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; nam…

Accepted Answer

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]

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;
    }
```

}

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;            
    }
}
```

How to bind Xamarin iOS Obj-C library with marshalling

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