[
ToolboxData("<{0}:MyCustomHierarchicalControl runat=server> </{0}:MyCustomHierarchicalControl>")
] 
public class MyCustomHierarchicalControl : TreeView 
{        
    private object _dataSource;

    [TypeConverter(typeof(HierarchicalDataSourceConverter))]
    public override object DataSource
    {
        get
        {
            return _dataSource;
        }
        set
        {
            if (value != null) 
            {
                ValidateDataSource(value);
            }
            _dataSource = value;
            OnDataPropertyChanged();
        }
    }

    // Define rest of custom control implementation.
    // ...
}