c# inter process acquire/release resource count

Question

I've got a resource that is shared across potentially many processes and needs to be cleaned up when no longer in use.

What's the easiest way in C# to maintain a usage count across processes so that the resource may be cleaned up when no longer used?

(If the last process to use the resources dies, it is ok to leave the resource unfreed so long as the next to use has a valid usage count).

Answer 1

The code I used:

internal sealed class InterProcessResource {
    private static readonly string MutexNameThisProcess = "IPR-" + Guid.NewGuid().ToString();
    private static readonly Mutex MutexThisProcess = new Mutex(true, MutexNameThisProcess);

    private readonly MemoryMappedFile mmf;
    private readonly string mutexName;

    public InterProcessResource(string resourceName) {
        this.mutexName = resourceName + "-mtx";
        this.mmf = MemoryMappedFile.CreateOrOpen(resourceName + "-mmf", 16 * 1024, MemoryMappedFileAccess.ReadWrite);
    }


    public void Acquire(Action initAction) {
        using (new Mutex(true, this.mutexName)) {
            var currentList = ReadStringList(mmf);
            if (currentList.Count == 0) {
                initAction();
            }
            var newList = PruneMutexList(currentList);
            newList.Add(MutexNameThisProcess);
            WriteStringList(this.mmf, newList);
        }
    }

    public void Release(Action freeAction) {
        using (new Mutex(true, this.mutexName)) {
            var currentList = ReadStringList(this.mmf);
            var newList = PruneMutexList(currentList);
            WriteStringList(this.mmf, newList);
            if (newList.Count == 0) {
                freeAction();
            }
        }
    }

    private static List<string> ReadStringList(MemoryMappedFile mmf) {
        var list = new List<string>();
        using (var stream = mmf.CreateViewStream()) {
            var reader = new BinaryReader(stream);
            int count = reader.ReadInt32();
            for (int i = 0; i < count; i++) {
                list.Add(reader.ReadString());
            }
        }
        return list;
    }

    private static void WriteStringList(MemoryMappedFile mmf, List<string> newList) {
        using (var stream = mmf.CreateViewStream()) {
            var writer = new BinaryWriter(stream);
            int count = newList.Count;
            writer.Write(count);
            for (int i = 0; i < count; i++) {
                writer.Write(newList[i]);
            }
        }
    }

    // removes our mutex name AND any dead processes mutex names
    private static List<string> PruneMutexList(List<string> list) {
        var newList = new List<string>();
        foreach (var s in list) {
            if (s != MutexNameThisProcess) {
                Mutex m;
                if (Mutex.TryOpenExisting(s, out m)) {
                    newList.Add(s);
                    m.Dispose();
                }
            }
        }
        return newList;
    }
}