Windows 8.1 开发过程中遇到的小问题

最近在开发Windows 8 应用的时候碰到了一个莫名的问题，错误内容如下：（其中 **.DLL是本地创建的项目，在主项目中添加了引用，其中大部分代码是MVVM light 框架库的代码）

System.InvalidCastException”类型的异常在 **.DLL 中发生，但未在用户代码中进行处理

其他信息: Unable to cast COM object of type 'System.ComponentModel.PropertyChangedEventHandler' to class type 'System.ComponentModel.PropertyChangedEventHandler'. Instances of types that represent COM components cannot be cast to types that do not represent COM components; however they can be cast to interfaces as long as the underlying COM component supports QueryInterface calls for the IID of the interface.

前提是：在charm栏中选择分享，并选择分享到的应用；正常启动应用是不会有事的。（我很纳闷为什么，渴望大神赐教）

出错代码是这里（红色字体部分）：

[SuppressMessage("Microsoft.Design", "CA1030:UseEventsWhereAppropriate",
            Justification = "This cannot be an event")]
        protected virtual void RaisePropertyChanged(string propertyName)
        {
#if WIN8
            if (string.IsNullOrEmpty(propertyName))
            {
                throw new NotSupportedException(
                    "Raising the PropertyChanged event with an empty string or null is not supported in the Windows 8 developer preview");
            }
            else
            {
#endif
                //VerifyPropertyName(propertyName);

                var handler = PropertyChangedHandler;

                if (handler != null)
                {
                    handler(this, new PropertyChangedEventArgs(propertyName));
                }
#if WIN8
            }
#endif
        }

我以为是框架本身的问题，于是在相应的代码中，自己重新实现INotifyPropertyChanged接口，并实现此接口：

    public class LoginViewModel : ViewModelBase,INotifyPropertyChanged
    {
        event PropertyChangedEventHandler PropertyChanged;
        void NotifyPropertyChanged(string name)
        {
            if (PropertyChanged != null)
                PropertyChanged(this,new PropertyChangedEventArgs(name);
        }
    }

我差了相关帖子，看到了这个回复：

Just cast it to INotifyCollectionChanged

var collectionChanged = propertyObject as INotifyCollectionChanged;
if (collectionChanged != null)
    collectionChanged.CollectionChanged += ...

于是我修改代码为如下：

    public class LoginViewModel : ViewModelBase,INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        void NotifyPropertyChanged(string name)
        {
            var obj = this as INotifyPropertyChanged;
            if (obj != null)
                obj.PropertyChanged += (s, e) => {
                    PropertyChanged(s, new PropertyChangedEventArgs(name));
                };
        }
    }    

OK，问题解决，关键是谁能告诉我这是为什么，什么情况下会出现这个错误，为什么会在分享中出现，而正常状态下不会呢？