C# Dynamic Event Subscription

Question

How would you dynamically subscribe to a C# event so that given a Object instance and a String name containing the name of the event, you subscribe to that event and do something (write to the console for example) when that event has been fired?

It would seem using Reflection this isn't possible and I would like to avoid having to use Reflection.Emit if possible, as this currently (to me) seems like the only way of doing it.

/EDIT: I do not know the signature of the delegate needed for the event, this is the core of the problem

/EDIT 2: Although delegate contravariance seems like a good plan, I can not make the assumption necessary to use this solution

Willmert

Answer 1

You can compile expression trees to use void methods without any arguments as event handlers for events of any type. To accommodate other event handler types, you have to map the event handler's parameters to the events somehow.

 using System;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;

 class ExampleEventArgs : EventArgs
 {
    public int IntArg {get; set;}
 }

 class EventRaiser
 { 
     public event EventHandler SomethingHappened;
     public event EventHandler<ExampleEventArgs> SomethingHappenedWithArg;

     public void RaiseEvents()
     {
         if (SomethingHappened!=null) SomethingHappened(this, EventArgs.Empty);

         if (SomethingHappenedWithArg!=null) 
         {
            SomethingHappenedWithArg(this, new ExampleEventArgs{IntArg = 5});
         }
     }
 }

 class Handler
 { 
     public void HandleEvent() { Console.WriteLine("Handler.HandleEvent() called.");}
     public void HandleEventWithArg(int arg) { Console.WriteLine("Arg: {0}",arg);    }
 }

 static class EventProxy
 { 
     //void delegates with no parameters
     static public Delegate Create(EventInfo evt, Action d)
     { 
         var handlerType = evt.EventHandlerType;
         var eventParams = handlerType.GetMethod("Invoke").GetParameters();

         //lambda: (object x0, EventArgs x1) => d()
         var parameters = eventParams.Select(p=>Expression.Parameter(p.ParameterType,"x"));
         var body = Expression.Call(Expression.Constant(d),d.GetType().GetMethod("Invoke"));
         var lambda = Expression.Lambda(body,parameters.ToArray());
         return Delegate.CreateDelegate(handlerType, lambda.Compile(), "Invoke", false);
     }

     //void delegate with one parameter
     static public Delegate Create<T>(EventInfo evt, Action<T> d)
     {
         var handlerType = evt.EventHandlerType;
         var eventParams = handlerType.GetMethod("Invoke").GetParameters();

         //lambda: (object x0, ExampleEventArgs x1) => d(x1.IntArg)
         var parameters = eventParams.Select(p=>Expression.Parameter(p.ParameterType,"x")).ToArray();
         var arg    = getArgExpression(parameters[1], typeof(T));
         var body   = Expression.Call(Expression.Constant(d),d.GetType().GetMethod("Invoke"), arg);
         var lambda = Expression.Lambda(body,parameters);
         return Delegate.CreateDelegate(handlerType, lambda.Compile(), "Invoke", false);
     }

     //returns an expression that represents an argument to be passed to the delegate
     static Expression getArgExpression(ParameterExpression eventArgs, Type handlerArgType)
     {
        if (eventArgs.Type==typeof(ExampleEventArgs) && handlerArgType==typeof(int))
        {
           //"x1.IntArg"
           var memberInfo = eventArgs.Type.GetMember("IntArg")[0];
           return Expression.MakeMemberAccess(eventArgs,memberInfo);
        }

        throw new NotSupportedException(eventArgs+"->"+handlerArgType);
     }
 }


 static class Test
 {
     public static void Main()
     { 
        var raiser  = new EventRaiser();
        var handler = new Handler();

        //void delegate with no parameters
        string eventName = "SomethingHappened";
        var eventinfo = raiser.GetType().GetEvent(eventName);
        eventinfo.AddEventHandler(raiser,EventProxy.Create(eventinfo,handler.HandleEvent));

