Objective-C → C# Binding Definition Skill

This skill encodes the complete knowledge needed to translate Objective-C header declarations into Xamarin.iOS / .NET for iOS C# binding API definitions (the ApiDefinition.cs contract format).

The binding contract uses C# interfaces (not classes) decorated with attributes. The binding generator (btouch / generator) turns these interfaces into actual classes at build time.

File Structure

A binding project has two key files:

Rule: ApiDefinition.cs contains ONLY interfaces and delegate declarations. All enums, structs, and other value types go in StructsAndEnums.cs.

Class Binding (@interface)

Basic Class

@interface MyClass : NSObject
@end

[BaseType (typeof (NSObject))]
interface MyClass
{
}

Rules:

• •Every interface MUST have [BaseType] specifying the ObjC superclass
• •The C# interface name matches the ObjC class name
• •If the ObjC name doesn't match .NET conventions, use [BaseType (typeof (NSObject), Name = "ObjCName")]

Class with Protocol Conformance

@interface MyClass : UIView <UITextInput, NSCoding>
@end

[BaseType (typeof (UIView))]
interface MyClass : IUITextInput, INSCoding
{
}

Rule: Protocol conformance uses I-prefixed interface names in the inheritance list.

Disabling Default Constructor

When a class marks init as NS_UNAVAILABLE:

[DisableDefaultCtor]
[BaseType (typeof (NSObject))]
interface MyClass
{
    [DesignatedInitializer]
    [Export ("initWithName:")]
    NativeHandle Constructor (string name);
}

Property Binding (@property)

Basic Property

@property (nonatomic, copy) NSString *title;

[Export ("title")]
string Title { get; set; }

Rules:

• •Property name is PascalCase in C# (the [Export] carries the ObjC name)
• •The [Export] selector is the ObjC property name (the getter selector)
• •The runtime auto-derives the setter as setTitle: from the export name

ArgumentSemantic

@property (nonatomic, copy) NSString *name;
@property (nonatomic, strong) UIView *contentView;
@property (nonatomic, weak) id<MyDelegate> delegate;
@property (nonatomic, assign) NSInteger count;
@property (nonatomic) UIColor *tintColor;   // no explicit semantic

[Export ("name", ArgumentSemantic.Copy)]
string Name { get; set; }

[Export ("contentView", ArgumentSemantic.Strong)]
UIView ContentView { get; set; }

[NullAllowed, Export ("delegate", ArgumentSemantic.Weak)]
IMyDelegate Delegate { get; set; }

[Export ("count")]
nint Count { get; set; }

[Export ("tintColor")]   // no default semantic for object pointers
UIColor TintColor { get; set; }

Readonly Properties

@property (nonatomic, readonly) NSInteger count;

[Export ("count")]
nint Count { get; }

Rule: Omit the setter for readonly properties.

Class Properties (Static)

@property (class, nonatomic, readonly) MyClass *sharedInstance;

[Static]
[Export ("sharedInstance")]
MyClass SharedInstance { get; }

Custom Getter Name

@property (nonatomic, getter=isEnabled) BOOL enabled;

[Export ("enabled")]
bool Enabled { [Bind ("isEnabled")] get; set; }

Block-Type Properties

@property (nonatomic, copy) void (^completionHandler)(BOOL success);

[Export ("completionHandler", ArgumentSemantic.Copy)]
Action<bool> CompletionHandler { get; set; }

Rule: Block properties use Action<> for void-returning blocks, Func<> for value-returning blocks.

