Problem

Issue #937 reported that the Microsoft Store version 1.2.0 crashes after RDP connections, while the sideloaded package works correctly. Investigation revealed a critical race condition in the RDP client disposal logic that was exposed more frequently by the Microsoft Store version's threading characteristics.

Root Cause

The RdpClientDispose() method in AxMsRdpClient09Host.xaml.cs used asynchronous disposal via Execute.OnUIThread(), which allowed code execution to continue before the RDP client was fully disposed. This created several race conditions:

// Before fix - ASYNC disposal
Execute.OnUIThread(DisposeRdpClient);  // Schedules disposal asynchronously
// Code continues immediately, before disposal completes!

Race condition scenarios:

1. Reconnection race: ReConn() calls RdpClientDispose() then immediately attempts to create a new RDP connection before the old client is fully disposed
2. Disconnect callback race: Disconnect handler calls RdpClientDispose() then invokes OnClosed callback before disposal completes
3. Double disposal race: Multiple threads could attempt to dispose the client simultaneously without proper synchronization

The log message "RDP Host: _rdpClient.Disposed." was printing before actual disposal completed, masking the timing issue.

Solution

Changed to synchronous disposal to ensure the RDP client is fully disposed before any subsequent operations:

// After fix - SYNC disposal
Execute.OnUIThreadSync(DisposeRdpClient);  // Waits for disposal to complete
// Code continues only after disposal is done

Additionally, introduced a dedicated lock object _rdpClientDisposeLock to ensure thread-safe access during creation and disposal, replacing the lock(this) pattern which can lead to deadlocks.

Changes

• Changed Execute.OnUIThread() to Execute.OnUIThreadSync() in RdpClientDispose() for synchronous disposal
• Added private readonly object _rdpClientDisposeLock for thread-safe disposal
• Updated DisposeRdpClient(), CreateRdpClient(), and OnScreenResolutionChanged() to use the dedicated lock

Impact

These minimal, surgical changes ensure:

• RDP client is fully disposed before reconnection attempts or callbacks execute
• Thread-safe disposal prevents concurrent access crashes
• Microsoft Store version should have the same stability as the sideloaded version
• No API or behavior changes for existing functionality

Testing

Due to the WPF/Windows-specific nature of this code, testing requires a Windows environment. Recommended test scenarios:

• RDP reconnection (disconnect and reconnect)
• Rapid connection/disconnection cycles
• Alternate address switching
• Verify no regressions in normal RDP operations

Fixes #937

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