# Wednesday, 23 January 2008
How To Get an Explicit Interface Implementation Method

Suppose you have a Type that you know implements IEnumerable, how do you get the GetEnumerator method?

Both the developer(s) at Microsoft and the Mono developer(s) that wrote System.Xml.Serialization managed to find the same wrong answer to this question. Here's the code from Mono, but Microsoft does somethig very similar:

MethodInfo met = type.GetMethod ("GetEnumerator", Type.EmptyTypes);
if (met == null) {
  // get private implemenation
  met = type.GetMethod ("System.Collections.IEnumerable.GetEnumerator",
                        BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance,
                        null, Type.EmptyTypes, null);

The reason this is wrong is that the name of the private method is an implementation detail of the compiler that was used to compile the code. C# happens to name the private method this way, but other languages may not.

For example, try the following VB webservice:

<%@ WebService Language="VB" Class="Service1" %>

Imports System.Collections
Imports System.Web.Services

Public Class Service1
  Inherits WebService

  <WebMethod()> Public Function HelloWorld() As Frob
    Return New Frob()
  End Function
End Class

Public Class Frob
  Implements IEnumerable

  ' Note that this method is private
  Private Function GetEnumerator() As IEnumerator Implements IEnumerable.GetEnumerator
    Return "frob".GetEnumerator()
  End Function

  Public Sub Add(ByVal obj As Object)
    Throw New NotSupportedException()
  End Sub
End Class

If you run this webservice on .NET, you'll an exception inside System.Xml.Serialization.TypeScope.GetTypeDesc(), because it expects to find a public GetEnumerator method or a private System.Collections.IEnumerable.GetEnumerator method. However, VB allows you to pick the method name (in this case simply GetEnumerator). If you make the GetEnumerator method public, the webservice will work.

The Right Way

Here's the right way to get the method:

MethodInfo getEnumerator = typeof(IEnumerable).GetMethod("GetEnumerator", Type.EmptyTypes);
InterfaceMapping map = type.GetInterfaceMap(typeof(IEnumerable));
int index = Array.IndexOf(map.InterfaceMethods, getEnumerator);
MethodInfo meth = map.TargetMethods[index];

Meta Question

Of course, the real question remains unanswered. Why do we need the MethodInfo in the first place? Wouldn't Xml serialization be better off by simply using the IEnumerable interface?

Wednesday, 23 January 2008 07:37:35 (W. Europe Standard Time, UTC+01:00)  #    Comments [8]
# Monday, 14 January 2008
IKVM 0.36 Update 1 Release Candidate 1

As I've said when I released 0.36, this is a version that will be maintained for a while because it is the last version to support .NET 1.1. This means that I will strive to release updates relatively soon after bugs are reported (in the supported areas). This is the first of such update releases.


  • Changed version to
  • Fix for reflection bug on .NET generic nested types that caused this.
  • Fix for bug #1865922.
  • java.awt.image.Raster fix.

Binaries available here: ikvmbin-
Sources (+ binaries):ikvm-

The external sources are the same as the ones with the first 0.36 release + java.awt.image.Raster patch referenced above.

Monday, 14 January 2008 06:27:59 (W. Europe Standard Time, UTC+01:00)  #    Comments [0]
# Monday, 07 January 2008
New Development Snapshot

The major change is that I dropped support for .NET 1.1 and started taking advantage of .NET 2.0 specific features. The most significant result of that is that it is now possible to run (some) applications in partial trust and that IKVM.OpenJDK.ClassLibrary.dll and IKVM.Runtime.dll can be put into the GAC and called from partially trusted code. Note that if you want to put the binaries in the GAC you'll have to build your own binaries (to get them strong named) as the development snapshot binaries are not strong named.


