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On Android, a user defined PreserveAttribute (see definition) is only honored when applied at the assembly level (see line 8) but has no effect when applied at class or member level (see line 22, 25). The impact of this is significant for a Forms developer because of two common late-bound idioms: [1] types activated late-bound from XAML (see line 17) and [2] properties accessed by BindableProperties on objects used as a BindingContext (see line 44). Without use of a PreserveAttribute these types and properties must be referenced in a much more clunky fashion (see line 16, 31).

On Android, all types declared in the shim dll are preserved (see line 41 in https://github.com/kingces95/King.Linker/blob/master/Application.Android/MainActivity.cs). The impact of this is low. An easy work around is not to include any significant logic in the shim. This is naturally the case with a Forms application where all the logic lives in the PCL. Still, this behavior is not mirrored on iOS so is confusing.

On Android, enums passed as arguments to custom attribute parameters typed as object are not preserved. (see line 13 given attribute definition : https://github.com/kingces95/King.Linker/blob/master/Library.Pcl.Target/MyClass.cs). While the impact of this is generally low (no Forms idioms pass enums to parameters typed as objects -- we encountered this because of testing infrastructure does this [see line 13 : https://github.com/xamarin/Duplo/blob/ff7a3a84a7de2433a2f415516c7d55c8bf1e7d71/Xamarin.Forms.Controls/TestCasesPages/Issue1545.xaml.cs]) I'd love to see this fixed because when building custom loaders its very useful to be able to pass enums to parameters typed as objects.

On iOS, types derived from NSObject are never shaken out by the linker (see line 20 : https://github.com/kingces95/King.Linker/blob/master/Application.iOS/Main.cs). All renders derive from NSObject and so none of them will be shaken out. This will prevent any significant savings on iOS.

Consequently, enabling the linker on a Xamarin.Forms.iOS project will have little effect on the size of 3ed party libraries which supply customer renders which will all derive from NSObject.