Miguel posted a nice example of how to use Gtk# from Java using IKVM/Mono on his blog. In response Pablo posted a question to the Mono list and Jonathan Pryor replied with a nice explanation of how delegates are handled to the IKVM and Mono lists (quoted with permission, slightly edited):

From: Jonathan Pryor
Sent: Friday, March 19, 2004 02:35
To: Pablo Baena
Cc: Miguel de Icaza; mono-list@lists.ximian.com; ikvm-developers@lists.sourceforge.net
Subject: Re: [Mono-list] Java and C#

Below...

On Thu, 2004-03-18 at 16:07, Pablo Baena wrote:
> Miguel: I saw your blog about IKVM. One thing I haven't been able to 
> investigate is, how useful can be Gtk# with Java. Because, for example, I 
> couldn't find a clue on how to attach a Java 'listener' to a C# event, or any 
> way to use attributes in Java.

They really need to document this better...

However, grepping through the ikvm.zip file (from their website), we
see:

// file: classpath/java/lang/VMRuntime.java
cli.System.AppDomain.get_CurrentDomain().add_ProcessExit (
  new cli.System.EventHandler (
    new cli.System.EventHandler.Method () {
      public void Invoke (Object sender, cli.System.EventArgs e) {
        Runtime.getRuntime().runShutdownHooks();
      }
    }
  )
);

>From this (and prior knowledge), we can draw the following statements:

1. Properties are actually functions with `get_' and `set_' prefixed to
them. Thus C# property System.AppDomain.CurrentDomain is the static
Java function cli.System.AppDomain.get_CurrentDomain().

2. Events are actually functions with `add_' and `remove_' prefixed to
their name. Thus C# event System.AppDomain.ProcessExit is the static
Java function cli.System.AppDomain.add_ProcessExit().

3. There is no equivalent to C# delegates in Java, so these are
translated into a class + interface pair. The EventHandler class is the
standard C# type name (cli.System.EventHandler), which takes as an
argument an interface to invoke, named "cli." + C# delegate type name +
".Method", hence cli.System.EventHandler.Method. The EventHandler.Method
interface has a function Invoke() which must be implemented, and this
method will be invoked when the event is signaled.

I suspect that there is no way to add attributes in Java. Microsoft's
Visual J# permits the use of Attributes (IIRC), but it's through their
Visual J++ syntax -- through a specially formed JavaDoc comment. 
Something like (from memory):

/**
* @attribute-name (args...)
*/
public void myMethod () {/* ... */}

Of course, that's compiler specific, and no standard Java compiler will
support that. So when it comes to attributes, you're probably up the
creek.

- Jon

I replied saying that I believe that the attribute construct in JDK 1.5 can probably be used to expose .NET attributes to Java (and use them in Java code that is target to run on IKVM).