How to bind Xamarin iOS Obj-C library with marshalling

Question

I want to study how to bind iOS library with handling pointer to Xamarin.iOS. It need marshaling.

I prepare such library for test,

MarshalTest.h

typedef struct
{
    float x,y,z;
} Marshal3D;


@interface MarshalTest : NSObject

-(id)initWithMarshal:(Marshal3D *)marshal;
-(id)initWithMarshals:(Marshal3D *)marshals num:(int)numCoord;

-(void)addMarshal:(Marshal3D *)marshal;
-(void)addMarshals:(Marshal3D *)marshals num:(int)numCoord;

-(int)getMarshals:(Marshal3D **)getMarshals;

-(int) storedNumber;
-(float)checkMarshalValue:(int)index xyz:(int)xyz;

@end

and prepare Binding c# codes like this:

StructsAndEnums.cs

using System;
using System.Runtime.InteropServices;

namespace MarshalTestNet
{
    [StructLayout(LayoutKind.Sequential)]
    public struct Marshal3D
    {
        public float x;
        public float y;
        public float z;
    };
}

ApiDefinition.cs

using System;
using System.Drawing;
using System.Runtime.InteropServices;

using MonoTouch.ObjCRuntime;
using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace MarshalTestNet
{
    [BaseType (typeof (NSObject))]
    public partial interface MarshalTest {

        [Export ("initWithMarshal:")]
        IntPtr Constructor ([In, MarshalAs(UnmanagedType.LPStruct)]Marshal3D marshal);

        [Export ("initWithMarshals:num:")]
        IntPtr Constructor ([In, MarshalAs(UnmanagedType.LPStruct)]Marshal3D marshals, int numCoord);

        [Export ("addMarshal:")]
        void AddMarshal ([In, MarshalAs(UnmanagedType.LPStruct)]Marshal3D marshal);

        [Export ("addMarshals:num:")]
        void AddMarshals ([In, MarshalAs(UnmanagedType.LPStruct)]Marshal3D marshals, int numCoord);

        [Export ("getMarshals:")]
        int GetMarshals ([Out, MarshalAs(UnmanagedType.LPStruct)]Marshal3D getMarshals);

        [Export ("storedNumber")]//, Verify ("ObjC method massaged into getter property", "/Users/kokogiko/Desktop/MarshalTest.h", Line = 32)]
        int StoredNumber { get; }

        [Export ("checkMarshalValue:xyz:")]
        float CheckMarshalValue (int index, int xyz);
    }
}

But this not work, just calling

var mars3 = new Marshal3D{ 
    x = 1.0f,
    y = 2.0f,
    z = 3.0f
};
var mars = new MarshalTest (mars3);

will terminated without showing error message.

How to bind iOS library with marshalling? There are very less information for doing this...

[Edited after Rolf's answer]

Thank you Rolf, I tested many cases so sorry for replying so late.

1.

[Export ("initWithMarshal:")]
IntPtr Constructor (ref Marshal3D marshal);

[Export ("addMarshal:")]
void AddMarshal (ref Marshal3D marshal);

works very well. Thank you!

2.

[Export ("initWithMarshals:num:")]
IntPtr Constructor (Marshal3D[] marshals, int numCoord);

[Export ("addMarshals:num:")]
void AddMarshals (Marshal3D[] marshals, int numCoord);

not works, with showing this error in compile stage:

obj/Debug/ios/MarshalTestNet/MarshalTest.g.cs(140,31): error CS1502: The best overloaded method match for `MonoTouch.Foundation.NSArray.FromNSObjects(params MonoTouch.Foundation.NSObject[])' has some invalid arguments
    /Developer/MonoTouch/usr/lib/mono/2.1/monotouch.dll (Location of the symbol related to previous error)
obj/Debug/ios/MarshalTestNet/MarshalTest.g.cs(140,46): error CS1503: Argument `#1' cannot convert `MarshalTestNet.Marshal3D[]' expression to type `MonoTouch.Foundation.NSObject[]'

so I change this by myself,

ApiDefinition.cs

[Export ("addMarshals:num:")]
void AddMarshals (IntPtr marshals, int numCoord);

Extra.cs

public unsafe void AddMarshals (Marshal3D[] marshals) {
    var count = marshals.Length;
    fixed (Marshal3D* ptr = &marshals[0])
        AddMarshals ((IntPtr)ptr, count);         
}

this works well.

