Sorry I'm writing in english. I'm having probably the same problem with the control property (collection Tabs) - the error message: '' could not be set on property 'Tabs'.

After setting the control property Tabs as Readonly (with the Get only) the error message changed to 'Tabs' could not be initialized. The error message appears only after rebuilding the project and returning back to design mode in VS.NET.

When inserting my new control on the page in design mode I can set all its properties (incl. the collection properties) and the final HTML page looks pretty good. The error appears just only when returning back to design mode. Could you help me please?

Vladimir CZ

the collection item:
Public Class mZalozka
Public Property TextNadpis() As String ...
Public Property NavigateUrl() As String ...
Public Property CommandArgument() As String ...
End Class

the collection:
Public Class mZalozky
Inherits CollectionBase
Default Public ReadOnly Property Item(ByVal index As Integer) As mZalozka...
Public Sub Add(ByRef Zalozka As mZalozka)....
Public Sub Remove(ByVal index As Integer)....
End Class

my control class:
<ToolboxData("<{0}:mTab runat=server></{0}:mTab>"), ParseChildren(True), PersistChildren(False)> _
Public Class mTab
Inherits System.Web.UI.WebControls.WebControl
Implements IPostBackEventHandler...

local:
Private _zalozky As New mZalozky

the collection property:
<Category("Misc"), Browsable(True), PersistenceMode(PersistenceMode.InnerProperty), _
DesignerSerializationVisibility(DesignerSerializationVisibility.Content), NotifyParentProperty(True), _
Editor(GetType(mZalozky), GetType(System.ComponentModel.Design.CollectionEditor))> _
Public ReadOnly Property Tabs() As mZalozky
Get
Return _zalozky
End Get
End Property

and finally the aspx control snippet:
<mc:mTab id="MTab47" runat="server">
<Tabs>
<mc:mZalozka NavigateUrl="" TextNadpis="Text" CommandArgument=""></mc:mZalozka>
<mc:mZalozka NavigateUrl="" TextNadpis="Text" CommandArgument=""></mc:mZalozka>
</Tabs>
</mc:mTab>

Ooh - with the same result - after returning back to VS designer the error message 'Tabs' could not be initialized. I had to give up after three days and had to change the collection property to simple string delimited with *. Since now it works well and I can do the rest of my job :(

Vladimir