API Design

Perspective on the Xamarin.iOS API design

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Overview

In addition to the core Base Class Libraries that are part of Mono, Xamarin.iOS ships with bindings for various iOS APIs to allow developers to create native iOS applications with Mono.

At the core of Xamarin.iOS, there is an interop engine that bridges the C# world with the Objective-C world as, well as bindings for the iOS C-based APIs like CoreGraphics and OpenGLES.

The low-level runtime to communicate with Objective-C code is in the MonoTouch.ObjCRuntime. On top of this, bindings for Foundation, CoreFoundation and UIKit are provided.

Design Principles

These are some of our design principles for the Xamarin.iOS binding (these also apply to Xamarin.Mac, the Mono bindings for Objective-C on OS X):

  • Follow the Framework Design Guidelines
  • Allow developers to subclass Objective-C classes:
    • Derive from an existing class
    • Call base constructor to chain
    • Overriding methods should be done with C#'s override system
  • Subclass should work with C# standard constructs
  • Do not expose developers to Objective-C selectors
  • Provide a mechanism to call arbitrary Objective-C libraries
  • Make common Objective-C tasks easy, and hard Objective-C tasks possible
  • Expose Objective-C properties as C# properties
  • Expose a strongly typed API:
  • Increase type-safety
  • Minimize runtime errors
  • Get IDE intellisense on return types
  • Allows for IDE popup documentation
  • Encourage in-IDE exploration of the APIs:
  • Native C# types:
    • Example: instead of exposing the weakly-typed array like this:
      NSArray *getViews

      we expose them with strong types, like this:

      NSView [] Views { get; set; }

      This gives Xamarin Studio the ability to do auto-completion while browsing the API and also allows all of the System.Array operations to be available on the returned value and allows the return value to participate in LINQ
  • NSString becomes string
  • Turn int and uint parameters that should have been enums as C# enumerations and C# enumerations with [Flags] attributes
  • Instead of type-neutral NSArray objects expose arrays as strongly typed array.
  • Events and notifications, give users a choice between:
    • Strongly typed version is the default
    • Weakly typed version for advance use cases
  • Support the Objective-C delegate pattern:
    • C# event system
    • Expose C# delegates (lambdas, anonymous methods and System.Delegate) to Objective-C APIs as "blocks"

Assemblies

Xamarin.iOS includes a number of assemblies that constitute the Xamarin.iOS Profile. The Assemblies page has more information.

Major Namespaces

ObjCRuntime

The ObjCRuntime namespace allows developers to bridge the worlds between C# and Objective-C. This is a new binding, designed specifically for the iOS, based on the experience from Cocoa# and Gtk#.

Foundation

The Foundation namespace provides the basic data types designed to interoperate with the Objective-C Foundation framework that is part of the iOS and it is the base for object oriented programming in Objective-C.

Xamarin.iOS mirrors in C# the hierarchy of classes from Objective-C. For example, the Objective-C base class NSObject is usable from C# via Foundation.NSObject.

Although this namespace provides bindings for the underlying Objective-C Foundation types, in a few cases we have mapped the underlying types to .NET types. For example:

  • Instead of dealing with NSString and NSArray, the runtime exposes these as C# strings and strongly typed arrays throughout the API.

  • Various helper APIs are exposed here to allow developers to bind third party Objective-C APIs, other iOS APIs or APIs that are not currently bound by Xamarin.iOS.

For more details on binding APIs, see the Xamarin.iOS Binding Generator section.

NSObject

The NSObject type is the foundation for all the Objective-C bindings. Xamarin.iOS types mirror two classes of types from the iOS CocoaTouch APIs: the C types (typically refered to as CoreFoundation types) and the Objective-C types (these all derive from the NSObject class).

For each type that mirrors an unmanaged type, it is possible to obtain the native object through the Handle property.

