# SESSION_NOT_OPEN Error - Root Cause and Fix

## Problem Summary
The application was failing with `SESSION_NOT_OPEN` error when trying to start the live view camera feed:
```
Failed to start live view: SESSION_NOT_OPEN at EOSDigital.API.STAThread.Invoke(Action action) 
at EOSDigital.API.Camera.OpenSession() 
at CamBooth.App.Features.Camera.CameraService.OpenSession()
```

## Root Cause Analysis

### Issue 1: Uninitialized Camera Instance at Startup
In `App.xaml.cs`, the `ICamera` dependency is registered as a **Singleton at application startup**:
```csharp
services.AddSingleton<ICamera, Camera>();
```

This creates a single Camera instance **before any actual physical camera is connected**, and this uninitialized instance is never properly updated when cameras are detected later.

### Issue 2: Stale Camera Reference
The `CameraService` constructor receives this uninitialized `_mainCamera` instance:
```csharp
public CameraService(..., ICamera mainCamera, ICanonAPI APIHandler)
{
    this._mainCamera = mainCamera;  // This is the uninitialized singleton
    ...
}
```

### Issue 3: Incorrect Flow in ConnectCamera()
The original `ConnectCamera()` method called `OpenSession()` **before reassigning** `_mainCamera`:
```csharp
public void ConnectCamera()
{
    this.RefreshCamera();  // Gets camera list
    List<ICamera> cameraList = this._APIHandler.GetCameraList();
    if (cameraList.Any())
    {
        this.OpenSession();  // ← Uses OLD _mainCamera, not the newly detected one!
        // ...
    }
}
```

The `OpenSession()` method did try to reassign `_mainCamera`, but the error was thrown **before reaching that point** in some error conditions.

## The Fix

### Change 1: Update ConnectCamera() Method
Moved the `_mainCamera` assignment **before** calling `OpenSession()`:

```csharp
public void ConnectCamera()
{
    ErrorHandler.SevereErrorHappened += this.ErrorHandler_SevereErrorHappened;
    ErrorHandler.NonSevereErrorHappened += this.ErrorHandler_NonSevereErrorHappened;

    try
    {
        this.RefreshCamera();
        
        // Retry logic for camera detection (some systems need time to initialize)
        int maxRetries = 3;
        int retryDelay = 500; // milliseconds
        
        for (int attempt = 0; attempt < maxRetries; attempt++)
        {
            if (this.CamList != null && this.CamList.Any())
            {
                break;
            }
            
            if (attempt < maxRetries - 1)
            {
                this._logger.Info($"No cameras found on attempt {attempt + 1}/{maxRetries}, retrying in {retryDelay}ms...");
                System.Threading.Thread.Sleep(retryDelay);
                this.RefreshCamera();
            }
        }
        
        if (this.CamList == null || !this.CamList.Any())
        {
            this.ReportError("No cameras / devices found");
            throw new InvalidOperationException("No cameras / devices found after multiple attempts");
        }

        this._logger.Info($"Found {this.CamList.Count} camera(s)");
        string cameraDeviceNames = string.Join(", ", this.CamList.Select(cam => cam.DeviceName));
        this._logger.Info(cameraDeviceNames);

        // ← UPDATE _mainCamera BEFORE calling OpenSession()
        this._mainCamera = this.CamList[0];
        this._logger.Info($"Selected camera: {this._mainCamera.DeviceName}");

        this.OpenSession();
        this.SetSettingSaveToComputer();
        this.StarLiveView();
    }
    catch (Exception ex)
    {
        this._logger.Error($"Error connecting camera: {ex.Message}");
        throw;
    }
}
```

**Key improvements:**
1. **Explicit camera assignment before OpenSession()** - Ensures the correct physical camera is used
2. **Retry logic** - Handles SDK initialization delays on some systems
3. **Better error messages** - Clearer logging of the connection process

### Change 2: Simplify OpenSession() Method
Removed redundant camera reassignment and simplified validation:

```csharp
private void OpenSession()
{
    try
    {
        if (this._mainCamera == null)
        {
            throw new InvalidOperationException("Camera reference is null. Make sure ConnectCamera is called first.");
        }

        // Check if session is already open
        if (this._mainCamera.SessionOpen)
        {
            this._logger.Info($"Camera session already open for {this._mainCamera.DeviceName}");
            return;
        }

        this._logger.Info($"Opening session for camera: {this._mainCamera.DeviceName}");
        this._mainCamera.OpenSession();
        this._logger.Info("Camera session opened successfully");

        this._mainCamera.StateChanged += this.MainCamera_StateChanged;
        this._mainCamera.DownloadReady += this.MainCamera_DownloadReady;

        this.AvList = this._mainCamera.GetSettingsList(PropertyID.Av);
        this.TvList = this._mainCamera.GetSettingsList(PropertyID.Tv);
        this.ISOList = this._mainCamera.GetSettingsList(PropertyID.ISO);
    }
    catch (Exception ex)
    {
        this._logger.Error($"Failed to open camera session: {ex.Message}");
        this.ReportError($"Failed to open camera session: {ex.Message}");
        throw;
    }
}
```

**Improvements:**
1. **No redundant camera assignment** - Camera is already set by ConnectCamera()
2. **Clearer null check** - Validates that ConnectCamera() was called first
3. **Maintained duplex open check** - Still checks if session is already open

## Files Modified
- `CamBooth.App\Features\Camera\CameraService.cs`
  - `ConnectCamera()` method
  - `OpenSession()` method

## Testing Recommendations
1. **Test with camera connected** - Verify live view starts successfully
2. **Test with camera disconnected** - Verify graceful error handling with retries
3. **Test with delayed camera detection** - Ensure retry logic gives SDK time to initialize
4. **Test camera reconnection** - Disconnect and reconnect camera to verify session management

## Additional Notes
- The singleton pattern for `ICamera` in dependency injection is not ideal, but this fix ensures the correct instance is used once a physical camera is detected
- The retry logic (3 attempts, 500ms delay) can be adjusted based on system requirements
- Consider future refactoring to use factory pattern for camera initialization instead of singleton
