Sending an instance Callback as a function pointer results in “this” being “null” in C#

Question

I'm working on writing a FileSystemWatcher using Mono but for some reason the callback i send to the FSEvents library isn't mantaining the this reference, it is always null inside the callback even tho the callback is an instance method.

Any idea why it happens?

    using System;
    using System.IO;
    using System.Collections.Generic;
    using System.Runtime.InteropServices;
    using MonoMac.Foundation;
    using System.Threading;
    using NUnit.Framework;
    using System.Text;

    namespace Test
    {
        class MainClass
        {
            public void Main ()
            {
                string testFolder = Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.MyDocuments), "mono-test");

                if (Directory.Exists (testFolder)) {
                    Directory.Delete (testFolder, true);
                }
                Directory.CreateDirectory (testFolder);

                IntPtr path = CFStringCreateWithCString (IntPtr.Zero, testFolder, 0);
                IntPtr paths = CFArrayCreate (IntPtr.Zero, new IntPtr [1] { path }, 1, IntPtr.Zero);

                IntPtr stream = FSEventStreamCreate (IntPtr.Zero, this.Callback, IntPtr.Zero, paths, FSEventStreamEventIdSinceNow, 0, FSEventStreamCreateFlags.WatchRoot | FSEventStreamCreateFlags.FileEvents);

                CFRelease (paths);
                CFRelease (path);

                Thread runLoop = new Thread (delegate() {
                    FSEventStreamScheduleWithRunLoop (stream, CFRunLoopGetCurrent (), kCFRunLoopDefaultMode);
                    FSEventStreamStart (stream);
                    CFRunLoopRun ();
                });
                runLoop.Name = "FSEventStream";
                runLoop.Start ();

                Thread.Sleep (3000);

                string file1 = Path.Combine (testFolder, "file1.txt");
                //Thread.Sleep(1000);
                using (System.IO.File.Create(file1)) {
                }
                //Thread.Sleep(1000);
                System.IO.File.WriteAllText (file1, "file1");
                //Thread.Sleep(1000);
                System.IO.File.Delete (file1);          

            }

            public static void Main (string[] args)
            {
                new MainClass().Main();
            }

            private static IDictionary<IntPtr, MainClass> thisDict = new Dictionary<IntPtr, MainClass>();

            private void Callback (IntPtr streamRef, IntPtr clientCallBackInfo, int numEvents, IntPtr eventPaths, IntPtr eventFlags, IntPtr eventIds)
            {
                MainClass thisObj;
                if (this != null) {
                    thisDict.Add(streamRef, this);
                    thisObj = this;
                } else {
                    thisObj = thisDict[streamRef];
                }
                Console.WriteLine("\n{0}", this != null ? "this is not null" : "this is null");
                Console.WriteLine("{0}\n", thisObj != null ? "thisObj is not null" : "thisObj is null");

                string[] paths = new string[numEvents];
                UInt32[] flags = new UInt32[numEvents];
                UInt64[] ids = new UInt64[numEvents];
                unsafe
                {
                    char** eventPathsPointer = (char**) eventPaths.ToPointer();
                    uint* eventFlagsPointer = (uint*) eventFlags.ToPointer();
                    ulong* eventIdsPointer = (ulong*) eventIds.ToPointer();
                    for (int i = 0; i < numEvents; i++)
                    {
                        paths[i] = Marshal.PtrToStringAuto(new IntPtr(eventPathsPointer[i]));
                        flags[i] = eventFlagsPointer[i];
                        ids[i] = eventIdsPointer[i];
                    }
                }
                Console.WriteLine("Number of events: {0}", numEvents);
                for (int i = 0; i < numEvents; i++)
                {
                    Console.WriteLine("{0} {1:x8} {2}", ids[i], flags[i], paths[i]);
                    Console.WriteLine("Modified: {0:x8}", (flags[i] & (uint) FSEventStreamEventFlagItem.Modified));
                    Console.WriteLine("Created:  {0:x8}", (flags[i] & (uint) FSEventStreamEventFlagItem.Created));
                    Console.WriteLine("Removed:  {0:x8}", (flags[i] & (uint) FSEventStreamEventFlagItem.Removed));
                    Console.WriteLine("Renamed:  {0:x8}", (flags[i] & (uint) FSEventStreamEventFlagItem.Renamed));
                    Console.WriteLine();
                }
            }

