So I decided to see what would happen if I tried .NET on my project. Here's what I got:

C:\Users\Tom\Documents\Workspace\fan-math>fan -version --Dfan.runtime=dotnet
Fan Launcher
Copyright (c) 2006-2008, Brian Frank and Andy Frank
Licensed under the Academic Free License version 3.0

.NET Runtime:
  clr.version: 2.0.50727.1434
  fan.home:    C:\Users\Tom\Applications\fan
  sys.version: 1.0.44

C:\Users\Tom\Documents\Workspace\fan-math>fan --Dfan.runtime=dotnet build.fan
System.IO.FileLoadException: Could not load file or assembly 'QUT.SymbolWriter, Version=0.0.0.0, Culture=neutral, PublicKeyToken=8b16d9e060688056' or one of its dependencies. The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log for more detail. (Exception from HRESULT: 0x800736B1)
  at PERWAPI.PDBWriter.WritePDBFile (Unknown Source)
  at PERWAPI.PEWriter.MakeFile (PERWAPI.cs:13620)
  at PERWAPI.PEFile.WritePEFile (PERWAPI.cs:17446)
  at Fanx.Emit.Emitter.commit (Emitter.cs:93)
  at Fanx.Emit.FTypeEmit.emitPod (FTypeEmit.cs:111)
  at Fanx.Emit.FTypeEmit.emitAndLoad (FTypeEmit.cs:32)
  sys::ClassType.emit (ClassType.cs:503)
 Could not load file or assembly 'QUT.SymbolWriter, Version=0.0.0.0, Culture=neutral, PublicKeyToken=8b16d9e060688056' or one of its dependencies. The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log for more detail. (Exception from HRESULT: 0x800736B1)
sys::Err: Cannot emit: compiler::Main
  sys::ClassType.emit (ClassType.cs:511)
  sys::ClassType.finish (ClassType.cs:537)
ERROR: cannot compile script sys::Err: Method not mapped to System.Reflection correctly compiler::Main.compileScript
  sys::Method.MethodFunc._isStatic (Method.cs:448)
  sys::Method.MethodFunc.call (Method.cs:274)
  sys::Method.call (Method.cs:153)
  at Fanx.Util.ScriptUtil.compile (ScriptUtil.cs:89)
  at Fanx.Util.ScriptUtil.compile (ScriptUtil.cs:47)
  sys::Sys.compile (Sys.cs:571)
  at Fanx.Tools.Fan.executeFile (Fan.cs:52)

C:\Users\Tom\Documents\Workspace\fan-math>fan --Dfan.runtime=dotnet build.fan
ERROR: cannot compile script
sys::Err: Method 'flags' in type 'Fan.Compiler.TypeDef' from assembly 'compiler, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation.
  compiler::UsingAndTypeScanner.parseType (Unknown Source)
  compiler::UsingAndTypeScanner.parse (Unknown Source)
  compiler::ScanForUsingsAndTypes.run (Unknown Source)
  sys::List.each (List.cs:505)
  compiler::ScanForUsingsAndTypes.run (Unknown Source)
  compiler::Compiler.frontend (Unknown Source)
  compiler::Compiler.compile (Unknown Source)
  compiler::Main.compileScript (Unknown Source)

Is it possible I'm just doing something wrong?

That is the same version on .NET I am running.

What happens if you run this:

fant --Dfan.runtime=dotnet testSys

I was wise enough to try Google now. Got some clue maybe. I am on 64-bit Vista, and that seems like it might be related.