Method Binding

Instance Methods

- (void)performAction:(NSString *)action withCompletion:(void (^)(BOOL))handler;

[Export ("performAction:withCompletion:")]
void PerformAction (string action, Action<bool> handler);

Rules:

• •The [Export] selector includes ALL colons: "performAction:withCompletion:"
• •Method name uses only the first selector part (PascalCased), with trailing parameter context stripped
• •addAnnotation:options:animated: → AddAnnotation
• •cancelSearchAnimated: → CancelSearch (strips "Animated")
• •For protocol/delegate methods, strip the sender class name prefix: myController:didSelectItem: → DidSelectItem
• •For single-part protocol selectors with embedded sender prefix + verb: myViewControllerDidCancel: → DidCancel
• •For non-void methods with parameters that don't start with a verb, add Get prefix: annotationForIndexPath: → GetAnnotation
• •Normalize acronyms: URL→Url, PDF→Pdf, HUD→Hud, HTML→Html, JSON→Json
• •[Async] is only added to class methods with completion handlers, never protocol methods
• •Constructor detection handles nonnull instancetype and nullable instancetype, not just bare instancetype
• •Parameter names come from the ObjC parameter names

Class Methods (Static)

+ (instancetype)documentWithURL:(NSURL *)url;

[Static]
[Export ("documentWithURL:")]
MyClass FromUrl (NSUrl url);

Rule: Add [Static] before [Export] for class methods (+).

NullAllowed on Parameters

- (void)setText:(nullable NSString *)text;

[Export ("setText:")]
void SetText ([NullAllowed] string text);

NullAllowed on Return Values

- (nullable UIView *)viewForKey:(NSString *)key;

[return: NullAllowed]
[Export ("viewForKey:")]
UIView ViewForKey (string key);

Constructor Binding

Basic Constructor

- (instancetype)initWithFrame:(CGRect)frame;

[Export ("initWithFrame:")]
NativeHandle Constructor (CGRect frame);

Rules:

• •Constructors return NativeHandle (was IntPtr in older Xamarin)
• •Method name is always Constructor
• •The [Export] carries the full ObjC selector

Designated Initializer

- (instancetype)initWithTitle:(NSString *)title NS_DESIGNATED_INITIALIZER;

[DesignatedInitializer]
[Export ("initWithTitle:")]
NativeHandle Constructor (string title);

Auto-Generated Constructors

The binding generator automatically creates these constructors for every bound class:

1. •Foo () — default, maps to ObjC init
2. •Foo (NSCoder) — for NIB deserialization (initWithCoder:)
3. •Foo (NativeHandle) — handle-based creation (runtime use)
4. •Foo (NSEmptyFlag) — for derived classes (prevents double init)

Use [DisableDefaultCtor] to suppress the parameterless constructor.

Protocol Binding (@protocol)

Protocol with Required and Optional Methods

@protocol MyDelegate <NSObject>
@required
- (void)didFinish:(MyClass *)sender;
@optional
- (BOOL)shouldStart:(MyClass *)sender;
@end

// 1. Empty stub interface (REQUIRED for typed protocol references)
interface IMyDelegate {}

// 2. Protocol definition
[Protocol, Model]
[BaseType (typeof (NSObject))]
interface MyDelegate
{
    // @required → [Abstract]
    [Abstract]
    [Export ("didFinish:")]
    void DidFinish (MyClass sender);

    // @optional → no [Abstract]
    [Export ("shouldStart:")]
    bool ShouldStart (MyClass sender);
}

Critical Rules:

• •Every @protocol needs an empty interface IProtocolName {} stub BEFORE the definition
• •The stub enables typed protocol references (IMyDelegate as parameter/property type)
• •@required methods get [Abstract]
• •@optional methods do NOT get [Abstract]
• •Protocols ending in Delegate or DataSource get [Protocol, Model] + [BaseType (typeof (NSObject))]
• •Other protocols get [Protocol] only (no [Model], no [BaseType])

Protocol Properties — Required vs Optional

Protocol properties are handled differently based on @required vs @optional:

@protocol MyProtocol <NSObject>
@required
@property (nonatomic, readonly, getter=isActive) BOOL active;
@property (nonatomic) NSString *name;
@optional
@property (nonatomic, getter=isSelected) BOOL selected;
@property (nonatomic) NSObject *value;
@end

