exception occurring

Answer 1

Let's take a look at your constructor:

Public Sub New(ByRef Robot As cRobot)
    InitializeComponent()
    thisRobot = Robot '<-- Problem is here
End Sub

On the line indicated above, you are making a copy of the reference, and so the ByRef no longer helps you.

Thinking about how to get around this problem, you might be able to do it by nesting the Robot inside another class:

Public Class RobotContainer
     Public Property Robot As Robot
End Class

Pass a RobotContainer instance to your constructor in the normal (ByVal) way and keep a reference to that entire object in your class. Now, both your frmCreate type and the calling code have a reference to the same object . When you update the Robot property on that object, it will be updated for both locations.

But really, this whole design doesn't smell right. Normally I would suggest a method that return the created robot, rather than trying to assign it to an outside location directly, but I understand that working with Windows Forms controls this may not be option. To suggest a better solution we'd need to see a lot more of your code.

---

Hmm... looking back at this I wanted to do something to make the RobotContainer more useful:

Public Class ReferenceContainer(Of T)
    Public Property Item As T
End Class

Answer 2

No, the "ByRef"-ness is only relevant for the method in which the parameter is declared. The value of the thisRobot variable is still just a reference value . Changing the value of that variable later on won't change the caller's variable.