WPF UncommonField 类型是什么

internal static readonly UncommonField<EventHandlersStore> EventHandlersStoreField = new UncommonField<EventHandlersStore>();

     //这个类可以减少内存使用，比使用 DependencyObject 少的内存，这个类在框架使用，不在外面使用
  [FriendAccessAllowed] // Built into Base, used by Core and Framework
    internal class UncommonField<T>
    {
        /// <summary>
        ///     Create a new UncommonField.
        /// </summary>
        public UncommonField() : this(default(T))
        {
        }
 
        /// <summary>
        ///     Create a new UncommonField.
        /// </summary>
        /// <param name="defaultValue">The default value of the field.</param>
        public UncommonField(T defaultValue)
        {
            _defaultValue = defaultValue;
            _hasBeenSet = false;
 
            lock (DependencyProperty.Synchronized)
            {
            	//注册方法和依赖属性相同
                _globalIndex = DependencyProperty.GetUniqueGlobalIndex(null, null);
 
                DependencyProperty.RegisteredPropertyList.Add();
            }
        }
 
        /// <summary>
        ///     从下面代码可以看到，设置值代码和依赖属性相同
        ///     Write the given value onto a DependencyObject instance.
        /// </summary>
        /// <param name="instance">The DependencyObject on which to set the value.</param>
        /// <param name="value">The value to set.</param>
        public void SetValue(DependencyObject instance, T value)
        {
        	//如果传入的值是空，会有异常
            if (instance != null)
            {
                EntryIndex entryIndex = instance.LookupEntry(_globalIndex);
 
                //设置的值如果和默认的相同，那么就直接跳过
                // Set the value if it's not the default, otherwise remove the value.
                if (!object.ReferenceEquals(value, _defaultValue))
                {
                	//下面的代码进行设置值
                    instance.SetEffectiveValue(entryIndex, null /* dp */, _globalIndex, null /* metadata */, value, BaseValueSourceInternal.Local);
                    _hasBeenSet = true;
                }
                else
                {
                    instance.UnsetEffectiveValue(entryIndex, null /* dp */, null /* metadata */);
                }
            }
            else
            {
                throw new ArgumentNullException("instance");
            }
        }
 
        /// <summary>
        ///     如果没有设置值，就从默认获取，或者上一级，方法和依赖属性相同
        ///     Read the value of this field on a DependencyObject instance.
        /// </summary>
        /// <param name="instance">The DependencyObject from which to get the value.</param>
        /// <returns></returns>
        public T GetValue(DependencyObject instance)
        {
            if (instance != null)
            {
                if (_hasBeenSet)
                {
                    EntryIndex entryIndex = instance.LookupEntry(_globalIndex);
 
                    if (entryIndex.Found)
                    {
                        object value = instance.EffectiveValues[entryIndex.Index].LocalValue;
 
                        if (value != DependencyProperty.UnsetValue)
                        {
                            return (T)value;
                        }
                    }
                    return _defaultValue;
                }
                else
                {
                    return _defaultValue;
                }
            }
            else
            {
                throw new ArgumentNullException("instance");
            }
        }
 
 
        /// <summary>
        ///     Clear this field from the given DependencyObject instance.
        /// </summary>
        /// <param name="instance"></param>
        public void ClearValue(DependencyObject instance)
        {
            if (instance != null)
            {
                EntryIndex entryIndex = instance.LookupEntry(_globalIndex);
 
                instance.UnsetEffectiveValue(entryIndex, null /* dp */, null /* metadata */);
            }
            else
            {
                throw new ArgumentNullException("instance");
            }
        }
 
        internal int GlobalIndex
        {
            get
            {
                return _globalIndex;
            }
        }
 
        #region Private Fields
 
        private T _defaultValue;
        private int _globalIndex;
        private bool _hasBeenSet;
 
        #endregion
    }