While Mono will provide garbage collection for all of your objects, the Foundation.NSObject implements the System.IDisposable interface. This means that you can explicitly release the resources of any given NSObject without having to wait for the Garbage Collector to kick-in. This is important when you are using heavy NSObjects, for example, UIImages that might hold pointers to large blocks of data.

If your type needs to perform deterministic finalization, override the NSObject.Dispose(bool) method The parameter to Dispose is "bool disposing", and if set to true it means that your Dispose method is being called because the user explicitly called Dispose () on the object. If the value is false, this means that your Dispose(bool disposing) method is being called from the finalizer on the finalizer thread.

Categories

Starting with Xamarin.iOS 8.10 it is possible to create Objective-C categories from C#.

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 ("shouldAudoRotate")]
        static bool GlobalRotate ()
        {
            return true;
        }
    }

One scenario where this is useful is adding a method to an entire set of classes in your codebase, for example, this would make all UIViewController instances report that they can rotate:

[Category (typeof (UINavigationController))]
class Rotation_IOS6 {
      [Export ("shouldAutorotate:")]
      static bool ShouldAutoRotate (this UINavigationController self)
      {
          return true;
      }
}

PreserveAttribute

PreserveAttribute is a custom attribute that is used to tell mtouch – the Xamarin.iOS deployment tool – to preserve a type, or a member of a type, during the phase when the application is processed to reduce its size.

Every member that is not statically linked by the application is subject to be removed. Hence, this attribute is used to mark members that are not statically referenced, but that are still needed by your application.

For instance, if you instantiate types dynamically, you may want to preserve the default constructor of your types. If you use XML serialization, you may want to preserve the properties of your types.

You can apply this attribute on every member of a type, or on the type itself. If you want to preserve the whole type, you can use the syntax [Preserve (AllMembers = true)] on the type.

UIKit

The UIKit namespace contains a one-to-one mapping to all of the UI components that make up CocoaTouch in the form of C# classes. The API has been modified to follow the conventions used in the C# language.

C# delegates are provided for common operations. See the delegates section for more information.

OpenGLES

For OpenGLES, we distribute a modified version of the OpenTK API, an object-oriented binding to OpenGL that has been modified to use CoreGraphics data types and structures, as well as only exposing the functionality that is available on iOS.

OpenGLES 1.1 functionality is available through the ES11.GL type, documented here type.

OpenGLES 2.0 functionality is available through the ES20.GL type, documented here type.

OpenGLES 3.0 functionality is available through the ES30.GL type, documented here type.

Binding Design

Xamarin.iOS is not merely a binding to the underlying Objective-C platform. It extends the .NET type system and dispatch system to better blend C# and Objective-C.

Just as P/Invoke is a useful tool to invoke native libraries on Windows and Linux, or as IJW support can be used for COM interop on Windows, Xamarin.iOS extends the runtime to support binding C# objects to Objective-C objects.

The discussion in the next few sections is not necessary for users that are creating Xamarin.iOS applications, but will help developers understand how things are done and will assist them when creating more complicated applications.

Types

Where it made sense, C# types are exposed instead of low-level Foundation types, to the C# universe. This means that the API uses the C# "string" type instead of NSString and it uses strongly typed C# arrays instead of exposing NSArray.

In general, in the Xamarin.iOS and Xamarin.Mac design, the underlying NSArray object is not exposed. Instead, the runtime automatically converts NSArrays to strongly typed arrays of some NSObject class. So, Xamarin.iOS does not expose a weakly-typed method like GetViews to return an NSArray:

NSArray GetViews ();

Instead, the binding exposes a strongly typed return value, like this:

UIView [] GetViews ();

There are a few of methods exposed in NSArray, for the corner cases where you might want to use an NSArray directly, but their use is discouraged in the API binding.

Additionally, in the Classic API instead of exposing CGRect, CGPoint and CGSize from the CoreGraphics API, we replaced those with the System.Drawing implementations RectangleF, PointF and SizeF as they would help developers preserve existing OpenGL code that uses OpenTK. When using the new 64-bit Unified API, the CoreGraphics API should be used.

