simple identitymap interface

IIdentityMap.cs

    /// <summary>
    /// identity map pattern
    /// </summary>
    /// <typeparam name="T">class</typeparam>
    /// <typeparam name="TIdentity">key type there index <typeparamref name="T"/></typeparam>
    public interface IIdentityMap<T, TIdentity> where T : class
    {
        /// <summary>
        /// add a product to the map
        /// </summary>
        /// <param name="item">a instance of <typeparamref name="T"/></param>
        /// <param name="identity">the identity of <typeparamref name="T"/></param>
        /// <exception cref="ArgumentNullException">item can't be null</exception>
        /// <exception cref="InvalidProgramException">can't add the same item more then ones</exception>
        void AddItem(TIdentity identity, T item);

        /// <summary>
        /// get a item from the map if can't find one in the map will look in dao for it and add it to the map
        /// </summary>
        /// <param name="id">identity for a item</param>
        /// <returns>a instance of <typeparamref name="T"/> or null</returns>
        T GetById(TIdentity id);
    }