What happens if you run this: fant --Dfan.runtime=dotnet testSys

C:\Users\Tom\Documents\Workspace\fan-math>fant --Dfan.runtime=dotnet testSys

-- Run:  testSys::ActorTest.testMake...
System.IO.FileLoadException: Could not load file or assembly 'QUT.SymbolWriter, Version=0.0.0.0, Culture=neutral, PublicKeyToken=8b16d9e060688056' or one of its dependencies. The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log for more detail. (Exception from HRESULT: 0x800736B1)
  at PERWAPI.PDBWriter.WritePDBFile (Unknown Source)
  at PERWAPI.PEWriter.MakeFile (PERWAPI.cs:13620)
  at PERWAPI.PEFile.WritePEFile (PERWAPI.cs:17446)
  at Fanx.Emit.Emitter.commit (Emitter.cs:93)
  at Fanx.Emit.FTypeEmit.emitPod (FTypeEmit.cs:111)
  at Fanx.Emit.FTypeEmit.emitAndLoad (FTypeEmit.cs:32)
  sys::ClassType.emit (ClassType.cs:503)
 Could not load file or assembly 'QUT.SymbolWriter, Version=0.0.0.0, Culture=neutral, PublicKeyToken=8b16d9e060688056' or one of its dependencies. The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log for more detail. (Exception from HRESULT: 0x800736B1)
sys::Err: Cannot emit: testSys::ActorTest
  sys::ClassType.emit (ClassType.cs:511)
  sys::ClassType.finish (ClassType.cs:537)

ERROR: Cannot make test testSys::ActorTest
sys::Err: Method not mapped to System.Reflection correctly testSys::ActorTest.make
  sys::Method.MethodFunc._isStatic (Method.cs:448)
  sys::Method.MethodFunc.callList (Method.cs:184)
  sys::Type.make (Type.cs:253)
  sys::ClassType.make (ClassType.cs:111)
  sys::Type.make (Type.cs:243)
  at Fanx.Tools.Fant.runTest (Fant.cs:136)
-- Run:  testSys::ActorTest.testBasics...
   Pass: testSys::ActorTest.testBasics [505]
-- Run:  testSys::ActorTest.testOrdering...
   Pass: testSys::ActorTest.testOrdering [99800]
-- Run:  testSys::ActorTest.testMessaging...
   Pass: testSys::ActorTest.testMessaging [12]
-- Run:  testSys::ActorTest.testTimeoutCancel...
   Pass: testSys::ActorTest.testTimeoutCancel [6]
-- Run:  testSys::ActorTest.testStop...
   Pass: testSys::ActorTest.testStop [329]
-- Run:  testSys::ActorTest.testKill...
System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state.
  at System.Threading.Monitor.Enter (Unknown Source)
  at Fanx.Util.ThreadPool.ready (ThreadPool.cs:180)
  at Fanx.Util.ThreadPool.Worker.run (ThreadPool.cs:281)
System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state.
  at System.Threading.Monitor.Enter (Unknown Source)
  at Fanx.Util.ThreadPool.ready (ThreadPool.cs:180)
  at Fanx.Util.ThreadPool.Worker.run (ThreadPool.cs:281)
System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state.
  at System.Threading.Monitor.Enter (Unknown Source)
  at Fanx.Util.ThreadPool.ready (ThreadPool.cs:180)
  at Fanx.Util.ThreadPool.Worker.run (ThreadPool.cs:281)
System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state.
  at System.Threading.Monitor.Enter (Unknown Source)
  at Fanx.Util.ThreadPool.ready (ThreadPool.cs:180)
  at Fanx.Util.ThreadPool.Worker.run (ThreadPool.cs:281)
System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state.
  at System.Threading.Monitor.Enter (Unknown Source)
  at Fanx.Util.ThreadPool.ready (ThreadPool.cs:180)
  at Fanx.Util.ThreadPool.Worker.run (ThreadPool.cs:281)
System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state.
  at System.Threading.Monitor.Enter (Unknown Source)
  at Fanx.Util.ThreadPool.ready (ThreadPool.cs:180)
  at Fanx.Util.ThreadPool.Worker.run (ThreadPool.cs:281)

