This is from my answer on StackOverflow from a question that got closed. As not to lose it to the SO ‘on-hold/close’ flush, I am re-posting it.

Question

How can you write an Android FileObserver in C#?

i.e. Native Java example:

FileObserver observer = new FileObserver(imageUri.getPath()) {
    public String basePath;

    @Override
    public void onEvent(int event, String path) {
        String fullPath = basePath;
        if(path != null) {
            fullPath += path;
        }
        Log.d(TAG, "FILE: "+fullPath);
    }
};
observer.basePath = imageUri.getPath();
observer.startWatching();

Answer

Create a class that inherits from Android.OS.FileObserver, you only need to implement the OnEvent() and one(+) Constructors. Its a really simple pattern after you see it once… ;-)

Quick Notes:

• Watch on a path, if you need to filter by file, do it in the OnEvent
• Do not let your FileObserver object get GC’d or your OnEvents will magically stop :-/
• Remember to call StartWatching() in order to receive OnEvent calls

C# FileObserver Class:

using System;
using Android.OS;
using Android.Util;

namespace MyFileObserver
{
    public class MyPathObserver : Android.OS.FileObserver
    {
        static FileObserverEvents _Events = (FileObserverEvents.AllEvents);
        const string tag = "StackoverFlow";

        public MyPathObserver (String rootPath) : base(rootPath, _Events)
        {
            Log.Info(tag, String.Format("Watching : {0}", rootPath)); 
        }

        public MyPathObserver (String rootPath, FileObserverEvents events) : base(rootPath, events)
        {
            Log.Info(tag, String.Format("Watching : {0} : {1}", rootPath, events)); 
        }

        public override void OnEvent(FileObserverEvents e, String path)
        {
            Log.Info(tag, String.Format("{0}:{1}",path, e)); 
        }
    }
}

Example Usage:

var pathToWatch = System.Environment.GetFolderPath (System.Environment.SpecialFolder.Personal);
// Do not let myFileObserver get GC'd, stash it's ref in an activty, or ...
myFileObserver = new MyPathObserver (pathToWatch);
myFileObserver.StartWatching (); // and StopWatching () when you are done...
var document = Path.Combine(pathToWatch, "StackOverFlow.txt");
button.Click += delegate {
    if (File.Exists (document)) {
        button.Text = "Delete File";
        File.Delete (document);
    } else {
        button.Text = "Create File";
        File.WriteAllText (document, "Foobar");
    }
};

adb logcat Output (when clicking on the test button):

I/StackoverFlow( 3596): StackOverFlow.txt:Create
I/StackoverFlow( 3596): StackOverFlow.txt:Open
I/StackoverFlow( 3596): StackOverFlow.txt:Modify
I/StackoverFlow( 3596): StackOverFlow.txt:CloseWrite
I/StackoverFlow( 3596): StackOverFlow.txt:Delete
I/StackoverFlow( 3596): StackOverFlow.txt:Create
I/StackoverFlow( 3596): StackOverFlow.txt:Open
I/StackoverFlow( 3596): StackOverFlow.txt:Modify
I/StackoverFlow( 3596): StackOverFlow.txt:CloseWrite
I/StackoverFlow( 3596): StackOverFlow.txt:Delete
I/StackoverFlow( 3596): StackOverFlow.txt:Create
I/StackoverFlow( 3596): StackOverFlow.txt:Open
I/StackoverFlow( 3596): StackOverFlow.txt:Modify
I/StackoverFlow( 3596): StackOverFlow.txt:CloseWrite