  • .NET 1.1 is no longer supported.
  • Removed .NET 2.0 warnings (except for the "unreachable code" ones).
  • Removed most .NET 1.1 specific code.
  • Removed conditional compilation of .NET 2.0 specific code.
  • Updated FileChannelImpl to use SafeFileHandle and GC.Add|RemoveMemoryPressure.
  • Added GC.KeepAlive to "native" methods of MappedByteBuffer.
  • Improved VFS.
  • The VFS file "lib/security/cacerts" is now dynamically generated from the .NET X509 store.
  • Implemented native methods of java.io.Console (some Win32 only).
  • Implemented support for InternalsVisibleToAttribute.
  • Moved JNI implementation into a separate assembly (IKVM.Runtime.JNI.dll) to make IKVM.Runtime.dll verifiable.
  • Made all "native" method classes internal.
  • Restructured VM <-> Library interface to take advantage of InternalsVisibleTo to remove public methods and reflection usage.
  • Added support for adding constructors in map.xml.
  • Changed <clinit> method to SmartConstructorMethodWrapper to support calling it from map.xml
  • Removed call to unnecessary call to MatchTypes (to support partial trust)
  • Ignore SecurityException in CanonicalizePath.
  • Moved some calls to methods with a LinkDemand (that fails in partial trust) to a separate methods.
  • Added stuff to map.xml to remove the need for reflection in VM / Library bootstrap.
  • Inverted IKVM.Runtime.JNI dependency in stack walking code to avoid needlessly loading the JNI assembly.
  • Fixed bug that caused nested .NET generic types to report a declaring type.
  • Added -skiperror option to ikvmstub (by Kevin Grigorenko).
  • Replaced GCHandle in java.lang.ref.Reference with WeakReference to support partial trust.
  • Added check to prevent sun.misc.Unsafe.objectFieldOffset() from working on static fields.
  • Added support for registering a delegate with a DynamicTypeWrapper that gets called after the type is baked.
  • Intrinsified AtomicReferenceFieldUpdater.newUpdater().
  • Added virtual opcode to explicitly trigger class initialization from map.xml.
  • Added accessor methods for "slot" to Method & Constructor.
  • Implemented System.setIn0, setOut0, setErr0 in map.xml.
  • Hacked sun.misc.SharedSecrets in map.xml to replace Unsafe.ensureClassInitialize() with direct calls.
  • Replaced java.nio.Bits.byteOrder() in map.xml with simple System.BitConver.IsLittleEndian based implementation.
  • Disabled DynamicMethodSupport when running in partial trust.
  • Don't trigger load of JNI assembly when "loading" a fake system library.
  • Added ikvmc -platform option.
  • Added SecurityCritical and AllowPartiallyTrustedCallers annotation to IKVM.OpenJDK.ClassLibrary.dll.
  • Added SecurityCritical and AllowPartiallyTrustedCallers attributes to IKVM.Runtime.dll.


Development snapshots are intended for evaluating and keeping track of where the project is going, not for production usage. The binaries have not been extensively tested and are not strong named.

Binaries available here: ikvmbin-0.37.2928.zip

Monday, 07 January 2008 08:56:57 (W. Europe Standard Time, UTC+01:00)  #    Comments [4]
# Monday, 31 December 2007
Argh! Part 2
using System;

    GCHandle handle;

        handle = GCHandle.Alloc(this, GCHandleType.WeakTrackResurrection);

        Console.WriteLine(handle.Target != null);

    static void Main()
        new Program();

Judging from the comments, many people had trouble understanding the previous code snippet. The above code is equivalent (except that this actually works on .NET 2.0). Maybe this will help clarify.

Monday, 31 December 2007 22:18:31 (W. Europe Standard Time, UTC+01:00)  #    Comments [1]
using System;

    WeakReference wr;

        wr = new WeakReference(this, true);


    static void Main()
        new Program();

What should the output of this program be? On .NET 1.1 it's True (as expected), but on .NET 2.0 it is False. On .NET 2.0 the GC has special support for finalizing WeakReferences, but it fails to take into account whether GC.SuppressFinalize() has been called (and this may be by design, but it's still broken).

This may look academic, but it is actually a real issue in the implementation of ReferenceQueues on IKVM. Previously I used GCHandle to keep a weak reference back to the reference object that was being monitored, but because that requires full trust, I changed the code to use WeakReference, but that broke it because of the above mentioned behavior. The work around is easy, but ugly and inefficient, simply keep these weak references in a global hashtable to make sure they are always strongly reachable.

.NET really needs a better mechanism for doing post-mortem cleanup (i.e. something like java.lang.ref.ReferenceQueue).

P.S. By my new policy, I won't be filing a bug with Microsoft since they have amply demonstrated not to care about external bug reports.

Monday, 31 December 2007 14:33:49 (W. Europe Standard Time, UTC+01:00)  #    Comments [6]
# Friday, 28 December 2007
Hello World

Five and a half years ago, Hello World ran for the first time... Now, for the first time a statically compiled HelloWorld.exe runs in partial trust (LocalIntranet zone in this case). Here's an exciting screenshot:

Hello World


Obviously this doesn't mean things are done, but hopefully it won't take another five years ;-)

Friday, 28 December 2007 16:13:47 (W. Europe Standard Time, UTC+01:00)  #    Comments [0]
# Monday, 10 December 2007
IKVM 0.36 Released

After what feels like an eternity, I've finally released IKVM 0.36. The binaries are identical to the ones in rc6.