But, this approach cannot use for

-(id)initWithMarshals:(Marshal3D *)marshals num:(int)numCoord;

because it is constructor, not allow to add any logic before calling parent method.

Is there any way to fix this?
Using "Factory pattern" is best?

3.

[Export ("getMarshals:")]
[Internal]
int GetMarshals (IntPtr getMarshals);

and

public partial class MarshalTest : NSObject {
    public unsafe Marshal3D [] GetMarshals () {
        var count = this.StoredNumber();
        var array = new Marshal3D [count];

        fixed (Marshal3D* ptr = &array[0])
            GetMarshals ((IntPtr) ptr);

        return array;            
    }
}

not works.
Terminated without showing any errors in runnig phase.

I think

Marshal3D* ptr = &array[0]

is a pointer, but

-(int)getMarshals:(Marshal3D **)getMarshals;

needs pointer-of-pointer.
Isn't it right?

Answer 1

Xamarin.iOS binding projects do not support custom marshalling.

But I don't think you need it:

[BaseType (typeof (NSObject))]
public partial interface MarshalTest {

    [Export ("initWithMarshal:")]
    IntPtr Constructor (ref Marshal3D marshal);

    [Export ("initWithMarshals:num:")]
    IntPtr Constructor (Marshal3D[] marshals, int numCoord);

    [Export ("addMarshal:")]
    void AddMarshal (ref Marshal3D marshal);

    [Export ("addMarshals:num:")]
    void AddMarshals (Marshal3D[] marshals, int numCoord);

    [Export ("getMarshals:")]
    [Internal]
    int GetMarshals (IntPtr getMarshals);

    // ...
}

However to support GetMarshals nicely, you'll have to add a custom binding (this is typically done in the StructsAndEnums.cs file):

public partial class MarshalTest : NSObject {
    public unsafe Marshal3D [] GetMarshals () {
        var count = // how many are there? there's no API to fetch this in your sample
        var array = new Marshal3D [count];

        fixed (Marshal3D* ptr = &array[0])
            GetMarshals ((IntPtr) ptr);

        return array;            
    }
}

[Updated to answer second set of questions]

1. For the constructor, I think this will work:

   ApiDefinition.cs

   [BaseType (typeof (NSObject))]
   public partial interface MarshalTest {
       [Export ("initWithMarshals:num:")]
       [Internal]
       IntPtr Constructor (IntPtr marshals, int numCoord);
   }
   

   Extra.cs

   public partial class MarshalTest : NSObject {
       public MarshalTest (Marshal3D[] marshals, int numCoord)
           : this (GetPointer (marshals), numCoord)
       {
       }
   
       static IntPtr GetPointer (Marshal3D[] marshals)
       {
           fixed (Marshal3D* ptr = &marshals[0])
               return (IntPtr) ptr;
       }
   

   }

2. You're right, it's expecting a pointer to a array. Try this instead:

   ApiDefinition.cs

   [BaseType (typeof (NSObject))]
   public partial interface MarshalTest {
       [Export ("getMarshals:")]
       [Internal]
       int GetMarshals (ref IntPtr getMarshals);
   }
   

   Extra.cs

   public partial class MarshalTest : NSObject {
       public unsafe Marshal3D [] GetMarshals () {
           var ptr = IntPtr.Zero;
           var count = this.StoredNumber ();
           var array = new Marshal3D [count];
   
           GetMarshals (ref ptr);
   
           unsafe {
               Marshal3D* ptr3d = (Marshal3D *) ptr;
               for (int i = 0; i < count; i++)
                   array [i] = *ptr3d++;
           }
   
           return array;            
       }
   }