How to PInvoke AppKit methods from .Net Core application?

Question

Is it possible to use PInvoke to call into AppKit methods when running on .Net Core on Mac OS? Specifically, I would like to use the NSWorkspace.Recycle method.

According to the documentation PInvoke is supported on MacOS, it is just not clear how one would use this to interact with frameworks such as AppKit.

Answer 1

PInvoke is for C methods only but since the Objective C runtime is based on C methods, we can use them. Xamarin would provide all the necessary setup and convenience method but this is possible. Note that this example uses unsafe code and thus needs the following property set in the csproj file:

<AllowUnsafeBlocks>true</AllowUnsafeBlocks>

Below is the full code, note that objc_msgSend needs to imported multiple time with the correct signature. In C this would be done by casting it to different function pointers with the correct signature. The frameworks also need to be imported by via a full path since the CoreCLR has no probing logic for frameworks, it would search for a lib{sth}.dylib instead.

using System;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;

namespace NSWorkspaceExample
{
    public class Program
    {
        static void Main(string[] args)
        {
            var testFile = Path.Combine(Directory.GetCurrentDirectory(), "test.txt");
            File.WriteAllText(testFile, "example file content");

            var cfstrTestFile = CreateCFString(testFile);
            var nsURL = objc_getClass("NSURL");
            var fileUrl = objc_msgSend_retIntPtr_IntPtr(nsURL, GetSelector("fileURLWithPath:"), cfstrTestFile);
            CFRelease(cfstrTestFile);

            var urlArray = CreateCFArray(new IntPtr[] {fileUrl});

            var nsWorkspace = objc_getClass("NSWorkspace");
            var sharedWorkspace = objc_msgSend_retIntPtr(nsWorkspace, GetSelector("sharedWorkspace"));

            objc_msgSend_retVoid_IntPtr_IntPtr(sharedWorkspace, GetSelector("recycleURLs:completionHandler:"), urlArray, IntPtr.Zero);

            CFRelease(urlArray);
            CFRelease(fileUrl);
            CFRelease(sharedWorkspace);

            // sleep since we didn't go through the troubles of creating a block object as a callback
            Thread.Sleep(1000);
        }

        public static IntPtr GetSelector(string name)
        {
            IntPtr cfstrSelector = CreateCFString(name);
            IntPtr selector = NSSelectorFromString(cfstrSelector);
            CFRelease(cfstrSelector);
            return selector;
        }

        private const string FoundationFramework = "/System/Library/Frameworks/Foundation.framework/Foundation";
        private const string AppKitFramework = "/System/Library/Frameworks/AppKit.framework/AppKit";

        public unsafe static IntPtr CreateCFString(string aString)
        {
            var bytes = Encoding.Unicode.GetBytes(aString);
            fixed (byte* b = bytes) {
                var cfStr = CFStringCreateWithBytes(IntPtr.Zero, (IntPtr)b, bytes.Length, CFStringEncoding.UTF16, false);
                return cfStr;
            }
        }

        // warning: this doesn't call retain/release on the elements in the array
        public unsafe static IntPtr CreateCFArray(IntPtr[] objectes)
        {
            fixed(IntPtr* vals = objectes) {
                 return CFArrayCreate(IntPtr.Zero, (IntPtr)vals, objectes.Length, IntPtr.Zero);
            }
        }

        [DllImport(FoundationFramework)]
        public static extern IntPtr CFStringCreateWithBytes(IntPtr allocator, IntPtr buffer, long bufferLength, CFStringEncoding encoding, bool isExternalRepresentation);

        [DllImport(FoundationFramework)]
        public static extern IntPtr CFArrayCreate(IntPtr allocator, IntPtr values, long numValues, IntPtr callbackStruct);

        [DllImport(FoundationFramework)]
        public static extern void CFRetain(IntPtr handle);

        [DllImport(FoundationFramework)]
        public static extern void CFRelease(IntPtr handle);

        [DllImport(AppKitFramework, CharSet = CharSet.Ansi)]
        public static extern IntPtr objc_getClass(string name);

        [DllImport(AppKitFramework)]
        public static extern IntPtr NSSelectorFromString(IntPtr cfstr);

        [DllImport(FoundationFramework, EntryPoint="objc_msgSend")]
        public static extern IntPtr objc_msgSend_retIntPtr(IntPtr target, IntPtr selector);

        [DllImport(FoundationFramework, EntryPoint="objc_msgSend")]
        public static extern void objc_msgSend_retVoid_IntPtr_IntPtr(IntPtr target, IntPtr selector, IntPtr param1, IntPtr param2);

        [DllImport(FoundationFramework, EntryPoint="objc_msgSend")]
        public static extern IntPtr objc_msgSend_retIntPtr_IntPtr(IntPtr target, IntPtr selector, IntPtr param);

        [DllImport(FoundationFramework, EntryPoint="objc_msgSend")]
        public static extern void objc_msgSend_retVoid(IntPtr target, IntPtr selector);

        public enum CFStringEncoding : uint
        {
            UTF16 = 0x0100,
            UTF16BE = 0x10000100,
            UTF16LE = 0x14000100,
            ASCII = 0x0600
        }
    }
}