Mono open source ECMA CLI, C# and .NET implementation.


Mono is a software platform designed to allow developers to easily
create cross platform applications. It is an open source
implementation of Microsoft’s .NET Framework based on the ECMA
standards for C# and the Common Language Runtime.

The Mono project is part of the .NET Foundation

Join us on Discord in the #monovm channel:


  1. Compilation and Installation
  2. Using Mono
  3. Directory Roadmap
  4. Contributing to Mono
  5. Reporting bugs
  6. Configuration Options
  7. Working with Submodules

Build Status

Public CI: Azure Pipelines

Legacy Jenkins CI (no longer available publicly):

OS Architecture Status
Debian 9 amd64 debian-9-amd64
Debian 9 i386 debian-9-i386
Debian 9 armel debian-9-armel
Debian 9 armhf debian-9-armhf
Debian 9 arm64 debian-9-arm64
OS X amd64 osx-amd64
OS X i386 osx-i386
Windows amd64 windows-amd64
Windows i386 windows-i386
CentOS s390x (cs) centos-s390x
Debian 9 ppc64el (cs) debian-9-ppc64el
AIX 6.1 ppc64 (cs) aix-ppc64
FreeBSD 12 amd64 (cs) freebsd-amd64

(cs) = community supported architecture

Compilation and Installation

Building the Software

Please see our guides for building Mono on
Mac OS X,
Linux and

Note that building from Git assumes that you already have Mono installed,
so please download and install the latest Mono release
before trying to build from Git. This is required because the Mono build
relies on a working Mono C# compiler to compile itself
(also known as bootstrapping).

If you don’t have a working Mono installation

If you don’t have a working Mono installation, you can try a slightly
more risky approach: getting the latest version of the ‘monolite’ distribution,
which contains just enough to run the ‘mcs’ compiler. You do this with:

# Run the following line after ./
make get-monolite-latest

This will download and place the files appropriately so that you can then
just run:


The build will then use the files downloaded by make get-monolite-latest.

Testing and Installation

You can run the mono and mcs test suites with the command: make check.

Expect to find a few test suite failures. As a sanity check, you
can compare the failures you got with

You can now install mono with: make install

You can verify your installation by using the mono-test-install
script, it can diagnose some common problems with Mono’s install.
Failure to follow these steps may result in a broken installation.

Using Mono

Once you have installed the software, you can run a few programs:

  • mono program.exe runtime engine

  • mcs program.cs C# compiler

  • monodis program.exe CIL Disassembler

See the man pages for mono(1), mcs(1) and monodis(1) for further details.

Directory Roadmap

  • acceptance-tests/ - Optional third party test suites used to validate Mono against a wider range of test cases.

  • data/ - Configuration files installed as part of the Mono runtime.

  • docs/ - Technical documents about the Mono runtime.

  • external/ - Git submodules for external libraries (Newtonsoft.Json, ikvm, etc).

  • ikvm-native/ - Glue code for ikvm.

  • libgc/ - The (deprecated) Boehm GC implementation.

  • llvm/ - Utility Makefiles for integrating the Mono LLVM fork.

  • m4/ - General utility Makefiles.

  • man/ - Manual pages for the various Mono commands and programs.

  • mcs/ - The class libraries, compiler and tools

    • class/ - The class libraries (like System.*, Microsoft.Build, etc.)

    • mcs/ - The Mono C# compiler written in C#

    • tools/ - Tools like gacutil, ikdasm, mdoc, etc.

  • mono/ - The core of the Mono Runtime.

    • arch/ - Architecture specific portions.

    • benchmark/ - A collection of benchmarks.

    • btls/ - Build files for the BTLS library which incorporates BoringSSL.

    • cil/ - Common Intermediate Representation, XML
      definition of the CIL bytecodes.

    • dis/ - CIL executable Disassembler.

    • eglib/ - Independent implementation of the glib API.

    • metadata/ - The object system and metadata reader.

    • mini/ - The Just in Time Compiler.

    • profiler/ - The profiler implementation.

    • sgen/ - The SGen Garbage Collector implementation.

    • tests/ - The main runtime tests.

    • unit-tests/ - Additional runtime unit tests.

    • utils/ - Utility functions used across the runtime codebase.

  • msvc/ - Logic for the MSVC / Visual Studio based runtime and BCL build system.
    The latter is experimental at the moment.

  • packaging/ - Packaging logic for the OS X and Windows Mono packages.

