使用反射添加事件处理程序

Question

I have a .json file with so-called commands:

"Commands":[{
   "EventName": "MouseLeftButtonUp",
   "MethodToExecute": "NextJson",
   "Args": "Next.json"
},{
   "EventName": "MouseRightButtonUp",
   "MethodToExecute": "CloseApp"
}

I deserialize this json to this class:

    public class Command
    {
        [JsonPropertyName("EventName")]
        public string EventName { get; set; }

        [JsonPropertyName("MethodToExecute")]
        public string MethodToExecute { get; set; }

        [JsonPropertyName("Args")]
        public string Args { get; set; }

        /*Methods*/
    }

EventName is a name of UIElement class events. MethodToExecute is a name of method to call, when event triggered. Args are the args, passed to the MethodToExecute .

I don't want my users to be able to call any method in the application, so I don't use reflection to get MethodInfo , instead I create Dictionary : Dictionary<string, Delegate> MethodsDictionary . The key in this dictionary is the name of method ( MethodToExecute from Command class), and the value is something like this:

MethodsDictionary.Add(nameof(CloseApp), new Action(CloseApp));
MethodsDictionary.Add(nameof(NextJson), new Action<string>(NextJson));

Without using reflection, I'd added the event handler like this:

button.MouseLeftButtonUp += (sender, args) => MethodsDictionary[command.MethodToExecute].DynamicInvoke(command.Args);

But I'd like to make a dynamic binding of events. Well, of course I can make it through ugly switch-case on command.Name property, but I still would like to try the solution with reflection. The solutuion, as I see it, should looke something like:

foreach (var command in commands)
{
    command.Bind(uielement, MethodsDictionary[command.MethodToExecute]);
}

//And command.Bind method is like:

public void Bind(UIElement uielement, Delegate methodToExecute)
{
    //I know there's no such method like GetEventHandler, just an example
    var handler = uielement.GetEventHandler(EventName);
    handler += (sender, args) => methodToExecute.DynamicInvoke(Args);
}

I searched through several pretty similar questions:

Subscribe to an event with Reflection

https://social.msdn.microsoft.com/Forums/vstudio/en-US/d7f184f1-0964-412a-8659-6759a0e2db83/c-reflection-problem-subscribing-to-event?forum=netfxbcl

https://docs.microsoft.com/en-us/dotnet/framework/reflection-and-codedom/how-to-hook-up-a-delegate-using-reflection

AddEventHandler using reflection

Add Event Handler using Reflection ? / Get object of type?

But these doesn't help me to solve the problem the way I want to. I tried some of the solutions above, but they didn't work out for me, failing with different exceptions.

UPD.

I tried to implement handler binding through switch-case , as I mentioned above. It resulted in this method inside Command class:

public void Bind(UIElement element)
{
    switch (this.Name)
    {
        case "MouseRightButtonUp":
        {
            element.MouseRightButtonUp += (sender, args) => MethodsDictionary[this.MethodToExecute].DynamicInvoke(this.Args);
            break;
        }
        case "Click":
        {
            //UIElement doesn't have Click event
            var button = element as ButtonBase;
            button.Click += (sender, args) => MethodsDictionary[this.MethodToExecute].DynamicInvoke(this.Args);
            break;
        }
        /*And so on for each event*/
        default:
        {
            throw new NotSupportedException();
        }
    }
}

I don't like, how that part with adding new handlers is just a copy-paste of previous section, but I don't see another workaround in this situation. I'd like to use the reflection in this case, but I don't know if it's possible.

Answer 1

If you can't get reflection to work, you can use another Dictionary to store supported event subscriber methods:

Command.cs

public class Command
{
    [JsonPropertyName("EventName")]
    public string EventName { get; set; }

    [JsonPropertyName("MethodToExecute")]
    public string MethodToExecute { get; set; }

    [JsonPropertyName("Args")]
    public string Args { get; set; }

    /*Methods*/
}

JsonEventMapper.cs

class JsonEventMapper
{
  private Dictionary<string, Action<UIElement, EventHandler>>> SupportedEventSubscriberMap { get; }
  private Dictionary<string, EventHandler> RegisteredEventHandlerMap { get; }   

  public JsonEventMapper()
  {
    this.SupportedEventSubscriberMap = new Dictionary<string, Action<UIElement, EventHandler>>() 
    { 
      { 
        nameof(UIElement.MouseRightButtonUp), (uiElement, handler) => uiElement.MouseRightButtonUp += handler.Invoke 
      },
      { 
        nameof(UIElement.LostFocus), (uiElement, eventSubscriber) => uiElement.LostFocus += handler.Invoke 
      }
    };

    this.RegisteredEventHandlerMap = new Dictionary<string, EventHandler>()
    {
      {
        nameof(UIElement.MouseLeftButtonUp), CloseApp
      },
      {
        nameof(UIElement.LostFocus), NextJson
      }
    };
  }

  public void RegisterJsonCommands(IEnumerable<Command> commands, UIElement uiElement)
  {    
    foreach (var command in commands)
    {
      BindCommandToEvent(uiElement, command);
    }
  }

  private void BindCommandToEvent(UIElement uiElement, Command command)
  {    
    if (this.SupportedEventSubscriberMap.TryGetValue(command.EventName, out Action<UIElement, EventHandler> eventSubscriber)
      && this.RegisteredEventHandlerMap.TryGetValue(command.EventName, out EventHandler eventHandler))
    {
      eventSubscriber.Invoke(uiElement, eventHandler);
    }
  }

  private void CloseApp(object sender, EventArgs args)
  {    
    // Handle event
  }

  private void NextJson(object sender, EventArgs args)
  {    
    // Handle event
  }
}

Usage

IEnumerable<Command> commands = DeserializeJsonToCommands();
var eventMapper = new JsonEventMapper();
eventMapper.RegisterJsonCommands(commands, button);

You surely want to adjust details to your specific scenario.

Don't forget to unsubscribe from the events (eg, by defining another SupportedEventUnsubscriberMap ).

Answer 2

I think what you're trying to do is something like that:

public class CommandModel
{
    /// properties

    public void Bind(UIElement element)
    {
        EventBinder.Bind(() => MainViewModel.RunCommand(MethodToExecute, Args), element, EventName);
    }
}

public static class EventBinder
{
    public static void Bind(Action action, object eventSource, string eventName)
    {
        var eventInfo = eventSource.GetType().GetEvent(eventName);

        EventHandler eventHandler = (s, e) => action();

        eventInfo.AddEventHandler(eventSource, ConvertDelegate(eventHandler, eventInfo.EventHandlerType));
    }

    private static Delegate ConvertDelegate(Delegate originalDelegate, Type targetDelegateType)
    {
        return Delegate.CreateDelegate(
            targetDelegateType,
            originalDelegate.Target,
            originalDelegate.Method);
    }
}