Using Reflection.Emit to implement a interface

Question

Let's say that I have the following interface:

public interface IMyService
{
   void SimpleMethod(int id);
   int Hello(string temp);

}

And want to generate a class that looks like this (using reflection emit).

public class MyServiceProxy : IMyService
{
  IChannel _channel;

  public MyServiceProxy(IChannel channel)
  {
    _channel = channel;
  }

  public void SimpleMethod(int id)
  {
    _channel.Send(GetType(), "SimpleMethod", new object[]{id});
  }

  public int Hello(string temp)
  {
    return (int)_channel.Request(temp);
  }
}

How do I do it? I've checked various dynamic proxies and mock frameworks. They are bit complex and not very easy to follow (and I do not want an external dependency). It shouldn't be that hard to generate a proxy for an interface. Can anyone show me how?

Answer 1

All in all I'm going to agree with others' comments. I've used Castle's DynamicProxy and I think it's wonderful. You can do some really amazing and powerful stuff with it. That said, if you're still considering writing your own, read on:

If you're not excited about emitting IL, there are some new techniques using Lambda expressions that you can use to generate code. None of this is a trivial task, however.

• http://stackoverflow.com/questions/45779/c-dynamic-event-subscription
• http://stackoverflow.com/questions/939956/howto-emit-a-delegate-or-lambda-expression

Here's an example of how I've used Lambda expressions to generate a dynamic event handler for any .NET event. You could use a similar technique to generate a dynamic interface implementation.

    public delegate void CustomEventHandler(object sender, EventArgs e, string eventName);

    Delegate CreateEventHandler(EventInfo evt, CustomEventHandler d)
    {
        var handlerType = evt.EventHandlerType;
        var eventParams = handlerType.GetMethod("Invoke").GetParameters();

        //lambda: (object x0, EventArgs x1) => d(x0, x1)

        // This defines the incoming parameters of our dynamic method.  
        // The method signature will look something like this:
        // void dynamicMethod(object x0, EventArgs<T> x1)
        // Each parameter is dynamically determined via the 
        // EventInfo that was passed.
        var parameters = eventParams.Select((p, i) => Expression.Parameter(p.ParameterType, "x" + i)).ToArray();

        // Get the MethodInfo for the method we'll be invoking *within* our
        // dynamic method.  Since we already know the signature of this method,
        // we supply the types directly.
        MethodInfo targetMethod = d.GetType().GetMethod(
            "Invoke", 
            new Type[] { typeof(object), typeof(EventArgs), typeof(string) }
            );

        // Next, we need to convert the incoming parameters to the types
        // that are expected in our target method.  The second parameter,
        // in particular, needs to be downcast to an EventArgs object
        // in order for the call to succeed.
        var p1 = Expression.Convert(parameters[0], typeof(object));
        var p2 = Expression.Convert(parameters[1], typeof(EventArgs));
        var p3 = Expression.Constant(evt.Name);

        // Generate an expression that represents our method call.  
        // This generates an expression that looks something like:
        // d.Invoke(x0, x1, "eventName");
        var body = Expression.Call(
            Expression.Constant(d),
            targetMethod,
            p1,
            p2,
            p3
        );

        // Convert the entire expression into our shiny new, dynamic method.
        var lambda = Expression.Lambda(body, parameters.ToArray());

        // Convert our method into a Delegate, so we can use it for event handlers.
        return Delegate.CreateDelegate(handlerType, lambda.Compile(), "Invoke", false);
    }

Regards,

-Doug