Mono on MacOSX - “The runtime version supported by

I have just installed Mono v4.0.4 on MACOSX 10.10.5 Yosemite. The installer package was: MonoFramework-MDK-4.0.4.4.macos10.xamarin.x86.pkg downloaded from http://www.mono-project.com/download/ last night.

I have a webapi app that I would like to run with xsp4.exe. I use this command line:

/usr/bin/mono --runtime=v4.5 /Library/Frameworks/Mono.framework/Versions/4.0.4/lib/mono/4.5/xsp4.exe

I get this output:

WARNING: The runtime version supported by this application is unavailable.
Using default runtime: v4.0.30319
...

Why do I get this message? Why does it say, "the runtime version is unavailable"?

I believe I have v4.5 installed, with mono v4.0.4. In the directory /Library/Frameworks/Mono.framework/Versions/4.0.4/lib/mono, I can see

Does this NOT indicate that v4.5 of the runtime is available?

Inside the 4.5 directory, I see lots of DLLs, all symlinked to a gac directory. The targets of these symlinks exist.

I can't figure out how to tell mono, YEAH, v4.5 DOES EXIST. USE IT.

I have a separate, probably related problem. After getting the above warning, xsp4 runs. When I try to load the ASPX app in the browser, I get the dreaded CS0234 error,

System.Web.Compilation.CompilationException
CS0234: The type or namespace name `Http' does not exist in the namespace `System.Web'. Are you missing an assembly reference?

But in /Library/Frameworks/Mono.framework/Versions/4.0.4/lib/mono/4.5 , I can see

System.Web.Http.dll -> ../gac/System.Web.Http/4.0.0.0__31bf3856ad364e35/System.Web.Http.dll

And that DLL sure does exist in the gac directory. I think xsp4/mono cannot find the System.Web.Http.dll which is part of v4.5. Am I right?

I am doing something basic wrong. Can someone steer me right?

mono --runtime=

That option is to control the CLR runtime:

CLR version .NET version
1.0         1.0
1.1         1.1
2.0         2.0, 3.0, 3.5
4           4, 4.5, 4.6

If you disasm xsp4.exe you will see that it requires a host that can provide the CLR 4.0 runtime:

ikdasm xsp4.exe | head -n 2

// Metadata version: v4.0.30319

Or via xCorFlags:

>>corflags xsp4.exe

Mono/.NET Framework CorFlags Conversion Tool.  Version  1.0.5643.36015
Copyright (c) SushiHangover.  All rights reserved.

Version   : v4.0.30319
CLR Header: 2.5
PE        : PE32
CorFlags  : 0x9
ILONLY    : 1
32BITREQ  : 0
32BITPREF : 0
Signed    : 1

Mono will auto-detect the runtime required by probing the CIL that it is loading, so there is no need to specify it unless you really need to force a different runtime version.

FYI: Mono installs shell scripts for tools/utilities for xsp4 (and others) so you do not have to hand-type the paths:

cat `which xsp4`

#!/bin/sh
exec /Library/Frameworks/Mono.framework/Versions/4.2.1/bin/mono $MONO_OPTIONS "/Library/Frameworks/Mono.framework/Versions/4.2.1/lib/mono/4.5/xsp4.exe" "$@"