Inheritance

The Xamarin.iOS API design allows developers to extend native Objective-C types in the same way that they would extend a C# type, using the "override" keyword on a derived class, as well as chaining up to the base implementation using the "base" C# keyword.

This design allows developers to avoid dealing with Objective-C selectors as part of their development process, because the entire Objective-C system is already wrapped inside the Xamarin.iOS libraries.

Types and Interface Builder

When you create .NET classes that are instances of types created by Interface Builder, you need to provide a constructor that takes a single IntPtr parameter. This is required to bind the managed object instance with the unmanaged object. The code consists of a single line, like this:

public partial class void MyView : UIView {
   // This is the constructor that you need to add.
   public MyView (IntPtr handle) : base (handle) {}
}

Delegates

Objective-C and C# have different meanings for the word delegate in each language.

In the Objective-C world, and in the documentation that you will find online about CocoaTouch, a delegate is typically an instance of a class that will respond to a set of methods. This is very similar to a C# interface, with the difference being that the methods are not always mandatory.

These delegates play an important role in UIKit and other CocoaTouch APIs. They are used to accomplish various tasks:

  • To provide notifications to your code (Similar to event delivery in C# or Gtk+).
  • To implement models for data visualization controls.
  • To drive the behavior of a control.

The programming pattern was designed to minimize the creation of derived classes to alter behavior for a control. This solution is similar in spirit to what other GUI toolkits have done over the years: Gtk's signals, Qt slots, Winforms events, WPF/Silverlight events and so on. To avoid having hundreds of interfaces (one for each action) or requiring developers to implement too many methods they do not need, Objective-C supports optional method definitions. This is different than C# interfaces that require all methods to be implemented.

In Objective-C classes, you will see that classes that use this programming pattern expose a property, usually called delegate, which is required to implement the mandatory parts of the interface and zero, or more, of the optional parts.

In Xamarin.iOS three mutually exclusive mechanisms to bind to these delegates are offered:

  1. Via events .
  2. Strongly typed via a Delegateproperty .
  3. Loosely typed via a WeakDelegateproperty .

For example, consider the UIWebView class. This dispatches to a UIWebViewDelegate instance, which is assigned to the delegate property.

Via Events

For many types, Xamarin.iOS will automatically create an appropriate delegate which will forward the UIWebViewDelegate calls onto C# events. For UIWebView:

For example, this simple program records the start and end times when loading a web view:

DateTime startTime, endTime;
var web = new UIWebView (new CGRect (0, 0, 200, 200));
web.LoadStarted += (o, e) => startTime = DateTime.Now;
web.LoadFinished += (o, e) => endTime = DateTime.Now;

Via Properties

Events are useful when there might be more than one subscriber to the event. Also, events are limited to cases where there is no return value from the code.

For cases where the code is expected to return a value, we opted instead for properties. This means that only one method can be set at a given time in an object.

For example, you can use this mechanism to dismiss the keyboard on the screen on the handler for a UITextField:

void SetupTextField (UITextField tf)
{
    tf.ShouldReturn = delegate (textfield) {
        textfield.ResignFirstResponder ();
        return true;
    }
}

The UITextField's ShouldReturn property in this case takes as an argument a delegate that returns a bool value and determines whether the TextField should do something with the Return button being pressed. In our method, we return true to the caller, but we also remove the keyboard from the screen (this happens when the textfield calls ResignFirstResponder).

Strongly Typed via a Delegate Property

If you would prefer not to use events, you can provide your own UIWebViewDelegate subclass and assign it to the UIWebView.Delegate property. Once UIWebView.Delegate has been assigned, the UIWebView event dispatch mechanism will no longer function, and the UIWebViewDelegate methods will be invoked when the corresponding events occur.