        //void delegate with one parameter
        string eventName2 = "SomethingHappenedWithArg";
        var eventInfo2 = raiser.GetType().GetEvent(eventName2);
        eventInfo2.AddEventHandler(raiser,EventProxy.Create<int>(eventInfo2,handler.HandleEventWithArg));

        //or even just:
        eventinfo.AddEventHandler(raiser,EventProxy.Create(eventinfo,()=>Console.WriteLine("!")));  
        eventInfo2.AddEventHandler(raiser,EventProxy.Create<int>(eventInfo2,i=>Console.WriteLine(i+"!")));

        raiser.RaiseEvents();
     }
 }

Thermoscope

Hell, expression trees are so cool. I wrote similar code once via Reflection.Emit. What a pain. – Chrissa

Great piece of code. Can you maybe show how to change it to support arguments? I changed it to get the method with the arguments, but I get "variable 'x' of type 'System.String' referenced from scope '', but it is not defined" when I try to create the delegate. Thanks – Galatians

Done—I added another example. – Thermoscope

So how would this work for the normal pattern of Eventhandler: (object sender, Eventargs e)? – Freemanfreemartin

You would have to add an overload, Delegate Create<T1,T2>(EventInfo evt, Action<T1,T2> d). – Thermoscope

Say I have the below in my "Observable" eventInfo2.AddEventHandler(raiser,EventProxy.Create<int>(eventInfo2,i=>Console.WriteLine(i+"!"))); How exactly do I pass the arguments? I assume I pass it to Creat()? But what happens to it from there? THanks. – Blowing

Pass it to SomethingHappenedWithArg() – Thermoscope

Answer 2

It's not a completely general solution, but if all your events are of the form void Foo(object o, T args) , where T derives from EventArgs, then you can use delegate contravariance to get away with it. Like this (where the signature of KeyDown is not the same as that of Click) :

    public Form1()
    {
        Button b = new Button();
        TextBox tb = new TextBox();

        this.Controls.Add(b);
        this.Controls.Add(tb);
        WireUp(b, "Click", "Clickbutton");
        WireUp(tb, "KeyDown", "Clickbutton");
    }

    void WireUp(object o, string eventname, string methodname)
    {
        EventInfo ei = o.GetType().GetEvent(eventname);

        MethodInfo mi = this.GetType().GetMethod(methodname, BindingFlags.Public | BindingFlags.Instance | BindingFlags.NonPublic);

        Delegate del = Delegate.CreateDelegate(ei.EventHandlerType, this, mi);

        ei.AddEventHandler(o, del);

    }
    void Clickbutton(object sender, System.EventArgs e)
    {
        MessageBox.Show("hello!");
    }

Closefisted

Answer 3

It is possible to subscribe to an event using Reflection

var o = new SomeObjectWithEvent;
o.GetType().GetEvent("SomeEvent").AddEventHandler(...);

http://msdn.microsoft.com/en-us/library/system.reflection.eventinfo.addeventhandler.aspx

Now here is going to be the problem that you are going to have to solve. The delegates required for each event handler will have different signatures. You are going to have to find away to create these methods dynamically, which probably means Reflection.Emit, or you are going to have to limit your self to a certain delegate so that you can handle it with compiled code.

Hope this helps.

Lesialesion

Answer 4

public TestForm()
{
    Button b = new Button();

    this.Controls.Add(b);

    MethodInfo method = typeof(TestForm).GetMethod("Clickbutton",
    BindingFlags.NonPublic | BindingFlags.Instance);
    Type type = typeof(EventHandler);

    Delegate handler = Delegate.CreateDelegate(type, this, method);

    EventInfo eventInfo = cbo.GetType().GetEvent("Click");

    eventInfo.AddEventHandler(b, handler);

}

void Clickbutton(object sender, System.EventArgs e)
{
    // Code here
}

Depressor

Answer 5

Try LinFu--it has a universal event handler that lets you bind to any event at runtime. For example, here's you you can bind a handler to the Click event of a dynamic button:

// Note: The CustomDelegate signature is defined as:
// public delegate object CustomDelegate(params object[] args);
CustomDelegate handler = delegate
                         {
                           Console.WriteLine("Button Clicked!");
                           return null;
                         };

Button myButton = new Button();
// Connect the handler to the event
EventBinder.BindToEvent("Click", myButton, handler);

LinFu lets you bind your handlers to any event, regardless of the delegate signature. Enjoy!

