C# Exception in class library - event handler-CodePudding

I'm developing a class library which uses a Form to catch specific WndProc messages and process them afterwards. Processing includes file operations. The library then publishes the processed values as public properties or events.

Question: What do I do when then processing throws a exception?
If I'll let it bubble up to the caller, where does it need to be catched? Since it's not a function/method which needs to be called.

Library Class:

 public event EventHandler<CmmStateChangedEventArgs> CmmStateChanged;
 public event EventHandler<MeasurementInfoEventArgs> MeasurementInfoChanged;
 public event EventHandler<MeasurementPlanInfoEventArgs> MeasurementPlanInfoChanged;

 public MeasurementPlanInfo MeasurementPlanInfo { get; private set; }
 public MeasurementInfo MeasurementInfo { get; private set; }
 public Status CMMStatus { get; private set; }

 // event handler for internal form WndProc event
 // _messageForm is a empty invisible form which catches specific WndProc messages and invokes a event if necessary
 private async void _messageForm_CMMStateChanged(object sender, CmmStateChangedEventArgs e)
    {
        // this could throw
        var command = await _stateManager.GetCommandFileAsync(Configuration.CMMObserverFolderPath);
        var observer = await _stateManager.GetObserverFileAsync(Configuration.CMMObserverFolderPath);

        var measInfo = new MeasurementInfo();

        switch (e.Status)
        {
            case Status.Running:
                if (command.state == "set_cnc_start")
                {
                    // this could throw if observer is null
                    measInfo = new MeasurementInfo()
                    {
                        MeasurementPlanId = observer.planid,
                        ControllerType = observer.controllertyp,
                        DeviceGroup = observer.devicegroup,
                        FirmwareRevision = observer.firmWareRevision,
                        OperatorId = observer.operid,
                        PartNumber = observer.partnbinc,
                        MeasurementPlanFileName = command.planPath,
                    };

                    MeasurementInfo = measInfo;
                    MeasurementInfoChanged?.Invoke(this, new MeasurementInfoEventArgs() { MeasurementInfo = measInfo });

                }
                break;

            case Status.Finished:
                measInfo = new MeasurementInfo()
                {
                    MeasurementPlanId = observer.planid,
                    ControllerType = observer.controllertyp,
                    DeviceGroup = observer.devicegroup,
                    FirmwareRevision = observer.firmWareRevision,
                    OperatorId = observer.operid,
                    PartNumber = observer.partnbinc,
                    ChrFilePath = command.chrPath,
                    HdrFilePath = command.hdrPath,
                    FetFilePath = command.fetPath
                };

                MeasurementInfo = measInfo;
                MeasurementInfoChanged?.Invoke(this, new MeasurementInfoEventArgs() { MeasurementInfo = measInfo });
                break;

            case Status.Paused:
            case Status.Stopped:
            case Status.Exception:
            default:
                break;
        }

        CMMStatus = e.Status;
        CmmStateChanged?.Invoke(this, new CmmStateChangedEventArgs() { Status = e.Status });
    }

Calling Class:

    public MainForm()
    {
        InitializeComponent();

        apiConfiguration = new ApiConfiguration();

        // where catch possible exception ?
        api = new Calypso()
            .Configure(apiConfiguration)
            .Initialize();

        api.CmmStateChanged  = Api_CmmStateChanged;
        api.MeasurementInfoChanged  = Api_MeasurementInfoChanged;
        api.MeasurementPlanInfoChanged  = Api_MeasurementPlanInfoChanged;
    }

CodePudding user response：

It depends. Which scope can interpret the exception and take action (API or caller)? What is being thrown? Does the exception, when thrown, mean that the caller's state is now out-of-sync and things are about to get bad? Could the API try to fix such a problem? Will the next working change event give the caller a full recovery? Is the caller potentially interested in these "periodic glitches(?)" - perhaps add a message missed event.

If the the exception is a critical failure, and the question is where to handle this in the caller: I recommend catching the exception and adding a failure event, then the caller can respond the best it can. Alternatively, the initialization could accept a callback action to call.