Release Notes

This document lists the known issues and incompatibilities.


  • Code unloading (aka class GC) is not supported.
  • In Java static initializers can deadlock, on .NET some threads can see uninitialized state in cases where deadlock would occur on the JVM.
  • JNI
    • Only supported in the default AppDomain.
    • Only the JNICALL calling convention is supported! (On Windows, HotSpot appears to also support the cdecl calling convention).
    • Cannot call string contructors on already existing string instances
    • A few limitations in Invocation API support
      • The Invocation API is only supported when running on .NET.
      • JNI_CreateJavaVM: init options "-verbose[:class|:gc|:jni]", "vfprintf", "exit" and "abort" are not implemented. The JDK 1.1 version of JavaVMInitArgs isn't supported.
      • JNI_GetDefaultJavaVMInitArgs not implemented
      • JNI_GetCreatedJavaVMs only returns the JavaVM if the VM was started through JNI or a JNI call that retrieves the JavaVM has already occurred.
      • DestroyJVM is only partially implemented (it waits until there are no more non-daemon Java threads and then returns JNI_ERR).
      • DetachCurrentThread doesn't release monitors held by the thread.
    • Native libraries are never unloaded (because code unloading is not supported).
  • The JVM allows any reference type to be passed where an interface reference is expected (and to store any reference type in an interface reference type field), on IKVM this results in an IncompatibleClassChangeError.
  • monitorenter / monitorexit cannot be used on unitialized this reference.
  • Floating point is not fully spec compliant.
  • Volatile fields are not marked with modopt(volatile).
  • A method returning a boolean that returns an integer other than 0 or 1 behaves differently (this also applies to byte/char/short and for method parameters).
  • Synchronized blocks are not async exception safe.
  • Ghost arrays cannot be distinguished from object arrays (isinstance and casting don't differentiate and ArrayStoreException isn't thrown).
  • Class loading is more eager than on the reference VM.
  • Object instances don't get created at new, but only at the invokespecial of the constructor. This causes observable differences with classes that have a finalizer when the constructor arg evaluation throws an exception [with the JSR-133 change in finalization, this is no longer a spec issue, but since the reference VM doesn't correctly implement JSR-133 it still is an implementation difference].
  • Interface implementation methods are never really final (interface can be reimplemented by .NET subclasses).
  • JSR-133 finalization spec change is not implemented. The JSR-133 changes dictate that an object should not be finalized unless the Object constructor has run successfully.

Static Compiler (ikvmc)

  • Some subtle differences with ikvmc compiled code for public members inherited from non-public base classes (so called "access stubs"). Because the access stub lives in a derived class, when accessing a member in a base class, the derived cctor will be run whereas java (and ikvm) only runs the base cctor.
  • Try blocks around base class ctor invocation result in unverifiable code (no known compilers produce this type of code).
  • Try/catch blocks before base class ctor invocation result in unverifiable code (this actually happens with the Eclipse compiler when you pass a class literal to the base class ctor and compile with -target 1.4).
  • Only code compiled in a single assembly fully obeys the JLS binary compability rules.
  • An assembly can only contain one resource with a particular name.
  • Passing incorrect command line options to ikvmc may result in an exception rather than a proper error messages.

Class Library

Most class library code is based on OpenJDK build 13. Below is a list of divergences and IKVM specific implementation notes.

com.sun.security.auth.module        Not implemented.
java.applet GNU Classpath implementation. Not implemented.
java.awt GNU Classpath implementation with partial System.Windows.Forms based back-end. Not supported.
java.io.Console Not implemented.
java.lang.instrument Not implemented.
java.lang.management Not implemented.
java.net No IPv6 support implemented.
java.net.ProxySelector Getting the default system proxy for a URL is not implemented.
java.security Not supported.
java.text.Bidi GNU Classpath implementation. Not supported.
java.util.zip Partially based on GNU Classpath implementation.
javax.crypto IcedTea implementation. Not supported.
javax.imageio.plugins.jpeg Not implemented.
javax.management Not implemented.
javax.print Not implemented.
javax.script Not implemented.
javax.security Not supported.
javax.smartcardio Not implemented.
javax.sound Not implemented.
javax.swing GNU Classpath implementation. Not supported.
javax.tools Not implemented.
org.ietfs.jgss Not implemented.
sun.jdbc.odbc Not implemented.
sun.net.www.content.audio Audio content handlers not implemented.
sun.net.www.content.image Image content handlers not implemented.

