Type Registrar

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This document describes the type registration system used by Xamarin.iOS.

Registration of managed classes and methods

During startup, Xamarin.iOS will register:

  • Classes with a [Register] attribute as Objective-C classes.
  • Classes with a [Category] attribute as Objective-C categories.
  • Interfaces with a [Protocol] attribute as Objective-C protocols.

and in each cases members with an [Export] attribute are exported to the Objective-C. This allows managed classes to be created and managed methods to be called from Objective-C and is the way that methods and properties are linked between the C# world and the Objective-C one.

One very simple example is the AppDelegate class which every application has. You will recall that the managed Main method has a line like this one:

UIApplication.Main (args, null, "AppDelegate");

This tells the Objective-C runtime to create the type called "AppDelegate" as the application's delegate class. For the Objective-C runtime to know how to create an instance of the "AppDelegate" class written in C#, this class has to be registered.

Xamarin.iOS runtime will take care of the registration for you, and internally, this registration can be done entirely at runtime (dynamic registration) or it can be done at compilation time (static registration). The dynamic approach involves using reflection at startup to find all the classes and methods to register and pass those to the Objective-C runtime. The static approach inspects the assemblies used by the application at compile time. It determines the classes and methods to register with Objective-C and generates a map which is embedded into your binary. Then, at startup, we register the map with the Objective-C runtime.


Starting with Xamarin.iOS 8.10 it will be possible to create Objective-C categories using C# syntax.

This is done using the [Category] attribute, specifying the type to extend as an argument to the attribute. The following example will for instance extend NSString:

[Category (typeof (NSString))]

Each category method is using the normal mechanism for exporting methods to Objective-C using the [Export] attribute:

[Export ("today")]
public static string Today ()
    return "Today";

All managed extension methods must be static, but it’s possible to create Objective-C instance methods using the standard syntax for extension methods in C#:

[Export ("toUpper")]
public static string ToUpper (this NSString self)
    return self.ToString ().ToUpper ();

and the first argument to the extension method will be the instance on which the method was invoked.

Complete example:

[Category (typeof (NSString))]
public static class MyStringCategory
    [Export ("toUpper")]
    static string ToUpper (this NSString self)
        return self.ToString ().ToUpper ();

This example will add a native toUpper instance method to the NSString class, which can be invoked from Objective-C.

[Category (typeof (UIViewController))]
public static class MyViewControllerCategory
    [Export ("shouldAutoRotate")]
    static bool GlobalRotate ()
        return true;


Starting with Xamarin.iOS 8.10 interfaces with the [Protocol] attribute will be exported to Objective-C as protocols.


[Protocol ("MyProtocol")]
interface IMyProtocol
    [Export ("method")]
    void Method ();

class MyClass : IMyProtocol
    void Method ()

This will be exported to Objective-C as a protocol (MyProtocol), and a class (MyClass) which implements the protocol.

Dynamic registration

is used for simulator builds, as it accelerates the build/debug cycle. This is the result of eliminating the steps that generates the class mapping and the compilation of this map table into your application launcher every time you start your application, and instead a general purpose launcher is used every time. Your desktop computer has plenty of power to perform the runtime scanning of classes quickly, so performance is never an issue.

Static registration

is intended for device builds as mobile devices are slower than desktop computers, and performing runtime scanning is slow. Since device builds will always need to create a new binary, the build/debug cycle is not affected by the creation of the registration mapping.

New Registration System

Starting with the stable 6.2.6 version and the beta 6.3.4 version we've added a new static registrar. In the 7.2.1 version we made the new registrar the default.

This new registration system offers the following new features:

  • Compile time detection of programmer errors:

    • Two classes being registered with the same name.
    • More than one method exported to respond to the same selector
  • Can remove unused native code

    • The new registration system will add strong references to code used in static libraries, allowing the native linker to strip out unused native code from the resulting binary. On Xamarin's sample bindings, most applications become at least 300k smaller.
  • Support for generic subclasses of NSObject. See NSObject Generics for more information. Additionally the new registration system will catch unsupported generic constructs which would previously have caused random behavior at runtime.

