How to get inherited type of Form in ControlDesigner - vb.net

Question

I am building a custom designer that will associate a control with a business property on the form. The form DealUI has properties Instrument and Product, which are a business items:
Public Class DealUI
Inherits System.Windows.Forms.Form ' repetition of Inherits in Deal.Designed.vb, just to make the point
Sub New()
InitializeComponent()
End Sub
<Business(True)> _
Public Property Product As String
<Business(True)> _
Public Property Instrument As String
End Class
The Business attribute is simply
NotInheritable Class BusinessAttribute
Inherits Attribute
Private _isBusiness As Boolean
Sub New(isBusiness As Boolean)
_isBusiness = isBusiness
End Sub
End Class
The form contains a custom control, ProductTextBox of type PilotTextBox:
<DesignerAttribute(GetType(PilotControlDesigner)), _
ToolboxItem(GetType(PilotToolboxItem))> _
Public Class PilotTextBox
Inherits TextBox
Public Property Source As String
End Class
In the designer, when the selected control changes to ProductTextbox, I want to populate its Source property with the names of the Form's properties that have the BusinessAttribute (Instrument and Product), the user can then choose between Instrument and Product. The designer code is
Public Class PilotControlDesigner
Inherits ControlDesigner
Private Sub InitializeServices()
Me.selectionService = GetService(GetType(ISelectionService))
If (Me.selectionService IsNot Nothing) Then
AddHandler Me.selectionService.SelectionChanged, AddressOf selectionService_SelectionChanged
End If
End Sub
Private Sub selectionService_SelectionChanged(ByVal sender As Object, ByVal e As EventArgs)
If Me.selectionService IsNot Nothing Then
If Me.selectionService.PrimarySelection Is Me.Control Then
Dim form As Object = DesigningForm()
If form IsNot Nothing Then
For Each prop As PropertyInfo In form.GetType.GetProperties
Dim attr As Attribute = GetCustomAttribute(prop.ReflectedType, GetType(BusinessAttribute), False)
If attr IsNot Nothing Then
' we've found a Business attribute
End If
Next
End If
End If
End If
End Sub
Private Function DesigningForm() As Object ' in fact, a form, or more precisely something that inherits from Form
Dim host As IDesignerHost = CType(Me.Component.Site.GetService(GetType(IDesignerHost)), IDesignerHost)
Dim container As IContainer = host.Container
For Each comp As Component In container.Components
If comp.GetType.IsAssignableFrom(GetType(Form)) Then ' or anything that inherits 'Form'
return comp ' returns a Form, not a Deal!!
End If
Next comp
Return nothing
End Function
End Class
The selected control is a Deal (which inherits from Form), but the component in the designer is a Form, not a Deal (!! in the comment). I need to examine the Instrument and Product properties, which only exist on a Deal.
How can I obtain the Deal object in the designer?