For example, this simple type records the time it takes to load a web view:

class Notifier : UIWebViewDelegate  {
    DateTime startTime, endTime;

    public override LoadStarted (UIWebView webview)
    {
        startTime = DateTime.Now;
    }

    public override LoadingFinished (UIWebView webView)
    {
        endTime= DateTime.Now;
    }
}

The above is used in code like this:

var web = new UIWebView (new CGRect (0, 0, 200, 200));
     web.Delegate = new Notifier ();

The above will create a UIWebViewer and it will instruct it to send messages to an instance of Notifier, a class that we created to respond to messages.

This pattern is also used to control behavior for certain controls, for example in the UIWebView case, the UIWebView.ShouldStartLoad property allows the UIWebView instance to control whether the UIWebView will load a page or not.

The pattern is also used to provide the data on demand for a few controls. For example, the UITableView control is a powerful table-rendering control – and both the look and the contents are driven by an instance of a UITableViewDataSource

Loosely Typed via the WeakDelegate Property

In addition to the strongly typed property, there is also a weak typed delegate that allows the developer to bind things differently if desired. Everywhere a strongly typed Delegate property is exposed in Xamarin.iOS's binding, a corresponding WeakDelegate property is also exposed.

When using the WeakDelegate, you are responsible for properly decorating your class using the Export attribute to specify the selector. For example:

class Notifier : NSObject  {
    DateTime startTime, endTime;

    [Export ("webViewDidStartLoad:")]
    public void LoadStarted (UIWebView webview)
    {
        startTime = DateTime.Now;
    }

    [Export ("webViewDidFinishLoad:")]
    public void LoadingFinished (UIWebView webView)
    {
        endTime= DateTime.Now;
    }
}
[...]
     var web = new UIWebView (new CGRect (0, 0, 200, 200));
     web.WeakDelegate = new Notifier ();

Note that once the WeakDelegate property has been assigned, the Delegate property will not be used. Additionally, if you implement the method in an inherited base class that you wish to [Export], you must make it a public method.

Mapping of the Objective-C delegate pattern to C

When you see Objective-C samples that look like this:

foo.delegate = [[SomethingDelegate] alloc] init]

This instructs the language to create and construct an instance of the class "SomethingDelegate" and assign the value to the delegate property on the foo variable. This mechanism is supported by Xamarin.iOS and C# the syntax is:

foo.Delegate = new SomethingDelegate ();

In Xamarin.iOS we have provided strongly-typed classes that map to the Objective-C delegate classes. To use them, you will be subclassing and overriding the methods defined by Xamarin.iOS's implementation. For more information on how they work, see the section "Models" below.

Mapping Delegates to C

UIKit in general uses Objective-C delegates in two forms.

The first form provides an interface to a component's model. For example, as a mechanism to provide data on demand for a view, such as the data storage facility for a List view. In these cases, you should always create an instance of the proper class and assign the variable.

In the following example, we provide the UIPickerView with an implementation for a model that uses strings:

public class SampleTitleModel : UIPickerViewTitleModel {

    public override string TitleForRow (UIPickerView picker, nint row, nint component)
    {
        return String.Format ("At {0} {1}", row, component);
    }
}

[...]

pickerView.Model = new MyPickerModel ();

The second form is to provide notification for events. In those cases, although we still expose the API in the form outlined above, we also provide C# events, which should be simpler to use for quick operations and integrated with anonymous delegates and lambda expressions in C#.

For example, you can subscribe to UIAccelerometer events:

UIAccelerometer.SharedAccelerometer.Acceleration += (sender, args) => {
   UIAcceleration acc = args.Acceleration;
   Console.WriteLine ("Time={0} at {1},{2},{3}", acc.Time, acc.X, acc.Y, acc.Z);
}

The two options are available where they make sense, but as a programmer you must pick one or the other. If you create your own instance of a strongly typed responder/delegate and assign it, the C# events will not be functional. If you use the C# events, the methods in your responder/delegate class will never be called.