[Protocol]
[BaseType (typeof (NSObject))]
interface MyProtocol
{
    // @required → [Abstract] + C# property (with [Bind] for custom getter)
    [Abstract]
    [Export ("active")]
    bool Active { [Bind ("isActive")] get; }

    [Abstract]
    [Export ("name")]
    string Name { get; set; }

    // @optional → decomposed into getter/setter method pairs
    [Export ("isSelected")]
    bool GetSelected ();

    [Export ("setSelected:")]
    void SetSelected (bool selected);

    [Export ("value")]
    NSObject GetValue ();

    [Export ("setValue:")]
    void SetValue ([NullAllowed] NSObject value);
}

Critical Rules:

• •@required properties get [Abstract] and stay as C# properties
• •@required properties with custom getters use [Bind("isX")] syntax
• •@optional properties are decomposed into GetXxx()/SetXxx() method pairs
• •@optional properties with custom getters use the custom selector as the getter export (e.g., isSelected)
• •Regular class (@interface) properties are NEVER decomposed — they always use { get; set; }

Protocol Used as Parameter Type

- (void)setDelegate:(id<MyDelegate>)delegate;

[Export ("setDelegate:")]
void SetDelegate ([NullAllowed] IMyDelegate delegate);

Rule: Use I-prefixed stub interface for protocol-typed parameters.

Delegate/Event Pattern

To generate C# events from an ObjC delegate protocol:

[BaseType (typeof (NSObject),
    Delegates = new string [] { "WeakDelegate" },
    Events = new Type [] { typeof (MyDelegate) })]
interface MyClass
{
    [Wrap ("WeakDelegate")]
    [NullAllowed]
    IMyDelegate Delegate { get; set; }

    [Export ("delegate", ArgumentSemantic.Assign)]
    [NullAllowed]
    NSObject WeakDelegate { get; set; }
}

[Protocol, Model]
[BaseType (typeof (NSObject))]
interface MyDelegate
{
    [Export ("myClass:didSelectItem:"), EventArgs ("ItemSelected")]
    void DidSelectItem (MyClass sender, NSObject item);

    [Export ("myClassShouldClose:"), DelegateName ("Predicate<MyClass>"), DefaultValue (true)]
    bool ShouldClose (MyClass sender);
}

Rules:

• •Events and Delegates on [BaseType] enable C# event generation
• •[EventArgs ("Name")] generates a NameEventArgs class for multi-parameter events
• •[DelegateName] + [DefaultValue] for methods that return values
• •Provide both Delegate (strongly typed, [Wrap]) and WeakDelegate (weakly typed, [Export])

Nullability Rules

NS_ASSUME_NONNULL Scope

Most modern ObjC headers use:

NS_ASSUME_NONNULL_BEGIN
// Everything here is nonnull by default
NS_ASSUME_NONNULL_END

Inside NS_ASSUME_NONNULL scope:

• •All object pointers are assumed nonnull → do NOT add [NullAllowed]
• •Only explicitly nullable types get [NullAllowed]

Outside NS_ASSUME_NONNULL scope:

• •All object pointer types should get [NullAllowed]

Nullability Annotation Mapping

Where NullAllowed Goes

Enum Binding

NS_ENUM (Native Integer Enums)

typedef NS_ENUM(NSInteger, UITableViewCellStyle) {
    UITableViewCellStyleDefault,
    UITableViewCellStyleValue1,
    UITableViewCellStyleValue2,
    UITableViewCellStyleSubtitle
};

// In StructsAndEnums.cs
[Native]
public enum UITableViewCellStyle : long
{
    Default,
    Value1,
    Value2,
    Subtitle,
}

Rules:

• •NS_ENUM(NSInteger, ...) → [Native] + : long
• •NS_ENUM(NSUInteger, ...) → [Native] + : ulong
• •Strip the enum name prefix from member names: UITableViewCellStyleDefault → Default
• •Goes in StructsAndEnums.cs, NOT in ApiDefinition.cs

