drag and drop issue

[prem1er]

Having problem dragging and dropping a dynamically created set of PictureBoxes. I got most of this code of another VB forum and added my own to try and figure out what was happening, but now I'm getting a runtime error.

An error occurred creating the form. See Exception.InnerException for details. The error is: Object reference not set to an instance of an object.

Public Class frmMain
    Inherits System.Windows.Forms.Form
    Dim DragPoint As Point
    Private XOffset As Integer
    Private YOffset As Integer
    'Private gridOpp As New Grid(grpOppGrid)
    Private gridYou As New Grid(grpYourGrid)


    Private Sub frmMain_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim LV As Integer
        Dim Cheeses(23) As PictureBox
        'Our form will not be the only drag drop target this time.
        Me.AllowDrop = True
        'You can drag food to either of the two whelks.

        'Now let's create a couple dozen pieces of cheese.
        For LV = 0 To 23
            Cheeses(LV) = New PictureBox
            With Cheeses(LV)
                .Name = "Cheese" & LV.ToString("G")
                'Generate a quick name for it.
                '.Image = Cheese.Image
                'Give it the same image as the cheese image already on the form.
                .Size = New Size(24, 24)
                .SizeMode = PictureBoxSizeMode.StretchImage
                'Make the cheese big.
                .Location = New Point(24 * (LV Mod 4) + 250, 24 * (LV \ 4) + 20)
                'Move it into a grid fashion.
            End With
            Me.Controls.Add(Cheeses(LV))
            'Add this control to the form.
            AddHandler Cheeses(LV).MouseDown, New System.Windows.Forms.MouseEventHandler(AddressOf AnyShip_MouseDown)
            'Make each cheese event's mousedown be handled by the anycheese.
            'This will probably be the closest that you can get to a control array.
        Next
    End Sub

    Private Sub AnyShip_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs) Handles MyBase.MouseDown
        Dim PB As PictureBox
        PB = DirectCast(sender, System.Windows.Forms.PictureBox)
        DragPoint = New Point(e.X, e.Y)

        PB.DoDragDrop(PB, DragDropEffects.Move)
    End Sub

    Private Sub frmMain_DragEnter(ByVal sender As Object, ByVal e As System.Windows.Forms.DragEventArgs) Handles MyBase.DragEnter
        Dim Cheeses(23) As PictureBox
        If e.Data.GetDataPresent(Cheeses.GetType()) Then
            e.Effect = DragDropEffects.Move
        End If
    End Sub

    Private Sub frmMain_DragDrop(ByVal sender As Object, ByVal e As System.Windows.Forms.DragEventArgs) Handles MyBase.DragDrop
        Dim PB As GroupBox
        Dim Finalpoint As Point
        Dim Cheeses(23) As PictureBox

        PB = e.Data.GetData(Cheeses.GetType())
        Finalpoint = New Point(e.X, e.Y)
        Finalpoint = Me.PointToClient(Finalpoint)
        Finalpoint.Offset(-DragPoint.X, -DragPoint.Y)

        PB.Location = Finalpoint
    End Sub

    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
        Me.Close()
    End Sub

    Private Sub grpYourGrid_Enter(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles grpYourGrid.Enter
        Me.AllowDrop = True
    End Sub

    Private Sub grpOppGrid_Enter(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles grpOppGrid.Enter
        Me.AllowDrop = True
    End Sub
End Class

 

[JuggaloBrotha]

What's the run time error you're getting? We'll need to know what it is before looking at the code.

 

[prem1er]

An error occurred creating the form. See Exception.InnerException for details. The error is: Object reference not set to an instance of an object.

I put this in my previous post

 

[JuggaloBrotha]

i think i see the problem:

   Private Sub frmMain_DragDrop(ByVal sender As Object, ByVal e As System.Windows.Forms.DragEventArgs) Handles MyBase.DragDrop
        Dim PB As GroupBox
        Dim Finalpoint As Point
        Dim Cheeses(23) As PictureBox

        PB = e.Data.GetData(Cheeses.GetType())
        Finalpoint = New Point(e.X, e.Y)
        Finalpoint = Me.PointToClient(Finalpoint)
        Finalpoint.Offset(-DragPoint.X, -DragPoint.Y)

        PB.Location = Finalpoint
    End Sub

You're declaring a new array at the beginning of the event, then you're trying to get the type which wont work since it's not set to an instance of the picturebox object. What you should do is add the new picturebox's to the groupbox just like you added the picturebox's in the Load event

I'm not sure right off hand if that'll work, but I do know that's why you're getting the NullReference exception

 

[vis781]

You're right there JB and seeing as we're using .Net 2.0 we can make use of generics here which are nearly as fast as a normal array..

Dim Cheeses As New List(Of PictureBox)

Dim _Cheese As PictureBox
For I as Integer = 0 To 22
_Cheese = New PictureBox()
Me.Cheeses.Add(_Cheese)
Next

 

[JohnH]

I don't see that a NullReference exception can happen anywhere in that code, the form loads fine when I do it. The problem with the code you posted is that you send an object of type Picturebox (Cheeses variable is type Picturebox array, so dragdrop won't happen), so you have to use GetType(Picturebox), you also have to correct Dim PB As GroupBox in drop method which is also supposed to be type Picturebox. Doing this and the code works.