The previous example that used UIWebView can be written using C# 3.0 lambdas like this:

var web = new UIWebView (new CGRect (0, 0, 200, 200));
web.LoadStarted += () => { startTime = DateTime.Now; }
web.LoadFinished += () => { endTime = DateTime.Now; }

Responding to Events

In Objective-C code, sometimes event handlers for multiple controls and providers of information for multiple controls, will be hosted in the same class. This is possible because classes respond to messages, and as long as classes respond to messages, it is possible to link objects together.

As previously detailed, Xamarin.iOS supports both the C# event-based programming model, and the Objective-C delegate pattern, where you can create a new class that implements the delegate and overrides the desired methods.

It is also possible to support Objective-C's pattern where responders for multiple different operations are all hosted in the same instance of a class. To do this though, you will have to use low-level features of the Xamarin.iOS binding.

For example, if you wanted your class to respond to both the UITextFieldDelegate.textFieldShouldClear: message and the UIWebViewDelegate.webViewDidStartLoad: in the same instance of a class, you would have to use the [Export] attribute declaration:

public class MyCallbacks : NSObject {
    [Export ("textFieldShouldClear:"]
    public bool should_we_clear (UITextField tf)
    {
        return true;
    }

    [Export ("webViewDidStartLoad:")]
    public void OnWebViewStart (UIWebView view)
    {
        Console.WriteLine ("Loading started");
    }
}

The C# names for the methods are not important; all that matters are the strings passed to the [Export] attribute.

When using this style of programming, ensure that the C# parameters match the actual types that the runtime engine will pass.

Models

In UIKit storage facilities, or in responders that are implemented using helper classes, these are usually referred in the Objective-C code as delegates, and they are implemented as protocols.

Objective-C protocols are like interfaces, but they support optional methods – that is, not all of the methods need to be implemented for the protocol to work.

There are two ways of implementing a model. You can either implement it manually or use the existing strongly typed definitions.

The manual mechanism is necessary when you try to implement a class that has not been bound by Xamarin.iOS. It is very easy to do:

  • Flag your class for registration with the runtime
  • Apply the [Export] attribute with the actual selector name on each method you want to override
  • Instantiate the class, and pass it.

For example, the following implement only one of the optional methods in the UIApplicationDelegate protocol definition:

public class MyAppController : NSObject {
        [Export ("applicationDidFinishLaunching:")]
        public void FinishedLaunching (UIApplication app)
        {
                SetupWindow ();
        }
}

The Objective-C selector name ("applicationDidFinishLaunching:") is declared with the Export attribute and the class is registered with the [Register] attribute.

Xamarin.iOS provides strongly typed declarations, ready to use, that do not require manual binding. To support this programming model, the Xamarin.iOS runtime supports the [Model] attribute on a class declaration. This informs the runtime that it should not wire up all the methods in the class, unless the methods are is explicitly implemented.

This means that in UIKit, the classes that represent a protocol with optional methods are written like this:

[Model]
public class SomeViewModel : NSObject {
    [Export ("someMethod:")]
    public virtual int SomeMethod (TheView view) {
       throw new ModelNotImplementedException ();
    }
    ...
}

When you want to implement a model that only implements some of the methods, all you have to do is to override the methods that you are interested in, and ignore the other methods. The runtime will only hook up the overwritten methods, not the original methods to the Objective-C world.

The equivalent to the previous manual sample is:

public class AppController : UIApplicationDelegate {
    public override void FinishedLaunching (UIApplication uia)
    {
     ...
    }
}

The advantages are that there is no need to dig into the Objective-C header files to find the selector, the types of the arguments, or the mapping to C#, and that you get intellisense from Xamarin Studio, along with strong types

XIB Outlets and C

This section explains the IDE integration with outlets when using XIB files. When using the Xamarin Designer for iOS, this is all replaced by entering a name under Identity > Name in the Properties section of your IDE, as shown below:

For more information on the iOS Designer, please review the Introduction to the iOS Designer document.

