Filesystemwatcher two 'identical' implementations but different behaviour

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C#

C#4.0

forms

This question is not about the dodgy way events fire, because I don’t mind that.

I am doing something wrong here but once again I am code blind or missing a piece of the puzzle.
Setting: On Windows forms, one can implement a FileSystemWatcher in two ways.

Implementation 1: Dropping a FilesystemWatcher from the toolbox onto the Form.
I have these settings under properties:

Name.................FileSystemWatcher1
GenerateMembers......True
Modifiers ...........Private

EnableRaisingEvents..True
Filter   ..............*.*
IncludeSubFolders....True
NotifyFiler..........LastWrite
Path ...............           C:\somdir

Under event properties
Changed FileSystemWatcher1_created

With debugger I can see the SynchonizingObject = this.

My partial form class holds the FileSystemwatcher1_created implementation

Implementation 2: By coding the FileSystemwatcher

I put this code in my buttonclicked method from one of the form buttons. (Or even if I put it in the constructor of the form)

{
   FileSystemWatcher fs = new FileSystemWatcher(@"C:\somedir");
   fs.NotifyFilter = NotifyFilters.LastWrite;
   fs.Filter = "*.*";
   fs.IncludeSubdirectories = true;

   fs.SynchronizingObject = this;

   fs.Changed += new FileSystemEventHandler(OnChanged);
   fs.EnableRaisingEvents = true;
}

And a OnChanged implementation:

private void OnChanged(object source, FileSystemEventArgs args)
{
   int a = 5;
   // Doing my thing here, with being very busy.
}

Now for the difference:
1. When I copy a file to the watched folder both implementations fire an event and execute the code I want to execute as a result. So far so good.
2. But, in real life these file are created by 3rd party system which keeps on writing to these files. In this situation in Implementation 1. I get what I want. Event is fired and handler executed. In Implementation 2. Nothing happens until I close the 3rd party app, which never happens is real life.

Why this difference?

I tried with changing synchonizeobject = this or = null.

What I am doing wrong here?
Background info which you can skip if you want.

I have to migrate the app from FORMS to WPF. So I have to implement on code. I noticed the difference in behaviour so I made this test set to investigate my mistake. First I thought that it had to do with the missing ISynchronizeInvoke Interface in WPF.
But since I can reproduce the same behaviour in my Forms test set It must be something else that I am doing wrong.