Question

I have a problem with some code I need to refactor. Right now it uses lambdas as event handlers, but they are not removed properly. From what I have read, this is not even possible? Anyway I would like to rewrite it to use a delegate instead of an anonymous function, and now my problem is that right now it takes an action as parameter, and I can’t seem to figure out how to pass the action on to my new delegate. This is the code:

void RetrieveData(
            int pointId,
            int? chartCollectionId,
            Action action)
        {
            if (pointId <= 0)
                throw new ArgumentException("PointId not valid");

            LastPointId = NextPointId;
            NextPointId = pointId;

            Clear();

            _csr = new CustomerServiceRepository();

            _csr.ServiceClient.GetChartDataCompleted += (se, ea) =>
                                                            {
                                                                _cachedCharts = ea.Result;
                                                                ChartDataRetrieved(ea.Result);
                                                                if (action != null)
                                                                    action.Invoke();
                                                                _csr = null;
                                                            };
            _csr.ServiceClient.GetChartDataAsync(
                Settings.Current.Customer.CustomerName,
                pointId,
                chartCollectionId);

            _csr.ServiceClient.GetChartDataCompleted -= (se, ea) => //remove after usage
                                                            {
                                                                _cachedCharts = ea.Result;
                                                                ChartDataRetrieved(ea.Result);
                                                                if (action != null)
                                                                    action.Invoke();
                                                                _csr = null;
                                                            };
        }

I was thinking that maybe I could create the following:

public class extendedEventArgs : GetChartDataCompletedEventArgs
        {
            Action foo { get; set; }
        }

        void tang(object sender, extendedEventArgs e)
        {
            _cachedCharts = e.Result;
            ChartDataRetrieved(e.Result);
            if (action != null)
                action.Invoke();
            _csr = null;
        }

And the pass the action as a parameter in the extended event args, but when I try to use it like this

_csr.ServiceClient.GetChartDataCompleted += new EventHandler<extendedEventHandler>(tang);

It gives an error:

Cannot implicitly convert type  System.EventHandler<Conwx.Net.Client.CustomerClient.Controls.ChartControls.ChartListForecast.extendedEventArgs>'  to  System.EventHandler<Conwx.Net.Client.Framework.CustomerServiceReference.GetChartDataCompletedEventArgs>'

What am I doing wrong here? Alternative solutions are also welcome.

.K

Answer 1

As I read it, the key problem here is not being able to remove the handler; if so, all you need it to store the delegate (where in the below, YourDelegateType is meant to mean: the defined type of GetChartDataCompleted):

YourDelegateType handler = (se, ea) =>
    {
        _cachedCharts = ea.Result;
        ChartDataRetrieved(ea.Result);
        if (action != null)
            action.Invoke();
        _csr = null;
    };
_csr.ServiceClient.GetChartDataCompleted += handler;
...
_csr.ServiceClient.GetChartDataCompleted -= handler;

You can also make it self-unsubscribing (i.e. so that it unsubscribes when the event is raised):

YourDelegateType handler = null;
handler = (se, ea) =>
    {
        _cachedCharts = ea.Result;
        ChartDataRetrieved(ea.Result);
        if (action != null)
            action.Invoke();
        _csr.ServiceClient.GetChartDataCompleted -= handler;
        _csr = null;
    };
_csr.ServiceClient.GetChartDataCompleted += handler;