About the Android Support Libraries
Google has created support libraries to make new features available to older versions of Android. In general, Support Libraries are given a version number in their name, which is the lowest Android API Level they are compatible with (eg: Support-v13 can only be used on API Level 4 and higher).
Two of the support libraries:
Support-v13 can not be used together in the same app, that is, they are mutually exclusive. This is because
Support-v13 actually contains all of the types and implementation of
Support-v4. If you try and reference both in the same project you will encounter duplicate type errors.
Problems with Referencing
Since Support-v4 has become so popular, a lot of 3rd party libraries now depend on it. They could have chosen to depend on Support-v13 instead, but it's more common to depend on v4 since that gives any apps using these 3rd party libraries the option of supporting API levels all the way down to 4.
If a Xamarin 3rd party library references the
Xamarin.Android.Support.v4.dll binding to
Support-v4, any app that uses this library must also reference
Xamarin.Android.Support.v4.dll. This becomes a problem when the same app also wants to use some of the functionality from the
Xamarin.Android.Support.v13.dll binding to
Support-v13. If you reference both bindings, you will encounter duplicate type errors.
Type-Forwarded v4 Binding Assembly
To get around this problem, we have created a special
Xamarin.Android.Support.v4.dll assembly which has no implementation, but simply
[assembly: TypeForwardedTo (..)] attributes which forward all of the
Support-v4 types to the implementation within the
This means a developer can reference this type-forwarded assembly in their app which will satisfy the reference to
Xamarin.Android.Support.v4.dll by any 3rd party libraries, while still allowing
Xamarin.Android.Support.v13.dll to be used in the app.
While a developer could manually add the correct references necessary, we are able to use NuGet to help choose the right assembly (either the normal v4 binding or the type-forwarded v4 assembly) when the NuGet package is installed.
Xamarin.Android.Support.v4 NuGet package now contains the following logic:
If your app is targeting API Level 13 (Gingerbread 3.2) or higher:
Xamarin.Android.Support.v13NuGet will automatically be added as a dependency
- The type-forwarded
Xamarin.Android.Support.v4.dllwill be referenced in the project
If your app is targeting anything lower than API Level 13, you will get the normal
Xamarin.Android.Support.v4.dll binding referenced in your project.
Do I have to use Support-v13?
If your app is targeting API Level 13 or higher and you choose to use the
Xamarin Android Support-v4 NuGet package, then the
Xamarin Android Support v13 NuGet package is a required dependency.
We feel the very minor increase in app size (the two .jar files differ by 17kb) is well worth the compatibility and fewer headaches it results in.
If you are adamant about using
Support-v4 in an app that targets API Level 13 or higher, you can always manually download the
.nupkg, extract it, and reference the assembly.