These are some examples of the errors caught by the new registar:

Exporting the same selector more than once in the same class.

class MyDemo : NSObject {
    [Export ("foo:")]
    void Foo (NSString str);
    [Export ("foo:")]
    void Foo (string str)

Exporting more than one managed class with the same Objective-C name.

[Register ("Class")]
class MyClass : NSObject {}

[Register ("Class")]
class YourClass : NSObject {}

Exporting generic methods.

class MyDemo : NSObject {
    [Export ("foo")]
    void Foo<T> () {}

Some things to keep in mind about the new registrar: * Some third party libraries need to be updated to work with the new registration system, see the section required modifications below for more details. * A short term downside is also that Clang must be used if the Accounts framework is used (this is because Apple's accounts.h header can only be compiled by Clang). Add --compiler:clang to the additional mtouch arguments to use Clang if your're using Xcode 4.6 or earlier (Xamarin.iOS will automatically select Clang in Xcode 5.0 or later.)

<li>If Xcode 4.6 (or earlier) is used, GCC/G++ must be
selected if exported type names contain non-ascii characters
(this is because the version of Clang shipped with Xcode 4.6
does not support non-ascii characters inside identifiers in
Objective-C code). Add <code>--compiler:gcc</code> to the
additional mtouch arguments to use GCC.

Selecting a registrar

You can select a different registrar by adding one of the following options to the additional mtouch arguments in the project's iOS Build options:

  • --registrar:static : default for device builds
  • --registrar:dynamic : default for simulator builds
  • --registrar:legacystatic : default for device builds until Xamarin.iOS 7.2.1
  • --registrar:legacydynamic : default for simulator builds until Xamarin.iOS 7.2.1

Shortcomings in the Old Registration System

The old registration system has the following drawbacks:

  • There was no (native) static reference to Objective-C classes and methods in third-party native libraries, which meant that we couldn't ask the native linker to remove third-party native code that wasn't actually used (because everything would be removed). This is the reason for the "-force_load libNative.a" that every third-party binding had to do (or the equivalent ForceLoad=true in the [LinkWith] attribute).
  • You could export two managed types with the same Objective-C name, with no warning. A rare scenario was to end up with two AppDelegate classes (in different namespaces). At runtime it would be completely random which one was picked (in fact it varied between runs of an app that wasn't even rebuilt - which made for a very puzzling and frustrating debugging experience).
  • You could export two methods with the same Objective-C signature. Yet again which one would be called from Objective-C was random (but this problem wasn't as common as the previous one, mostly because the only way to actually experience this bug was to override the unlucky managed method).
  • The set of methods that was exported was slightly different between dynamic and static builds.
  • It does not work properly when exporting generic classes (which exact generic implementation executed at runtime would be random, effectively resulting in undetermined behavior).

New Registrar: Required Changes to Bindings

Existing Objective-C bindings might need to be updated to work with the new registar.

Here is a list of changes that need to be done.

Protocols must have the [Protocol] attribute

Protocols implementations must now have the [Protocol] attribute applied to them. If you do not do this, you will a native linker error like this one:

Undefined symbols for architecture i386: "_OBJC_CLASS_$_ProtocolName", referenced from: ...

Selectors must have a valid number of parameters

All selectors must indicate number of parameters correctly. Previously, these errors were ignored and could cause runtime problems.

In short, the number of colons must match the number of parameters.

For example:

  • No parameters: `foo'
  • One parameter: `foo:'
  • Two parameters: `foo:parameterName2:'

The following are incorrect uses:

// Invalid: export takes no arguments, but function expects one
[Export ("apply")]
void Apply (NSObject target);

// Invalid: exported as taking an argument, but the managed version does not have one:
[Export ("display:")]
void Display ();

Use IsVariadic parameter in Export

Variadic functions must say so in their Export attribute (this is because the selector is wrong about the number of arguments, i.e. point 2. from above is violated):

[Export ("variadicMethod:", IsVariadic = true)]
void VariadicMethod (NSObject first, IntPtr subsequent);

Must link to existing symbols

It is impossible to bind classes that don't exist in the native library.

You will get a native linker error if you try to bind non-existing classes.

This usually happens when a binding has existed for some time, and the native code has been modified during that time so that a particular native class has either been removed or renamed, while the binding has not been updated.

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