This is a low-level description of how Outlets integrate with C# and is provided for advanced users of Xamarin.iOS. When using Xamarin Studio the mapping is done automatically behind the scenes using generated code on the flight for you.

When you design your user interface with Interface Builder, you will only be designing the look of the application and will establish some default connections. If you want to programatically fetch information, alter the behavior of a control at runtime or modify the control at runtime, it is necessary to bind some of the controls to your managed code.

This is done in a few steps:

  1. Add the outlet declaration to your File's owner.
  2. Connect your control to the File's owner.
  3. Store the UI plus the connections into your XIB/NIB file.
  4. Load the NIB file at runtime.
  5. Access the outlet variable.

The steps (1) through (3) are covered in Apple's documentation for building interfaces with Interface Builder.

When using Xamarin.iOS, your application will need to create a class that derives from UIViewController. It is implemented it like this:

public class MyViewController : UIViewController {
    public MyViewController (string nibName, NSBundle bundle) : base (nibName, bundle)
    {
        // You can have as many arguments as you want, but you need to call
        // the base constructor with the provided nibName and bundle.
    }
}

Then to load your ViewController from a NIB file, you do this:

var controller = new MyViewController ("HelloWorld", NSBundle.MainBundle, this);

This loads the user interface from the NIB. Now, to access the outlets, it is necessary to inform the runtime that we want to access them. To do this, the UIViewController subclass needs to declare the properties and annotate them with the [Connect] attribute. Like this:

[Connect]
UITextField UserName {
    get {
        return (UITextField) GetNativeField ("UserName");
    }
    set {
        SetNativeField ("UserName", value);
    }
}

The property implementation is the one that actually fetches and stores the value for the actual native type.

You do not need to worry about this when using Xamarin Studio and InterfaceBuilder. Xamarin Studio automatically mirrors all the declared outlets with code in a partial class that is compiled as part of your project.

Selectors

A core concept of Objective-C programming is selectors. You will often come across APIs that require you to pass a selector, or expects your code to respond to a selector.

Creating new selectors in C# is very easy – you just create a new instance of the ObjCRuntime.Selector class and use the result in any place in the API that requires it. For example:

var selector_add = new Selector ("add:plus:");

For a C# method respond to a selector call, it must inherit from the NSObject type and the C# method must be decorated with the selector name using the [Export] attribute. For example:

public class MyMath : NSObject {
    [Export ("add:plus:")]
    int Add (int first, int second)
    {
         return first + second;
    }
}

Note that selector names must match exactly, including all intermediate and trailing colons (":"), if present.

NSObject Constructrors

Most classes in Xamarin.iOS that derive from NSObject will expose constructors specific to the functionality of the object, but they will also expose various constructors that are not immediately obvious.

The constructors are used as follows:

public Foo (IntPtr handle)

This constructor is used to instantiate your class when the runtime needs to map your class to an unmanaged class. This happens when you load a XIB/NIB file. At this point, the Objective-C runtime will have created an object in the unmanaged world, and this constructor will be called to initialize the managed side.

Typically, all you need to do is call the base constructor with the handle parameter, and in the body, do any initialization that is necessary.

public Foo ()

This is the default constructor for a class, and in Xamarin.iOS provided classes, this initializes the Foundation.NSObject class and all of the classes in between, and at the end, chains this to the Objective-C init method on the class.

public Foo (NSObjectFlag x)

This constructor is used to initialize the instance, but prevent the code from calling the Objective-C "init" method at the end. You typically use this when you already have registered for initialization (when you use [Export] on your constructor) or when you have already done your initialization through another mean.

public Foo (NSCoder coder)

This constructor is provided for the cases where the object is being initialized from an NSCoding instance. For more information, see Apple's Archives and Serialization Programming Guide.