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static IntPtr CFStringCreateWithCString (IntPtr allocator, string value, int encoding);

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static IntPtr CFArrayCreate (IntPtr allocator, IntPtr [] values, int numValues, IntPtr callBacks);

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static IntPtr CFArrayGetValueAtIndex(IntPtr array, int index);

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static void CFRelease(IntPtr cf);

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static IntPtr CFRunLoopGetCurrent ();

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static IntPtr CFRunLoopGetMain();

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static void CFRunLoopRun ();

            [DllImport ("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation")]
            extern static int CFRunLoopRunInMode (IntPtr mode, double seconds, int returnAfterSourceHandled);

            delegate void FSEventStreamCallback (IntPtr streamRef, IntPtr clientCallBackInfo, int numEvents, IntPtr eventPaths, IntPtr eventFlags, IntPtr eventIds);

            [DllImport ("/System/Library/Frameworks/CoreServices.framework/CoreServices")]
            extern static IntPtr FSEventStreamCreate (IntPtr allocator, FSEventStreamCallback callback, IntPtr context, IntPtr pathsToWatch, ulong sinceWhen, double latency, FSEventStreamCreateFlags flags);

            [DllImport ("/System/Library/Frameworks/CoreServices.framework/CoreServices")]
            extern static int FSEventStreamStart (IntPtr streamRef);

            [DllImport ("/System/Library/Frameworks/CoreServices.framework/CoreServices")]
            extern static void FSEventStreamStop (IntPtr streamRef);

            [DllImport ("/System/Library/Frameworks/CoreServices.framework/CoreServices")]
            extern static void FSEventStreamRelease (IntPtr streamRef);

            [DllImport ("/System/Library/Frameworks/CoreServices.framework/CoreServices")]
            extern static void FSEventStreamScheduleWithRunLoop (IntPtr streamRef, IntPtr runLoop, IntPtr runLoopMode);

            [DllImport ("/System/Library/Frameworks/CoreServices.framework/CoreServices")]
            extern static void FSEventStreamUnscheduleFromRunLoop (IntPtr streamRef, IntPtr runLoop, IntPtr runLoopMode);

            const ulong FSEventStreamEventIdSinceNow = ulong.MaxValue;

            private static IntPtr kCFRunLoopDefaultMode = CFStringCreateWithCString(IntPtr.Zero, "kCFRunLoopDefaultMode", 0);

            [Flags()]
            enum FSEventStreamCreateFlags : uint {
                None = 0x00000000,
                UseCFTypes = 0x00000001,
                NoDefer = 0x00000002,
                WatchRoot = 0x00000004,
                IgnoreSelf = 0x00000008,
                FileEvents = 0x00000010
            }

            [Flags()]
            enum FSEventStreamEventFlag : uint {
                None = 0x00000000,
                MustScanSubDirs = 0x00000001,
                UserDropped = 0x00000002,
                KernelDropped = 0x00000004,
                EventIdsWrapped = 0x00000008,
                HistoryDone = 0x00000010,
                RootChanged = 0x00000020,
                FlagMount  = 0x00000040,
                Unmount = 0x00000080
            }

            [Flags()]
            enum FSEventStreamEventFlagItem : uint {
                Created       = 0x00000100,
                Removed       = 0x00000200,
                InodeMetaMod  = 0x00000400,
                Renamed       = 0x00000800,
                Modified      = 0x00001000,
                FinderInfoMod = 0x00002000,
                ChangeOwner   = 0x00004000,
                XattrMod      = 0x00008000,
                IsFile        = 0x00010000,
                IsDir         = 0x00020000,
                IsSymlink     = 0x00040000
            }
        }
    }

UPDATE

The fixed code is here for anyone interested.

Answer 1

When you pass a delegate to a native method, the runtime has no way to know how long the native method will hold on to the function pointer, so it's up to you to keep the delegate alive. If you don't maintain a reference to it, the garbage collector might collect the delegate, resulting in undefined behaviour when the native code tries to call it.