NS_OPTIONS (Flags Enums)

typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {
    UIViewAutoresizingNone                 = 0,
    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,
    UIViewAutoresizingFlexibleWidth        = 1 << 1,
};

[Native]
[Flags]
public enum UIViewAutoresizing : ulong
{
    None = 0,
    FlexibleLeftMargin = 1 << 0,
    FlexibleWidth = 1 << 1,
}

Enum Backing Type Mapping

String-Backed Enums

enum NSRunLoopMode
{
    [DefaultEnumValue]
    [Field ("NSDefaultRunLoopMode")]
    Default,

    [Field ("NSRunLoopCommonModes")]
    Common,
}

Field Binding (Extern Constants)

String Constants / Notification Names

extern NSString * const MyClassDidFinishNotification;

[Field ("MyClassDidFinishNotification")]
NSString DidFinishNotification { get; }

Rules:

• •Fields go inside an interface decorated with [Static]
• •For statically linked libraries, use [Field ("name", "__Internal")]
• •For framework constants, use [Field ("name", "FrameworkName")]

Notification Binding

extern NSNotificationName const MyClassDidChangeNotification;

[Notification]
[Field ("MyClassDidChangeNotification")]
NSString DidChangeNotification { get; }

This generates a Notifications nested class with ObserveDidChange() methods.

For notifications with payload:

[Notification (typeof (MyChangedEventArgs))]
[Field ("MyClassDidChangeNotification")]
NSString DidChangeNotification { get; }

// Separate interface for EventArgs
interface MyChangedEventArgs
{
    [Export ("ChangedItemKey")]
    NSObject ChangedItem { get; set; }
}

Category Binding

Categories That Extend Known Types (Merge)

When the parent class is in your binding, merge the category members into the parent:

// PSPDFPageView+AnnotationMenu.h
@interface PSPDFPageView (AnnotationMenu)
- (void)showMenu;
@end

// Add directly into the PSPDFPageView interface definition:
[BaseType (typeof (UIView))]
interface PSPDFPageView
{
    // ... existing members ...

    // From AnnotationMenu category
    [Export ("showMenu")]
    void ShowMenu ();
}

Rule: Modern binding convention is to merge categories into their parent class.

Categories That Extend System Types (Extension Methods)

When extending a system type like UIView or NSString:

@interface UIView (MyExtensions)
- (void)makeBackgroundRed;
@end

[Category]
[BaseType (typeof (UIView))]
interface UIView_MyExtensions
{
    [Export ("makeBackgroundRed")]
    void MakeBackgroundRed ();
}

Rules:

• •Use [Category] + [BaseType (typeof (ExtendedType))]
• •Interface name convention: ExtendedType_CategoryName
• •These become C# extension methods
• •Avoid [Static] members in categories (generates broken code — use [Category (allowStaticMembers: true)] if needed)

Type Mapping Reference

Primitive Types

Object Types

Pointer / Handle Types

Block Types

const char * (C Strings)

Variadic Parameters

- (void)appendItems:(NSObject *)firstItem, ... NS_REQUIRES_NIL_TERMINATION;

[Export ("appendItems:"), Internal]
void AppendItems (NSObject firstItem, IntPtr varArgs);

// Then provide a public wrapper using params:
// public void AppendItems (params NSObject[] items) { ... }

Struct Binding

typedef struct {
    CGFloat x;
    CGFloat y;
    CGFloat width;
} MyRect;

// In StructsAndEnums.cs
[StructLayout (LayoutKind.Sequential)]
public struct MyRect
{
    public nfloat X;
    public nfloat Y;
    public nfloat Width;
}

Advanced Attributes

[Wrap] — Strongly Typed Convenience

[Export ("delegate", ArgumentSemantic.Assign)]
[NullAllowed]
NSObject WeakDelegate { get; set; }

[Wrap ("WeakDelegate")]
[NullAllowed]
IMyDelegate Delegate { get; set; }

[Async] — Async Wrapper Generation

[Export ("fetchDataWithCompletion:")]
[Async]
void FetchData (Action<NSData, NSError> completion);

