Xamarin.iOS MPMediaItem.AssetURL始终为null(Xamarin.iOS MPMediaItem.AssetURL is always null)

在Xamarin.iOS中，MPMediaItem.AssetURL始终为任何项返回null。 我研究了一下，然后尝试使用Objective-C句柄。 所以我用过

NSUrl url = new NSUrl(item.ValueForProperty(MPMediaItem.AssetURLProperty).Handle);     
 
 上面的代码尝试获取AssetURL属性的objective-c句柄，然后将其转换为Managed表示。 有用！ 
 
 所以，我决定研究单声道实现。 Mono已将其实施为 - 
 
return this.ValueForProperty (MPMediaItem.AssetURLProperty) as NSUrl;
 
 我手动尝试了这个，它实际上返回null。 所以，这意味着Xamarin的实现是错误的。 然而，有一件事让我感到困惑。 在我运行正确的代码（第一个代码段）之后，第二个剪辑不会返回null，而是正确的值。 我的第一个片段可以改变第二个片段的行为有什么可能？ 
In Xamarin.iOS, MPMediaItem.AssetURL always returns null for any item. I researched a bit, then tried to use the Objective-C handle. So I used 
 
NSUrl url = new NSUrl(item.ValueForProperty(MPMediaItem.AssetURLProperty).Handle);     
 
Above code tries to get the objective-c handle of AssetURL property and then converts that to Managed representation. It works!
 
So, I decided to look into mono implementation. Mono has implemented it as -
 
return this.ValueForProperty (MPMediaItem.AssetURLProperty) as NSUrl;
 
I tried this manually and it is actually returning null. So, that means Xamarin implementation is wrong. However, one thing flummoxed me. After I ran my correct code (first snippet), the second snipped doesn't return null, but correct value. What possibiliy could my first snippet have done to alter the behaviour of second snippet ? 
                
最满意答案
                
                    
                        
 听起来像从ValueForProperty返回的句柄不是NSUrl ，而as cast使其为null （例如，它可能是内部/私有类型）。 
 
 
 
 我的第一个片段可能会改变第二个片段的行为？ 
 
 
 简单。 您使用此特定句柄值创建了NSUrl 。 如果您尝试再次获取它，将返回相同的实例（除非它被垃圾收集），因此我们有一个托管实例代表一个本机实例。 
It sounds like the handle that is returned from ValueForProperty was not an NSUrl and the as cast makes it null (e.g. it could be an internal/private type).
 
 
 
What possibly could my first snippet have done to alter the behaviour of second snippet ?
 
 
Simple. You created an NSUrl with this specific handle value. If you try to get it again the same instance will be returned (unless it was garbage collected) so we have a single managed instance representing a single native instance.