Adding a User Control dynamically to a FlowLayoutPanel - vb.net

Question

I've created a UserControl and added it to a FlowLayoutPanel. The UserControl is being used to allow a user to enter a material, cost and delivery status to the form. When the user has filled it in, I want another UserControl to appear underneath it in the FlowLayoutPanel
The UserControl simply generates a string based on the text entered into two TextBox controls and the status of two Checkbox controls. it also has a property that indicates when the user has filled in enough information. I want to use this property to signal generating a new UserControl.
At the moment I have my first UserControl on the FlowLayoutPanel, it is successfully passing the String and CreateNew property back.
The problems I am encountering are:
How do I monitor to see if CreateNew has changed to True?
How do I add a control to the form and +1 to the controls name for future referencing
Once the new control is added, I need to monitor to see if the new CreateNew state changes to repeat the cycle
Can anyone point me in the right direction here, there's a lot of information out there on this, but I can't seem to find anything useful from other's problems/questions.
UPDATE 1
Thanks to user Zaggler for the comment, I have now managed to get the control to create a new instance of itself on the FlowLayoutPanel. But now I'm faced with a new problem of it only creating one new usercontrol, then it stops.
Here's the code I'm using:
UserControl Code
Public Class Alv_Product_Order_Control
Public OutString As String
Public Event CreateNew()
Dim CreateNewRaised As Boolean
Private Sub OutputString(sender As Object, e As EventArgs) Handles tbMaterial.TextChanged, tbCost.TextChanged,
cbDelivered.CheckedChanged, cbOrderPlaced.CheckedChanged
OutString = "¦¦" & tbMaterial.Text & "¦" & tbCost.Text & "¦"
If cbOrderPlaced.Checked = True Then
OutString = OutString & "Yes¦"
Else
OutString = OutString & "No¦"
End If
If cbDelivered.Checked = True Then
OutString = OutString & "Yes¦"
Else
OutString = OutString & "No¦"
End If
If tbCost.Text = "" Or tbMaterial.Text = "" Then
Else
If CreateNewRaised = False Then
RaiseEvent CreateNew() 'Raise the event that's used to signal adding a new control to the layout
CreateNewRaised = True 'Create A Latched Boolean that cannot change again in the future
End If
End If
End Sub
Public ReadOnly Property Alv_Product_Order_Control As String
Get
Return OutString 'Pass string back to main form
End Get
End Property
Main Form Code
Private Sub CreateSecondPOC() Handles POC1.CreateNew
FlowLayoutPanel1.Controls.Add(New Alveare_VB.Alv_Product_Order_Control)
End Sub
I'm guessing here that the problem is the CreateSecondPOC only handles the event for the the first POC1
How do I create a new Alveare_VB.Alv_Product_Order_Control, name it as POC2 and also add a handler to handle POC2.CreateNew and add another control?
Edit 2
I know I've found the answer, but i'd like to look at this memory leak that has been mentioned. I've changed the code supplied in the answer below to this:
Private Sub CreateSecondPOC(ByVal sender As Object, ByVal e As System.EventArgs) Handles POC1.CreateNew
Try
Dim oldPoc = DirectCast(sender, Alveare_VB.Alv_Product_Order_Control)
RemoveHandler oldPoc.CreateNew, AddressOf CreateSecondPOC
Catch ex As Exception
Debug.Print(ex.Message)
End Try
Dim newPoc As New Alveare_VB.Alv_Product_Order_Control
AddHandler newPoc.CreateNew, AddressOf CreateSecondPOC
FlowLayoutPanel1.Controls.Add(newPoc)
End Sub
And I get the following error on the "RemoveHandler" line:
Unable to cast object of type 'System.Windows.Forms.TextBox' to type 'Alveare_VB.Alv_Product_Order_Control'.
The CreateNew event is raised when a textbox is written in, this is getting passed back as the sender I assume? Not really sure where to go with this now.
Edit 3
The error was in my UserControl, I was sending the incorrect object back (in this case the textbox). I have now changed the RaiseEvent to return the UserControl as an object. Now all is working correctly.

Answer 1

You could change your handler to something like this
Private Sub CreateSecondPOC() Handles POC1.CreateNew
Dim newPoc As New Alveare_VB.Alv_Product_Order_Control
AddHandler newPoc.CreateNew, AddressOf CreateSecondPOC
FlowLayoutPanel1.Controls.Add(newPoc)
End Sub
I'm not sure if you want to keep handling the event, even after it has been populated once, i.e. can it be depopulated, then repopulated, then raise the event again? Maybe you want to lock it once it's populated, but this isn't clear.
You could also keep all your POC controls in a container and only create a new one when they are all populated.
Edit:
According to comments below, you should remove the event handler when you no longer need it in order to avoid memory leaks. Here is one way
Private Sub CreateSecondPOC(sender As Object) Handles POC1.CreateNew
Dim oldPoc = DirectCast(sender, Alveare_VB.Alv_Product_Order_Control)
RemoveHandler oldPoc, AddressOf CreateSecondPOC
Dim newPoc As New Alveare_VB.Alv_Product_Order_Control
AddHandler newPoc.CreateNew, AddressOf CreateSecondPOC
FlowLayoutPanel1.Controls.Add(newPoc)
End Sub
But the last POC control's event handler would never be unsubscribed. So you could also do something like this when closing the form
For Each poc In Me.FlowLayoutPanel1.Controls.OfType(Of Alveare_VB.Alv_Product_Order_Control)()
RemoveHandler poc.CreateNew, AddressOf CreateSecondPOC
Next poc
I mentioned above that You could also keep all your POC controls in a container, which would be a better way to keep track of your controls, instead of using the FlowLayoutPanel as logical container. Just do what works for you and consider removing the handlers.