# Wednesday, 31 July 2002
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I only just noticed that Stuart commented on the July 25 item:

The natural "con" that I can think of regarding your alternative implementation is that it would make it very difficult for Java code to catch System.Exception. At least it would require that code that wants to catch *any* exception would have to know that it might be running under IK.VM and explicitly catch System.Exception, where a normal Java program might expect to be able to catch Throwable.

I didn't mention this, but I would still map all usages of java.lang.Throwable to System.Exception (except in the case of the base class, when a class is derived from Throwable, it would be derived from Throwable). Sensible interop between .NET and Java exceptions is definitely something I'm aiming for.

The difficulty, it seems to me, is ensuring that you don't just have a good mapping from Java to C#, but that your mapping is fully *reversible*, and makes just as much sense going the other way. Java code expects to know exactly what exceptions are going to happen for a particular method, so you'll need to be remapping things like IOException already - and you'll need some way to ensure that any exception that's going to be thrown by a method where Java doesn't expect it gets mapped to a RuntimeException (or Error or direct Throwable subclass) with the original exception as the cause.

I'm not actually remapping exceptions like IOException, what happens is that the "native" code that implements an I/O function catches the System.IO.IOException and throws a java.io.IOException.

CLR generated exceptions get converted to the Java equivalent when they are first caught. At the moment I don't convert them back, so when .NET code calls Java code it can expect both System.NullReferenceException and java.lang.NullPointerException to be thrown (for example) depending on whether the Java code "caught" (a finally also triggers the conversion) the exception before it was propagated out the calling .NET code. This isn't very elegant, so I expect that in the future I'll be swapping the original exception back when the Java code rethrows the exception.

I suppose that your generated JAR files (described in your previous entry in response to my comment) will have to declare every method as "throws Exception", right?

To be honest, I hadn't even thought about this. My initial thought would be to make it seem (for the Java compiler) as if every .NET exception is derived from RuntimeException to get around this mess. This might cause additional complications, so I'm going to have to think about this a little more. Adding "throws Exception" doesn't appeal to me either, because that would make it very uncomfortable for the Java programmer trying to call .NET code.

Wednesday, 31 July 2002 15:29:13 (W. Europe Daylight Time, UTC+02:00)  #    Comments [2]