PersistenceModeAttribute Class

To implement custom control with collection property which can be used as follows:

<xxx:TheControl runat="server">
    <Items>
        <xxx:MyItem Name="Item1"/>
        <xxx:MyItem Name="Item2"/>
        <xxx:MyItem Name="Item3"/>
    </Items>
</xxx:TheControl>

you must implement your class as follows:<ParseChildren(True), PersistChildren(False)>

Public Class TheControl
    Inherits Control
    Private items As New List(Of MyItem)
    <PersistenceMode(PersistenceMode.InnerProperty)>
    Public ReadOnly Property Items As List(Of MyItem)
        Get
            Return items
        End Get
    End Property
End Class

Public Class MyItem
    Public Property Name As String
End Class

Especially important is type of the Items property. It seems that it must be statically typed to type which:

1. Implements the ICollection non-generic interface
2. Exposes the Add method

E.g. following types are OK: List(Of T), ObservableCollection(Of T), Collection(Of T), IList
Following types cause various errors: ICollection, ICollection(Of T), IList(Of T), T()

This is minimal C# implementation of class suitable as type of Items property (class implements ICollection and Add):

public class MyCollection : ICollection
{
    private ICollection i = newList<Object>();
    public void CopyTo(Array array, int index) { i.CopyTo(array, index); }
    public int Count { get { return i.Count; } }
    public bool IsSynchronized { get { return i.IsSynchronized; } }
    public object SyncRoot { get { return i.SyncRoot; } }
    public IEnumerator GetEnumerator() { return i.GetEnumerator(); }
    public void Add(object o) { ((IList)i).Add(o); }
}

Note: If you want to make it type-safe (or generic), you still have to implement non-generic interface ICollection).
This is valid (at least in C#), too (implementing Add as extension method):

public class MyCollection : ICollection
{
    internal ICollection i = newList<Object>();
    public void CopyTo(Array array, int index) { i.CopyTo(array, index); }
    public int Count { get { return i.Count; } }
    public bool IsSynchronized { get { return i.IsSynchronized; } }
    public object SyncRoot { get { return i.SyncRoot; } }
    public IEnumerator GetEnumerator() { return i.GetEnumerator(); }
}
publicstaticclassMyCollectionExtensions
{
    publicstaticvoid Add(thisMyCollection c, object o) { ((IList)c.i).Add(o); }
}