Wednesday, April 28, 2010

Subscribe to events using reflection

·When you use reflection to load and run assemblies, you cannot use language features like the C# += operator or the Visual Basic AddHandler statement to hook up events. This MSDN document (http://msdn.microsoft.com/en-us/library/ms228976.aspx) shows how to link an existing method to an event by getting all the necessary types through reflection and how to create a dynamic method using reflection emit and link it to an event.

A sample:

   1:          // Load an assembly, for example using the Assembly.Load
   2:          // method. In this case, the executing assembly is loaded, to
   3:          // keep the demonstration simple.
   4:          //
   5:          Assembly assem = Assembly.GetExecutingAssembly();
   6:   
   7:          // Get the type that is to be loaded, and create an instance 
   8:          // of it. Activator.CreateInstance has other overloads, if
   9:          // the type lacks a default constructor. The new instance
  10:          // is stored as type Object, to maintain the fiction that 
  11:          // nothing is known about the assembly. (Note that you can
  12:          // get the types in an assembly without knowing their names
  13:          // in advance.)
  14:          //
  15:          Type tExForm = assem.GetType("ExampleForm");
  16:          Object exFormAsObj = Activator.CreateInstance(tExForm);
  17:   
  18:          // Get an EventInfo representing the Click event, and get the
  19:          // type of delegate that handles the event.
  20:          //
  21:          EventInfo evClick = tExForm.GetEvent("Click");
  22:          Type tDelegate = evClick.EventHandlerType;
  23:   
  24:          // If you already have a method with the correct signature,
  25:          // you can simply get a MethodInfo for it. 
  26:          //
  27:          MethodInfo miHandler = 
  28:              typeof(Example).GetMethod("LuckyHandler", 
  29:                  BindingFlags.NonPublic | BindingFlags.Instance);
  30:              
  31:          // Create an instance of the delegate. Using the overloads
  32:          // of CreateDelegate that take MethodInfo is recommended.
  33:          //
  34:          Delegate d = Delegate.CreateDelegate(tDelegate, this, miHandler);
  35:   
  36:          // Get the "add" accessor of the event and invoke it late-
  37:          // bound, passing in the delegate instance. This is equivalent
  38:          // to using the += operator in C#, or AddHandler in Visual
  39:          // Basic. The instance on which the "add" accessor is invoked
  40:          // is the form; the arguments must be passed as an array.
  41:          //
  42:          MethodInfo addHandler = evClick.GetAddMethod();
  43:          Object[] addHandlerArgs = { d };
  44:          addHandler.Invoke(exFormAsObj, addHandlerArgs);


Some related links:


1 comment:

Aina said...

if i want the assembly loaded runs in other thread... how i do all this? (it can be a blocking app, like waiting for sockect communication).
thanks