The entire public API is available, so "Not implemented." for javax.print, for example, means that the API is there but there is no back-end to provide the actual printing support. "Not supported." means that the code is there and probably works at least somewhat, but that I'm less likely to fix bugs reported in these areas.

In addition to the issues listed above, the extended charset providers are not included (the ones in charsets.jar in the JRE).

Specific API notes:

  • java.lang.Thread.stop(Throwable t) doesn't support throwing arbitrary exceptions on other threads (only java.lang.ThreadDeath).
  • java.lang.Thread.holdsLock(Object o) causes a spurious notify on the object (this is allowed by the J2SE 5.0 spec).
  • java.lang.String.intern() strings are never garbage collected.
  • Weak/soft references and reference queues are inefficient and do not fully implement the required semantics.
  • Threads started outside of Java aren't "visible" (e.g. in ThreadGroup.enumerate()) until they first call Thread.currentThread().
  • java.lang.Thread.getState() returns WAITING or TIMED_WAITING instead of BLOCKING when we're inside Object.wait() and blocking to re-acquire the monitor.
  • java.io.File.getCanonicalPath() is broken on Mono on Windows.
  • java.nio.channel.FileChannel.lock() shared locks are only supported on Windows NT derived operating systems.
  • java.lang.ref.SoftReference: Soft references are not guaranteed to be cleared before an OutOfMemoryError is thrown.
  • java.lang.SecurityManager: Deprecated methods not implemented: classDepth(String), inClass(String), classLoaderDepth(), currentLoadedClass(), currentClassLoader(), inClassLoader()

Supported Platforms

This release has been tested on the following CLI implementations / platforms:

CLI Implementation       Architecture      Operating System
Mono 1.2.5 x86* Windows
Mono 1.2.5 x86* Linux
.NET 1.1 x86 Windows
.NET 2.0 x86 Windows
.NET 2.0 x64 Windows
.NET 2.0 SP1 x86 Windows
.NET 2.0 SP1 x64 Windows

* Running on Mono has only been tested on x86, but Mono includes the native (platform specific) parts of IKVM, so any platform where Mono runs should in theory be able to run this IKVM release.

Partial Trust

Partial trust is not supported. The IKVM class library and runtime require Full Trust to be able to run.

Monday, 10 December 2007 08:50:56 (W. Europe Standard Time, UTC+01:00)  #    Comments [1]
# Friday, 07 December 2007
Mono Summit 2007

Last week I attended the Mono Summit 2007 in Madrid. It was a very productive three days and very much fun as well! I got to meet Zoltan, Wade, Paolo, Massi and Dick for the first time and enjoyed seeing Jb (of Cecil and monolinker fame), Rodrigo (of Boo fame) and Miguel again.

Jb created a small repro of the monolinker bug that caused it too fail on IKVM.OpenJDK.ClassLibrary.dll, so hopefully that'll be fixed soon. I also talked to him about using Cecil in ikvmc to free me from the shackles of Reflection.Emit. This is a major chunk of work, but I think it will be worth it.

Zoltan found some weirdness in my usage of DynamicMethod (the DynamicMethod signature for the constructor reflection helper had an extra unused argument, the CLR didn't mind because it matched up with the unused this reference, but Mono didn't like it). Unfortunately, fixing this didn't resolve the problems I had with DynamicMethod on Mono 1.2.5 and 1.2.6, so fast reflection will remain disabled while running on Mono for the time being (on Zoltan's current code base it did work, so when the Mono release after 1.2.6 is out I'll be able to enable it).

Rodrigo did a very interesting and entertaining presentation on Boo. I must admit that I haven't seriously played with Boo yet, but it looks extremely interesting. It is a statically typed language that also supports more dynamic typing (like Erik Meijer's "Static Typing Where Possible, Dynamic Typing When Needed"). It also supports some very cool meta programming capabilities. I definitely need to check this out and I recommend you do too.

Paolo and I had a tentative discussion about adding Java bytecode support to the Mono JIT, so that the expensive IKVM just-in-time bytecode to IL conversion can be skipped when running Java code in dynamic mode on Mono. This would be really nice to have, but there aren't any specific plans yet, so don't hold your breath yet.