TEST FAILED
sys::TestErr: Test failed: 235ms
  sys::Test.fail (Test.cs:178)
  sys::Test.verify (Test.cs:64)
  testSys::ActorTest.testKill (Unknown Source)
-- Run:  testSys::ActorTest.testLater...
   Pass: testSys::ActorTest.testLater [11]
.
.
... many lines clipped ...
.
.
-- Run:  testSys::ZipTest.testCreate...
   Pass: testSys::ZipTest.testCreate [66]

Time: 62135ms

 -- failed: testSys::ActorTest.testMake
 -- failed: testSys::ActorTest.testKill
 -- failed: testSys::DateTimeTest.testNow
 -- failed: testSys::ErrTest.testTrace
 -- failed: testSys::FileTest.testMmap
 -- failed: testSys::FileTest.testSync
 -- failed: testSys::FuncTest.testCurryDefParams
 -- failed: testSys::MapTest.testOrdered
 -- failed: testSys::MapTest.testReadonly
 -- failed: testSys::MixinTest.testNet
 -- failed: testSys::RegexTest.testGroups
 -- failed: testSys::RegexTest.testSpecialChars
 -- failed: testSys::RegexTest.testClasses
 -- failed: testSys::RegexTest.testPosixClasses
 -- failed: testSys::RegexTest.testJavaCharacterClasses
 -- failed: testSys::RegexTest.testBoundaryMatchers
 -- failed: testSys::RegexTest.testPosessiveQuantifiers
 -- failed: testSys::RegexTest.testBackReferences
 -- failed: testSys::RegexTest.testQuotation
 -- failed: testSys::StreamTest.testISO_8859

***
*** 20 FAILURES [53 tests, 564 methods, 299341 verifies]
***

So some of it works, I guess.

On 32-bit XP I get the same -- sys::Err: cannot emit with reference to QUT.SymbolWriter but only on first invocation after a build. If I try again, it works. Weird.

Maybe my quick skim wasn't good enough, then. Meanwhile, mine (as shown above) wouldn't build even after multiple tries. Not sure what's up.

I think Andy will have to take a look when he gets back from vacation

Looks like .NET is not finding that DLL - which doesn't make sense cause it finds the Zip library we use (which is in the same directory). You're lib/dotnet/ directory should look like this:

lib/
  dotnet/
    ICSharpCode.SharpZipLib.dll
    QUT.SymbolWriter.dll
    sys.dll
    sys.pdb

Here's what it says:

Directory of C:\Users\Tom\Applications\fan\lib\dotnet

07/15/2009  10:26 PM    <DIR>          .
07/15/2009  10:26 PM    <DIR>          ..
05/14/2009  01:57 PM           143,360 ICSharpCode.SharpZipLib.dll
05/14/2009  01:57 PM            49,664 QUT.SymbolWriter.dll
06/25/2009  10:13 AM           679,936 sys.dll
06/25/2009  10:13 AM         2,487,808 sys.pdb
               4 File(s)      3,360,768 bytes

In skimming around some discussions on "side-by-side configuration" errors, it seems like several causes are possible. Some hints that it might depend on which system libraries are installed. Do y'all have things like Visual Studio installed by chance?

My /dotnet/ dir looks the same, and I also have Visual Studio (2008, haven't loaded the 2010 beta yet).

I've had to install a number of things in order to get various pieces of .NET working, so that may be the case. I have an Express version installed, but not the full Visual Studio.

I don't think I have any Express (and definitely no full) Studio installed. I'll have to double-check later.

I'm seeing this issue now. I had to reinstall from scratch a few weeks ago, and never installed Visual Studio (thankfully). Looking at the QUT.SymbolWriter.dll (which we use for generating debug info for .NET assemblies) it has a dependency on the Microsoft.VisualC namespace.

From the bit a read this appears to be a component of Visual Studio - which would explain why it worked for people with VS installed.

For now, I'm just going to disable debug support for .NET. I think long-term this runtime needs be rewritten using the System.Reflection.Emit APIs. But this will at least make sure we keep the .NET-runtime up-to-date with the JVM so we enable that when we're ready to bite it off.