You can find it here: http://www.codeproject.com/KB/cs/LinFuPart3.aspx

Accomplish

Answer 6

I recently wrote a series of blog posts describing unit testing events, and one of the techniques I discuss describes dynamic event subscription. I used reflection and MSIL (code emitting) for the dynamic aspects, but this is all wrapped up nicely. Using the DynamicEvent class, events can be subscribed to dynamically like so:

EventPublisher publisher = new EventPublisher();

foreach (EventInfo eventInfo in publisher.GetType().GetEvents())
{
    DynamicEvent.Subscribe(eventInfo, publisher, (sender, e, eventName) =>
    {
        Console.WriteLine("Event raised: " + eventName);
    });
}

One of the features of the pattern I implemented was that it injects the event name into the call to the event handler so you know which event has been raised. Very useful for unit testing.

The blog article is quite lengthy as it is describing an event unit testing technique, but full source code and tests are provided, and a detailed description of how dynamic event subscription was implemented is detailed in the last post.

http://gojisoft.com/blog/2010/04/22/event-sequence-unit-testing-part-1/

Georgia

@Robert Cheers for commenting, we had some outage yesterday after patching our web server. – Georgia

Answer 7

What you want can be achieved using dependency injection. For example Microsoft Composite UI app block does exactly what you described

Haversine

Answer 8

This method adds to an event, a dynamic handler that calls a method OnRaised, passing the event parameters as an object array:

void Subscribe(object source, EventInfo ev)
{
    var eventParams = ev.EventHandlerType.GetMethod("Invoke").GetParameters().Select(p => Expression.Parameter(p.ParameterType)).ToArray();
    var eventHandler = Expression.Lambda(ev.EventHandlerType,
        Expression.Call(
            instance: Expression.Constant(this),
            method: typeof(EventSubscriber).GetMethod(nameof(OnRaised), BindingFlags.NonPublic | BindingFlags.Instance),
            arg0: Expression.Constant(ev.Name),
            arg1: Expression.NewArrayInit(typeof(object), eventParams.Select(p => Expression.Convert(p, typeof(object))))),
        eventParams);
    ev.AddEventHandler(source, eventHandler.Compile());
}

OnRaised has this signature:

void OnRaised(string name, object[] parameters);

Philo

Answer 9

After some time we got this decision:

[Inject] private DiContainer di;

    public void SubscribeEvent(string className, string eventName)
    {
        Type type = GetTypeByName(className);
        object t = di.Resolve(type);
        EventInfo ev = t.GetType().GetEvent(eventName);
        ev.AddEventHandler(t,(Action<int>)D);
    }
    
    private void D(int i)
    {
        Debug.Log($"Event reached {i}");
    }

Accrue

Thank you for your interest in contributing to the Stack Overflow community. This question already has quite a few answers—including one that has been extensively validated by the community. Are you certain your approach hasn’t been given previously? If so, it would be useful to explain how your approach is different, under what circumstances your approach might be preferred, and/or why you think the previous answers aren’t sufficient. Can you kindly edit your answer to offer an explanation? – Haemato

Answer 10

Do you mean something like:

//reflect out the method to fire as a delegate
EventHandler eventDelegate = 
   ( EventHandler ) Delegate.CreateDelegate(
       typeof( EventHandler ),    //type of event delegate
       objectWithEventSubscriber, //instance of the object with the matching method
       eventSubscriberMethodName, //the name of the method
       true );

This doesn't do the subscription, but will give to the method to call.

Edit:

Post was clarified after this answer, my example won't help if you don't know the type.

However all events in .Net should follow the default event pattern, so as long as you've followed it this will work with the basic EventHandler.

Waddle