gistfile1.cs

// Provide an argument to a "multiplexing" event
class EventDispatcher {
  public delegate void EventHandler(MessageEvent e);
  public event EventHandler MultiplexedEvent;
}

class MessageEvent {
  int MessageType; // Guess what component I'm working on which does something very similar to this
  object Data;
}

EventDispatcher dispatcher = new EventDispatcher();

// Current semantics
dispatched.MultiplexedEvent +=
  (e) => Diagnostics.Trace.WriteLine(e.MessageType == 1 ? "Message type 1 handled." : "Message type 1 not received".);
  dispatched.MultiplexedEvent +=
  (e) => Diagnostics.Trace.WriteLine(e.MessageType == 2 ? "Message type 2 handled." : "Message type 2 not received".);
  
// Desired semantics
dispatched.MultiplexedEvent(1) += (e) => Diagnostics.Trace.WriteLine("Message type 1 handled.");
dispatched.MultiplexedEvent(2) += (e) => Diagnostics.Trace.WriteLine("Message type 1 handled.");

// Of course it would be most awesome to do this, but I recognize it's really only possible in God's language, JavaScript
dispatched.MessageType1 += (e) => ...
dispatched.MessageType2 += (e) => ...

Actually you can do that in c# using dynamic types, probably....