Exceptions

The Xamarin.iOS API design does not raise Objective-C exceptions as C# exceptions. The design enforces that no garbage be sent to the Objective-C world in the first place and that any exceptions that must be produced are produced by the binding itself before invalid data is ever passed to the Objective-C world.

Notifications

In both iOS and OS X, developers can subscribe to notifications that are broadcast by the underlying platform. This is done by using the NSNotificationCenter.DefaultCenter.AddObserver method. The AddObserver method takes two parameters; one is the notification that you want to subscribe to; the other is the method to be invoked when the notification is raised.

In both Xamarin.iOS and Xamarin.Mac, the keys for the various notifications are hosted on the class that triggers the notifications. For example, the notifications raised by the UIMenuController are hosted as static NSString properties in the UIMenuController classes that end with the name "Notification".

Memory Management

Xamarin.iOS has a garbage collector that will take care of releasing resources for you when they are no longer in use. In addition to the garbage collector, all objects that derive from NSObject implement the System.IDisposable interface.

NSObject and IDisposable

Exposing the IDisposable interface is a convenient way of assisting developers in releasing objects that might encapsulate large blocks of memory (for example, a UIImage might look like just an innocent pointer, but could be pointing to a 2 megabyte image) and other important and finite resources (like a video decoding buffer).

NSObject implements the IDisposable interface and also the .NET Dispose pattern. This allows developers that subclass NSObject to override the Dispose behavior and release their own resources on demand. For example, consider this view controller that keeps around a bunch of images:

class MenuViewController : UIViewController {
    UIImage breakfast, lunch, dinner;
    [...]
    public override void Dispose (bool disposing)
    {
        if (disposing){
             if (breakfast != null) breakfast.Dispose (); breakfast = null;
             if (lunch != null) lunch.Dispose (); lunch = null;
             if (dinner != null) dinner.Dispose (); dinner = null;
        }
        base.Dispose (disposing)
    }
}

When a managed object is disposed, it is no longer useful. You might still have a reference to the objects, but the object is for all intents and purposes invalid at this point. Some .NET APIs ensure this by throwing an ObjectDisposedException if you try to access any methods on a disposed object, for example:

var image = UIImage.FromFile ("demo.png");
image.Dispose ();
image.XXX = false;  // this at this point is an invalid operation

Even if you can still access the variable "image", it is really an invalid reference and no longer points to the Objective-C object that held the image.

But disposing an object in C# does not mean that the object will necessarily be destroyed. All you do is release the reference that C# had to the object. It is possible that the Cocoa environment might have kept a reference around for its own use. For example, if you set a UIImageView's Image property to an image, and then you dispose the image, the underlying UIImageView had taken its own reference and will keep a reference to this object until it is finished using it.

When to call Dispose

You should call Dispose when you need Mono in getting rid of your object. A possible use case is when Mono has no knowledge that your NSObject is actually holding a reference to an important resource like memory, or an information pool. In those cases, you should call Dispose to immediately release the reference to the memory, instead of waiting for Mono to perform a garbage collection cycle.

Internally, when Mono creates NSString references from C# strings, it will dispose them immediately to reduce the amount of work that the garbage collector has to do. The fewer objects around to deal with, the faster the GC will run.

When to Keep References to Objects

One side-effect that automatic memory management has is that the GC will get rid of unused objects as long as there are no references to them. This sometimes can have surprising side effects, for example, if you create a local variable to hold your toplevel view controller, or your toplevel window, and then having those vanish behind your back.

If you do not keep a reference in your static or instance variables to your objects, Mono will happily call the Dispose() method on them, and they will release the reference to the object. Since this might be the only outstanding reference, the Objective-C runtime will destroy the object for you.

Xamarin Workbook

If it's not already installed, install the Xamarin Workbooks app first. The workbook file should download automatically, but if it doesn't, just click to start the workbook download manually.