public abstract class NavigationModel
   {
   /// <summary>
   /// Gets the default NavigationModel of the ViewApp
   /// </summary>
   public static NavigationModel ViewAppNavigationModel { get; }

   /// <summary>
   /// Gets the root item of the model
   /// </summary>
   public abstract NavigationItem RootItem { get; }

   /// <summary>
   /// Gets the collection of navigation items.
   /// </summary>
   /// <returns> Returns an observable collection of immediate child navigation items
   /// </returns>
   public abstract ReadOnlyObservableNavigationItemCollection Items { get; }

   /// <summary>
   /// Gets the item for the given navigation item path
   /// </summary>
   /// <param name="navigationItemFullPath"> Full path or URI of a particular hierarchy item
   /// </param>
   /// <returns> Navigation item
   /// </returns>
   public abstract NavigationItem GetItemBypath(string navigationItemFullPath);

   /// <summary>
   /// Retrieves the content available under the navigation hierarchy path, based on the the given filter options
   /// </summary>
   /// <param name="NavigationHierarchyPath"> The navigation hierarchy path. This should be something like
   /// \\Home\NavigationItem1
   /// </param>
   /// <param name="FilterOption"> This is the filter criteria to retrieve content
   /// </param>
   /// <returns> Array of ContentData </returns>
   public abstract ContentData[] GetContentInHierarchy(string navigationHierarchyPath, FilterOptions filterOptions);
   }

   public abstract class NavigationItem
   {
   /// <summary>
   /// Gets or sets the actual name of the Navigation Item.
   /// </summary>
   public virtual string Name { get;}

   /// <summary> 
   /// Gets or sets the friendly name of the Navigation Item to be shown on
   /// navigation-aware control items.
   /// </summary>
   public virtual string Title { get;}

   /// <summary>
   /// Gets or sets a value indicating whether or not the Navigation Item has errors.
   /// </summary>
   /// <value>
   /// true if [HasErrors]; otherwise, false.
   /// </value>
   public virtual bool HasErrors { get;}

   /// <summary>
   /// Gets or sets the errors for this Navigation Item.
   /// </summary>
   public virtual IEnumerable Errors { get;}

   /// <summary>
   /// Gets or sets the parent of the Navigation Item.
   /// </summary>
   public NavigationItem Parent { get; }

   /// <summary>
   /// Gets a value indicating whether Navigation Item [has items].
   /// </summary>
   /// <value>
   /// true if [has items]; otherwise, false.
   /// <value>
   public abstract bool HasItems { get; }

   /// <summary>
   /// Gets the child item collection.
   /// </summary>
   /// <returns> An observable collection of navigation item that are immediate child items of this instance.
   /// </returns>
   public abstract ReadOnlyNavigationItemCollection Items { get; }

   /// <summary>
   /// Gets the previous sibling.
   /// </summary>
   /// <returns> NavigationItem
   /// </returns>
   navigationItem public NavigationItem PreviousSibling

   /// <summary>
   /// Gets the next sibling.
   /// </summary>
   /// <returns> NavigationItem
   /// </returns>
   public NavigationItem NextSibling

   /// <summary>
   /// Gets a value indicating the path of the item in its hierarchy.
   /// </summary>
   public string Path

   /// <summary>
   /// Gets or sets a value that indicates the Access Level of the configured Nav Item.
   /// </summary>
   public virtual int AccessLevel { get; }

   /// <summary>
   /// Gets the item located at a given hierarchy path.
   /// </summary>
   /// <param name="fullPath"> The full or relative path of the item to be retrieved.
   /// </param>
   /// <returns> Requested Navigation Item
   /// <returns>
   public abstract NavigationItem GetItem(string fullPath);
   }

   /// <summary>
   /// A custom exception class that wraps the exception/error information related to the Navigation Extension.
   /// </summary>
   [Serializable]
   public class NavigationException : Exception
   {
      /// <summary>
      /// Initializes a new instance of the <see cref="NavigationException"/> class.
      /// </summary>
      public NavigationException()
      {
      }
        /// <summary>
        /// Initializes a new instance of the NavigationException class.
        /// </summary>
        /// <param name="message">The message that describes the error.</param>
        public NavigationException(string message) : base(message)
        {
        }
         /// <summary>
        /// Initializes a new instance of the NavigationException class.
        /// </summary>
        /// <param name="message">The error message that explains the reason for the exception.
        /// </param>
        /// <param name="innerException">The exception that is the cause of the current exception, or a null reference
        /// (nothing in Visual Basic) if no inner exception is specified.
        /// </param>
        public NavigationException(string message, Exception innerException)
            : base(message, innerException)
        {
        }
         /// <summary>
        /// Initializes a new instance of the <see cref="NavigationException"/> class.
        /// </summary>
        /// <param name="info">The <see cref="T:System.Runtime.Serialization.SerializationInfo" /> that holds the
        /// serialized object data about the exception being thrown.
        /// </param>
        /// <param name="context">The <see cref="T:System.Runtime.Serialization.StreamingContext" /> that contains
        /// contextual information about the source or destination.
        /// </param>
        protected NavigationException(SerializationInfo info, StreamingContext context)
            : base(info, context)
         }
    }