32/64-bit platform considerations
While iOS and macOS have historically supported both 32 and 64-bit apps, Apple has gradually deprecated 32-bit support.
As of iOS 11, 32-bit apps will no longer launch, and all submissions to the App Store must support 64-bit.
Starting in January 2018, new apps submitted to the Mac App Store must support 64-bit, and existing apps must be updated by June 2018.
Xamarin's Classic API (XamMac.dll and monotouch.dll) supported only
32-bit applications. However, new Xamarin.iOS and Xamarin.Mac applications
use the Unified API
(Xamarin.iOS and Xamarin.Mac) by default, and can therefore target both
32 and 64-bit, as necessary.
iOS
Enabling 64-bit builds of Xamarin.iOS apps
Warning
This section is included for historic reasons, and to help move older Xamarin.iOS projects to the Unified API and support 64-bit. All new Xamarin.iOS projects will use the Unified API and target 64-bit by default.
For Xamarin.iOS mobile applications that have been converted to the Unified API, developers must manually update the build settings to target 64-bit:
In the Solution Pad, double-click the app's project to open the Project Options window.
Select iOS Build.
For the iPhone Simulator, in the Supported architectures dropdown, select either x86_64 or i386 + x86_64:
For physical devices, select one of the available ARM64 combinations:
Click OK.
Perform a clean build.
ARMv7s is supported only by the A6 processor included in the iPhone 5 (or greater). ARMv7 code is faster and smaller than the ARMv6, only works with the iPhone 3GS and later, and is required by Apple when targeting the iPad or a minimum iOS version of 5.0. ARMv6 works on all devices but is no longer supported by the compiler shipped with Xcode 4.5 and later.
ARM64 is required to support iOS 8 on iPhone 6 or other 64-bit devices and will be required by Apple when submitting new or updating exiting applications in the iTunes App Store.
For a comprehensive look at the capabilities of various iOS devices, check out Apple's Device Compatibility document.
64-bit and binary size increases
During Apple's transition from 32-bit to 64-bit, iOS apps will need to run on both 32-bit and 64-bit hardware. Because of this, Xamarin's Unified API allows developers to target both.
Targeting both 32-bit and 64-bit architectures will significantly increase the size of an application. However, doing so will allow newer devices to run optimized code while still supporting older devices.
Important
If you receive the following message when submitting an iOS application to the iTunes App Store, "WARNING ITMS-9000: Missing 64-bit support. Starting February 1, 2015, new iOS apps uploaded to the App Store must include 64-bit support and be built with the iOS 8 SDK, included in Xcode 6 or later. To enable 64-bit in your project, we recommend using the default Xcode build setting of “Standard architectures” to build a single binary with both 32-bit and 64-bit code." You need to switch the supported architectures to one of the available ARM64 combination (as shown above), recompile and resubmit.
Mac
Important
Starting in January 2018, all new Mac apps submitted to the Mac App Store must support 64-bit. Existing Mac App Store apps and their updates must support 64-bit starting in June 2018. See Apple's announcment and a guide that describes how to update your Xamarin.Mac apps to 64-bit.
Most modern Mac computers support both 32-bit and 64-bit applications. MacOS 10.6 (Snow Leopard) was the last operating system to run on 32-bit systems. Most Macs released since 2010 support both systems.
Unlike iOS, many of the new frameworks introduced in recent versions of macOS are only supported in 64-bit mode (CloudKit, EventKit, GameController, LocalAuthentication, MediaLibrary, MultipeerConnectivity, NotificationCenter, GLKit, SpriteKit, Social, and MapKit, among others).
The Unified API allow developers to choose what kind of applications they want to produce: 32-bit or 64-bit.
32-bit applications will run on both 32-bit and 64-bit Mac computers, have an address space limited to 32 bits, and require that all libraries are 32 bits.
You will typically use this mode if you have 32-bit dependencies that do not run in 64-bit mode, if you want to have a smaller download, or if there are no performance benefits in moving to 64-bit.
This mode is limiting as you wont be able to use many frameworks available in macOS Mavericks and macOS Yosemite.
64-bit applications will only run on 64-bit Mac devices.
For Mac, this is the preferred mode of operation as most Macs in use today support 64-bit mode, and you have access to the complete set of frameworks provided by Apple.
Enabling 64-bit builds of Xamarin.Mac apps
For information about building a 64-bit app using Xamarin.Mac, please see the Updating Xamarin.Mac Unified applications to 64-bit guide.