Finally, I tried to convice Jim Purbrick (of SecondLife, who was there because he wants to use Mono to speed up LSL and possibly support other languages) that he really should support Java as well ;-)

Friday, 07 December 2007 07:42:20 (W. Europe Standard Time, UTC+01:00)  #    Comments [0]
# Tuesday, 04 December 2007
IKVM 0.36 Release Candidate 6

This time I really expect this to be the final 0.36 release candidate :-)

The list of changes is relative to the 0.36 rc5, only the DynamicMethod constructor fix is new relative to the 0.37 development snapshot.


  • Implemented Custom Assembly Class Loaders.
  • Fixed URL.equals() for "ikvmres:" urls.
  • Fixed findResources() bug that caused resources in core assembly to show up twice if a .NET assembly explicitly referenced the core assembly.
  • Added check to prevent defineClass() on an assembly class loader from defining a class that already exists in the assembly.
  • Fixed Class.getModifiers() to mask out ACC_SUPER bit.
  • Fixed a bug in the DynamicMethod based reflection implementation that caused a NullPointerException when generating the setter for a final instance field for code that isn't compiled with -strictfinalfieldsemantics.
  • Fix to make sure that a ghost interface method call always goes thru the target reference wrapping path. (Calling java.lang.CharSequence methods through an interface that extends java.lang.CharSequence would generate incorrect code.)
  • Fixed ghost interface array casting to generate verifiable code.
  • Fixed FileChannelImpl to close the FileDescriptor after releasing the file locks.
  • Fixed DynamicMethod based constructor reflection invocation to have method match the delegate signature.

Binaries available here:ikvmbin-

Sources (+ binaries):ikvm-

External sources (haven't changed since rc1):classpath-0.95-stripped.zip,openjdk-b13-stripped.zip

Tuesday, 04 December 2007 08:55:12 (W. Europe Standard Time, UTC+01:00)  #    Comments [0]
# Monday, 19 November 2007
Splitting the Class Library Assembly

In a comment to the previous post Peter asks:

Do you plan to split the classpath assembly into multiple smaller assemblies at some point? The OpenJDK-based one is getting *huge* and when you only use a small subset of JDK (as I think most people who don't use IKVM as an alternative to JRE, but use it so that they can embed Java code in .NET app, do), it's pretty high overhead...

Cyprien Noel posted a feature request:

Would it be possible to split the dll with the jvm classes in 2 or more
parts ? E.g. a basic dll with java.lang, util, math, io, net, text, and
another dll with the other features. Thanks

Others have asked similar things. I'm very sympathetic to this request, because the bloat of the Java API has annoyed me ever since JDK 1.2.

Unfortunately this is a very hard problem without upstream support. Since Sun has never had any incentive to reduce dependencies inside the class library (because up until now it has always shipped as a single file), there is a pretty tight coupling between many packages in the Java class library. For example, just the static dependencies of java.lang.Object amount to 1893 classes in the IKVM 0.36 release codebase. Note that these are just the static dependencies and this set of classes won't even allow you to run "Hello World". However, adding a few more classes (to a total of 2676) gives a, what appears to be, workable subset and yields an assembly of approx. 5.5MB.

That looks like a great start, doesn't it? However, the problem is that it isn't at all clear what this assembly supports. For example, it supports the http protocol, but not the https protocol. Simply adding the https protocol handler drags in an additional 1895 classes!

Suppose I put the https support in a separate assembly and when you test your application you only test with http URLs so everything works with just the core assembly and you decide to ship only the core assembly. Now a user of your code tries an https URL and it doesn't work. This may seem like an obvious thing, but there are probably other scenarios that aren't as obvious.

The key point here is that I have no idea of most of the inner workings of the class library, so it would be difficult (or extremely time consuming) for me to figure out the best way to split the classes and to give any guidance as to when you'd need to ship what assembly.

Why I'm Probably Still Going To Do It

There is another argument for splitting the class library assembly: Performance. For example, depending on the scenario, having several smaller assemblies that are demand loaded may be more efficient than a single huge assembly. Especially because the assemblies are strong named and the CLR verifies the assembly hash when it loads the assembly (if the assembly isn't in the GAC).


As is usually the answer: I don't know. It'll be done when it's done. In the meantime you can always download the sources and build your own subset class library assembly :-)

Monday, 19 November 2007 08:51:30 (W. Europe Standard Time, UTC+01:00)  #    Comments [4]