Reflection - Add a Delegate to another Delegate

问题:

i am attempting to attach a Delegate to an invocation list of a different delegate. By that i am achieving a kind of Hook on existing events. I need to hook up something that runs after each event that is invoked.

The following example works as long as the Delegate exposed by the type and the Action i pass in have the exact same signature. (On1 and OnAll events are both declared with an Action delegate so it works).

Code : How i hook up an Action with an existing delegate exposed by an event modifier.

public static class ReflectionExtensions
{
    public static IEnumerable<EventInfo> GetEvents(this object obj)
    {
        var events = obj.GetType().GetEvents();
        return events;
    }

    public static void AddHandler(this object obj, Action action)
    {
        var events = obj.GetEvents();
        foreach (var @event in events)
        {                    
             @event.AddEventHandler(obj, action);
        }
    }
}

The Sample :

public class Tester 
{
    public event Action On1;
    public event Action On2;

    public void RaiseOn1()
    {
        On1();
    }

    public void RaiseOn2()
    {
        On2();
    }
}   

class Program
{
    static void Main(string[] args)
    {
        var t = new Tester();
        t.On1 += On1;
        t.On2 += On2;

        t.AddHandler(OnAll);

        t.RaiseOn1();
        t.RaiseOn2();
    }

    public void On1() { }
    public void On2() { }
    public void OnAll() { }
} 

The Problem : When the Delegate exposed with an event modifier in Tester does not have the same signature i get a well wanted and obvious exception which states (in my words) that Action can't be added to an invocation list of an Action<int> . makes sense.

Just to be clear I'm describing the following :

    public event Action<int> On1;    
    public void On1(int i){}

What I'm looking for is a way to create another Delegate of the same type as the EventHandlerType. In order to do that i need to create a method with the signature i of EventHandlerType which would internally invoke action.

something like :

 public static void AddHandler(this object obj, Action action)
 {
      var events = obj.GetEvents();
      foreach (var @event in events)
      {
          // method with the signeture of EventHandlerType which does action();
          MethodInfo wrapperMethod = WrapAction(@event.EventHandlerType, action);

          Delegate handler = Delegate.CreateDelegate(@event.EventHandlerType, action.Target, wrapperMethod);
          @event.AddEventHandler(obj, handler);
      }
 }

回答1:

This seems to work... There are various comments inside... I'm not sure if this is the best way to do it. I'm building an Expression tree to do the delegate invocation.

public static void AddHandler(this object obj, Action action)
{
    var events = obj.GetEvents();

    foreach (var @event in events)
    {
        // Simple case
        if (@event.EventHandlerType == typeof(Action))
        {
            @event.AddEventHandler(obj, action);
        }
        else
        {
            // From here: http://stackoverflow.com/a/429564/613130
            // We retrieve the parameter types of the event handler
            var parameters = @event.EventHandlerType.GetMethod("Invoke").GetParameters();

            // We convert it to ParameterExpression[]
            ParameterExpression[] parameters2 = Array.ConvertAll(parameters, x => Expression.Parameter(x.ParameterType));

            MethodCallExpression call;

            // Note that we are "opening" the delegate and using
            // directly the Target and the Method! Inside the 
            // LambdaExpression we will build there won't be a 
            // delegate call, there will be a method call!
            if (action.Target == null)
            {
                // static case
                call = Expression.Call(action.Method);
            }
            else
            {
                // instance type
                call = Expression.Call(Expression.Constant(action.Target), action.Method);
            }

            // If you are OK to create a delegate that calls another
            // delegate, you can:
            // call = Expression.Call(Expression.Constant(action), typeof(Action).GetMethod("Invoke"));
            // instead of the big if/else

            var lambda = Expression.Lambda(@event.EventHandlerType, call, parameters2);
            @event.AddEventHandler(obj, lambda.Compile());
        }
    }
}