Generates: Task<NSData> FetchDataAsync ()

[Sealed] — Prevent Overriding

[Sealed]
[Export ("uniqueIdentifier")]
string UniqueIdentifier { get; }

[New] — Hide Inherited Member

[New]
[Export ("init")]
NativeHandle Constructor ();

[Internal] — Hide from Public API

[Internal]
[Export ("_privateMethod")]
void PrivateMethod ();

[Verify] — Mark for Manual Review

[Verify (MethodToProperty)]
[Export ("count")]
nint Count { get; }

Used by Objective Sharpie to flag bindings that need human review.

[BindAs] — Better Type Conversion

[return: BindAs (typeof (bool?))]
[Export ("shouldDraw:")]
NSNumber ShouldDraw ([BindAs (typeof (CGRect))] NSValue rect);
// Generates: bool? ShouldDraw (CGRect rect)

Common Patterns and Pitfalls

1. Missing I-Prefix Protocol Stub

Wrong: Using protocol directly without stub

[Protocol, Model]
[BaseType (typeof (NSObject))]
interface MyDelegate { ... }

Right: Always add the empty stub interface

interface IMyDelegate {}

[Protocol, Model]
[BaseType (typeof (NSObject))]
interface MyDelegate { ... }

2. Forgetting ArgumentSemantic for Object Properties

Wrong: Missing semantic for object pointer property

[Export ("view")]
UIView View { get; set; }

Right: Only include ArgumentSemantic when explicitly declared in ObjC property attributes

[Export ("view")]
UIView View { get; set; }  // no default semantic needed

3. Wrong Selector for Multi-Part Methods

Wrong: Missing colons in selector

[Export ("setValueForKey")]
void SetValue (NSObject value, string key);

Right: Include ALL colons

[Export ("setValue:forKey:")]
void SetValue (NSObject value, string key);

4. Using int Instead of nint for NSInteger

Wrong:

[Export ("count")]
int Count { get; }

Right: NSInteger maps to nint (pointer-sized)

[Export ("count")]
nint Count { get; }

5. Enums in Wrong File

Wrong: Putting enums in ApiDefinition.cs

Right: All enums, structs, and delegates go in StructsAndEnums.cs

6. Constructor Returning Wrong Type

Wrong:

[Export ("initWithName:")]
IntPtr Constructor (string name);  // Old Xamarin style

Right (modern .NET for iOS):

[Export ("initWithName:")]
NativeHandle Constructor (string name);

Vendor Macro Handling

When reading ObjC headers from third-party frameworks, ignore these common macros:

Checklist for Binding an ObjC Header

1. •Identify the scope: Is the header inside NS_ASSUME_NONNULL_BEGIN/END?
2. •Map the class: @interface → [BaseType] interface
3. •Map the superclass: : NSObject → typeof (NSObject)
4. •Map protocol conformance: <P1, P2> → : IP1, IP2
5. •Map each property: type mapping, ArgumentSemantic, readonly, nullable
6. •Map each method: selector with colons, parameter types, return type, nullable
7. •Map constructors: init* methods → NativeHandle Constructor, add [DesignatedInitializer] if marked
8. •Map protocols: @protocol → interface IFoo {} stub + [Protocol] definition, [Abstract] for @required
9. •Map enums: NS_ENUM → [Native] enum, NS_OPTIONS → [Native] [Flags] enum
10. •Map extern constants: extern NSString * → [Field] in [Static] interface Constants
11. •Map categories: Merge into parent class or use [Category] for system types
12. •Verify nullability: Check every object pointer parameter, property, and return value
13. •Strip enum prefixes: UITableViewCellStyleDefault → Default
14. •Put types in correct files: Interfaces in ApiDefinition.cs, enums/structs in StructsAndEnums.cs

References

• •Binding Types Reference Guide
• •Binding Objective-C Libraries
• •type-mapping.md — Complete ObjC→C# type mapping table
• •attributes.md — Complete attribute reference