This bug report is empty and thus being closed. If there is a bug, please reopen with relevant information and a reproduction.
When in custom MarkupExtension you using IProvideValueTarget to get TargetProperty is always null, Because in the source code at https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Xaml/XamlServiceProvider.cs#L17 you always setted null the targetProperty parameter of XamlServiceProvider constructor.
I have push PR https://github.com/xamarin/Xamarin.Forms/pull/814
merged PR https://github.com/xamarin/Xamarin.Forms/pull/847
should be fixed in 2.3.5-pre1