WPF Custom Container Class - mvvm

Question

Well im trying to create a custom container class that contains in my case ViewMappingEntry's, so i created a class that inherits Freezable and implements ICollection.. but when creating an instance in Blend and browsing it i get this error: Cannot add content to an object of type 'ViewModelMapping'..
Note: It compiles fine and i get no error in Visual studio, so what am i missing?
public class ViewModelMapping : Freezable, ICollection<MappingEntry>
{
object locker = new object();
List<MappingEntry> internalList = new List<MappingEntry>();
#region ICollection<MappingEntry> Members
public void Add(MappingEntry item)
{
if (this.IsFrozen || this.IsSealed)
throw new InvalidOperationException("ViewModelMapping is frozen");
lock (locker)
internalList.Add(item);
}
public void Clear()
{
if (this.IsFrozen || this.IsSealed)
throw new InvalidOperationException("ViewModelMapping is frozen");
lock (locker)
internalList.Clear();
}
public bool Contains(MappingEntry item)
{
lock (locker)
return internalList.Contains(item);
}
public void CopyTo(MappingEntry[] array, int arrayIndex)
{
lock (locker)
internalList.CopyTo(array, arrayIndex);
}
public int Count
{
get
{
lock (locker)
return internalList.Count;
}
}
public bool IsReadOnly
{
get { return base.IsFrozen || base.IsSealed; }
}
public bool Remove(MappingEntry item)
{
if (this.IsFrozen || this.IsSealed)
throw new InvalidOperationException("ViewModelMapping is frozen");
lock (locker)
return internalList.Remove(item);
}
#endregion
#region IEnumerable<MappingEntry> Members
public IEnumerator<MappingEntry> GetEnumerator()
{
lock (locker)
return (IEnumerator<MappingEntry>)internalList.ToArray().GetEnumerator();
}
#endregion
#region IEnumerable Members
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
lock (locker)
return internalList.ToArray().GetEnumerator();
}
#endregion
protected override Freezable CreateInstanceCore()
{
ViewModelMapping viewModelMapping = new ViewModelMapping();
lock(locker)
viewModelMapping.internalList = new List<MappingEntry>(this.internalList.ToArray());
return viewModelMapping;
}
}

Answer 1

after trying around i found a solution if i implemented all these interfaces -> ICollection, IList, IList, ICollection, IEnumerable it started working in blend.