How to dynamically subscribe to an event?

Question

RELATED

• Watch control to determine events being fired?

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I need to detect when an event is fired. To do so I am trying to subscribe dynamically on the event.

The problem is that I have different types of delegates, not all events have the same signature. The solutions provided in here and here expects object sender, EventArgs e, which I am not using, so I get an exception telling the types doesn't match.

Here are some examples of a delegates I have:

public delegate void OnEventA(int id);
public delegate void OnEventB(double num, string name);

How can I create the correct delegate?

Answer 1

EventInfo has a method AddEventHandler that you can use. If you don't have a delegate instance, then you can create a delegate dynamically using Delegate.CreateDelegate:

var eh = Delegate.CreateDelegate(ei.EventHandlerType, target, methodInfo);
ei.AddEventHandler(owner, eh);

In this example target is the target object for the delegate and methodInfo is a MethodInfo of a method in the target object. Finally, owner is the object where the event ei belongs.

Answer 2

After some research I found some articles:

• How to: Hook Up a Delegate Using Reflection
• Delegate.CreateDelegate Method (Type, Object, MethodInfo, Boolean)

It helped me to understand what I was trying to do and I should do.

I need to use Delegate.CreateDelegate passing the EventHandlerType (the type of the event, the delegate), a instance of a class and the method info of the method (from the class in the previous parameter) that will handle the event. Target is the control that fires this event.

Delegate handler = Delegate.CreateDelegate(evt.EventHandlerType, abc, mi1, false);
evt.AddEventHandler(target, handler);

Further digging lead me to this method. I can subscribe to events using lambda expression. Using Action<T> I can subscribe with different types and numbers of parameters.

public static Delegate Create<T>(EventInfo e, Action<T> a)
{
    var parameters = e.EventHandlerType.GetMethod("Invoke").GetParameters().Select(p => Expression.Parameter(p.ParameterType, "p")).ToArray();
    var exp = Expression.Call(Expression.Constant(a), a.GetType().GetMethod("Invoke"), parameters);
    var l = Expression.Lambda(exp, parameters);
    return Delegate.CreateDelegate(e.EventHandlerType, l.Compile(), "Invoke", false);
}

Using this method (e is the EventInfo; EventManager is the class with the static method above)

e.AddEventHandler(this, EventManager.Create<int>(e, (x) => Console.WriteLine("Execute")));