[System] Make FileSystemWatcher backend non-static (#16922)

* [System] Make FileSystemWatcher backend non-static

The FileSystemWatcher `watcher` field is the backend IFileWatcher that's
supposed to be used for the actual operations.

Each backend has a `bool GetInstance (out watcher)` method that returns a
singleton object, so each new instance of FileSystemWatcher gets the same
IFileWatcher instance.  (They will get different `watcher_handle` instances
which are used by some backends to uniquely identify this FileSystemWatcher).

However when the first FileSystemWatcher instance is `Dispose`d, it will set
`watcher = null` which will prevent the remaining instances from calling

    watcher?.StopDispatching (watcher_handle);
    watcher?.Dispose (watcher_handle);

which means that the backend won't properly cleanup resources for any other FSW
instances that have other `watcher_handle` values.

Addresses https://github.com/mono/mono/issues/16709

1 files changed, 1 insertions, 1 deletions

diff --git a/mcs/class/System/System.IO/FileSystemWatcher.cs b/mcs/class/System/System.IO/FileSystemWatcher.cs
index 9c003a80e5a..cbf71e7a9a6 100644
--- a/mcs/class/System/System.IO/FileSystemWatcher.cs
+++ b/mcs/class/System/System.IO/FileSystemWatcher.cs
@@ -61,7 +61,7 @@ namespace System.IO {
 		SearchPattern2 pattern;
 		bool disposed;
 		string mangledFilter;
-		static IFileWatcher watcher;
+		IFileWatcher watcher;
 		object watcher_handle;
 		static object lockobj = new object ();