Design-time trouble with ASP.NET v2.0 Custom Control, with List<T> for child items - asp.net

Question

Folks,
I am having a devil of a time with a custom control. The control is very simple - it just displays a list of "StepItems" (rendered as table rows), each with an icon. When I first drag it onto a page, and add StepItems to its collection, it renders perfectly. If I provide some text for its Header property, that also renders perfectly.
If I then look at the HTML source view, and then back to the design view, I get an error where my control should be. There are two kinds of errors:
If I set the .Header property, the error reads "StepProgressControl1:'someheadertext' could not be set on property 'Header'.
If I don't set the .Header, but add StepItems to the collection, I get this: "ErrorStepProgressControl1:'StepItems' could not be initialized. Details: Method not found: 'System.Collections.Generic.List`1 StepProgressControl.TKC.Experiment.StepProgressControl.get_StepItems()'."
The complete code for my custom control is below. If you can provide any help, thank you a great deal!
Tom
'================================
Imports System
Imports System.Collections
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Security.Permissions
Imports System.ComponentModel
Namespace TKC.Experiment
' THIS IS THE INTERNAL "CHILD" ELEMENT
< _
PersistenceMode(PersistenceMode.InnerProperty), _
TypeConverter(GetType(StepItemConverter)) _
> _
Public Class StepItem
Private _name As String
Public Sub New()
Me.New("")
End Sub
Public Sub New(ByVal name As String)
Me._name = name
End Sub
Public Property Name() As String
Get
Return _name
End Get
Set(ByVal value As String)
_name = value
End Set
End Property
End Class
'=====================================================================
' THIS IS THE ACTUAL "PARENT" WEB CONTROL
< _
ParseChildren(True, "StepItems"), _
PersistChildren(False) _
> _
Public NotInheritable Class StepProgressControl
Inherits WebControl
Private _header As String = String.Empty
Private _stepItems As New List(Of StepItem)
Public Sub New()
Me.Header = "StepProgressControl"
End Sub
< _
PersistenceMode(PersistenceMode.Attribute) _
> _
Public Property Header() As String
Get
Return _header
End Get
Set(ByVal value As String)
_header = value
End Set
End Property
< _
DesignerSerializationVisibility(DesignerSerializationVisibility.Content), _
PersistenceMode(PersistenceMode.InnerProperty) _
> _
Public ReadOnly Property StepItems() As List(Of StepItem)
Get
If _stepItems Is Nothing Then
_stepItems = New List(Of StepItem)
End If
Return _stepItems
End Get
'Set(ByVal value As List(of stepitem))
' _stepItems = value
'End Set
End Property
Public Overrides Sub RenderControl(ByVal writer As System.Web.UI.HtmlTextWriter)
MyBase.RenderControl(writer)
Dim label As New Label()
label.Text = Header
label.RenderControl(writer)
Dim table As New Table()
Dim htr As New TableRow()
Dim hcell1 As New TableHeaderCell()
hcell1.Text = "Name"
htr.Cells.Add(hcell1)
Dim hcell2 As New TableHeaderCell()
hcell2.Text = "Title"
htr.Cells.Add(hcell2)
table.BorderWidth = Unit.Pixel(0)
Dim stepItem As StepItem
For Each stepItem In StepItems
Dim tr As New TableRow()
Dim cell1 As New TableCell()
Dim img As New HtmlImage
img.Src = ""
img.Alt = ""
cell1.Controls.Add(img)
tr.Cells.Add(cell1)
Dim cell2 As New TableCell()
cell2.Text = stepItem.Name
tr.Cells.Add(cell2)
table.Rows.Add(tr)
Next stepItem
table.RenderControl(writer)
End Sub
End Class
'========================================
'THIS IS A "TYPE CONVERTER" - JUST A COSMETIC THING, NOT CAUSING TROUBLE...
Public Class StepItemConverter
Inherits TypeConverter
Public Overloads Overrides Function ConvertTo(ByVal context As ITypeDescriptorContext, ByVal culture As System.Globalization.CultureInfo, ByVal value As Object, ByVal destinationType As Type) As Object
Dim obj As StepItem = DirectCast(value, StepItem)
Return obj.Name
End Function
End Class
End Namespace

Answer 1

You will want to implement your own Collection object to represent the list - otherwise the designer will not display it properly.
See the ICollection, IEnumerable, etc. interfaces.