/// <summary>
        /// Initializes a new instance of the <see cref="TransformerBuilder{TSource, TDestination}"/> class.
        /// </summary>
        /// <param name="observer">The observer.</param>
        /// <param name="propertyMappers">The property mappers.</param>
        internal TransformerBuilder(ITransformerObserver observer, IEnumerable <IPropertyMapper <TSource, TDestination> > propertyMappers)
        {
            this.observer = observer;

            this.propertyMappers = propertyMappers != null
                ? new HashSet <IPropertyMapper <TSource, TDestination> >(propertyMappers)
                : new HashSet <IPropertyMapper <TSource, TDestination> >();
        }
 /// <summary>
 ///
 /// </summary>
 static TransformerObserver()
 {
     instance   = new TransformerObserver();
     defaultKey = "default";
 }
 /// <summary>
 /// Initializes a new instance of the <see cref="TransformerBuilder{TSource, TDestination}"/> class.
 /// </summary>
 /// <param name="observer">The observer.</param>
 internal TransformerBuilder(ITransformerObserver observer)
 {
     this.observer        = observer;
     this.propertyMappers = new HashSet <IPropertyMapper <TSource, TDestination> >();
 }