My.Settings in-memory cache synchronization limits

Synchronization Limits of ApplicationSettingsBase

No, My.Settings.Reload does not provide a mechanism to merge external changes with unsaved in-memory modifications. When Reload is called, the current in-memory cache is completely discarded and replaced by the values currently persisted in the user.config file. Any changes made to settings properties that have not yet been committed via Save will be lost.

Additionally, there is no built-in event or property within My.Settings or ApplicationSettingsBase to monitor the underlying configuration file for external changes. The framework assumes the application instance has exclusive ownership of its user settings during its lifecycle.

Behavioral Analysis

The following behaviors are established for the ApplicationSettingsBase implementation (typically used in .NET Framework and .NET Core/5+ Windows Forms and WPF applications):

• Reload Behavior: Destructive overwrite of the in-memory cache.
• Save Behavior: Last-writer-wins. If Instance A and Instance B both load a value of "1", Instance A changes it to "2" and saves, and then Instance B changes it to "3" and saves, the value becomes "3".
• Cache Lifecycle: Values are loaded once upon first access and remain static until Reload is explicitly invoked.

Implementation for Automatic Invalidation

To synchronize settings across multiple instances or detect external edits, you must implement a file system monitor. Because user.config is stored in a versioned local app data folder, you must first resolve the actual path to the file.

1. Locate the Config File: Use ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.PerUserRoamingProfile)` or similar to find the physical path of the user.config file.
2. Implement FileSystemWatcher: Initialize a System.IO.FileSystemWatcher targeting the directory containing the config file, filtering for the specific filename.
3. Trigger Reload: In the Changed event of the watcher, call My.Settings.Reload().

' Example logic for the Changed event
Private Sub ConfigWatcher_Changed(sender As Object, e As FileSystemEventArgs)
    ' Note: FileSystemWatcher can fire multiple times for one save
    My.Settings.Reload()
End Sub

Verification and Safety

To verify this behavior, manually edit the user.config file in a text editor while the application is running. Call My.Settings.Reload() and inspect the properties via the debugger to confirm the cache has updated.

Warning: Using a FileSystemWatcher in a multi-instance environment can lead to race conditions. If two instances are both watching the file and calling Save and Reload rapidly, you may encounter IOException errors due to file locking.

To provide a more specific concurrency strategy, please specify if your application instances are running on the same machine or across a network share.