  • po/ - Translation files.

  • runtime/ - A directory that contains the Makefiles that link the
    mono/ and mcs/ build systems.

  • samples/ - Some simple sample programs on uses of the Mono
    runtime as an embedded library.

  • scripts/ - Scripts used to invoke Mono and the corresponding program.

  • support/ - Various support libraries.

  • tools/ - A collection of tools, mostly used during Mono development.

Contributing to Mono

Before submitting changes to Mono, please review the contribution
Please pay particular attention to the Important


Reporting bugs

To submit bug reports, please open an issue on the mono GitHub repo.

Please use the search facility to ensure the same bug hasn’t already
been submitted and follow our
on how to make a good bug report.

Configuration Options

The following are the configuration options that someone building Mono
might want to use:

  • --with-sgen=yes,no - Generational GC support: Used to enable or
    disable the compilation of a Mono runtime with the SGen garbage

    • On platforms that support it, after building Mono, you will have
      both a mono-boehm binary and a mono-sgen binary. mono-boehm uses Boehm,
      while mono-sgen uses the Simple Generational GC.
  • --with-libgc=[included, none] - Selects the default Boehm
    garbage collector engine to use.

    • included: (slightly modified Boehm GC) This is the default
      value for the Boehm GC, and it’s the most feature complete, it will
      allow Mono to use typed allocations and support the debugger.

    • none:
      Disables the inclusion of a Boehm garbage collector.

    • This defaults to included.

  • --enable-cooperative-suspend

    • If you pass this flag the Mono runtime is configured to only use
      the cooperative mode of the garbage collector. If you do not pass
      this flag, then you can control at runtime the use of the
      cooperative GC mode by setting the MONO_ENABLE_COOP_SUSPEND flag.
  • --with-tls=__thread,pthread

    • Controls how Mono should access thread local storage,
      pthread forces Mono to use the pthread APIs, while
      __thread uses compiler-optimized access to it.

    • Although __thread is faster, it requires support from
      the compiler, kernel and libc. Old Linux systems do
      not support with __thread.

    • This value is typically pre-configured and there is no
      need to set it, unless you are trying to debug a problem.

  • --with-sigaltstack=yes,no

    • Experimental: Use at your own risk, it is known to
      cause problems with garbage collection and is hard to
      reproduce those bugs.

    • This controls whether Mono will install a special
      signal handler to handle stack overflows. If set to
      yes, it will turn stack overflows into the
      StackOverflowException. Otherwise when a stack
      overflow happens, your program will receive a
      segmentation fault.

    • The configure script will try to detect if your
      operating system supports this. Some older Linux
      systems do not support this feature, or you might want
      to override the auto-detection.

  • --with-static_mono=yes,no

    • This controls whether mono should link against a
      static library (libmono.a) or a shared library

    • This defaults to yes, and will improve the performance
      of the mono program.

    • This only affects the `mono’ binary, the shared
      library will always be produced for
      developers that want to embed the runtime in their

  • --with-xen-opt=yes,no - Optimize code for Xen virtualization.

    • It makes Mono generate code which might be slightly
      slower on average systems, but the resulting executable will run
      faster under the Xen virtualization system.

    • This defaults to yes.

  • --with-large-heap=yes,no - Enable support for GC heaps larger than 3GB.

    • This only applies only to the Boehm garbage collector, the SGen garbage
      collector does not use this configuration option.

    • This defaults to no.

  • --enable-small-config=yes,no - Enable some tweaks to reduce memory usage
    and disk footprint at the expense of some capabilities.

    • Typically this means that the number of threads that can be created
      is limited (256), that the maximum heap size is also reduced (256 MB)
      and other such limitations that still make mono useful, but more suitable
      to embedded devices (like mobile phones).

    • This defaults to no.

  • --with-ikvm-native=yes,no - Controls whether the IKVM JNI interface library is
    built or not.

    • This is used if you are planning on
      using the IKVM Java Virtual machine with Mono.

    • This defaults to yes.

  • --with-profile4=yes,no - Whether you want to build the 4.x profile libraries
    and runtime.

    • This defaults to yes.
  • --with-libgdiplus=installed,sibling,<path> - Configure where Mono
    searches for libgdiplus when running System.Drawing tests.

    • It defaults to installed, which means that the
      library is available to Mono through the regular
      system setup.

    • sibling can be used to specify that a libgdiplus
      that resides as a sibling of this directory (mono)
      should be used.

  • Or you can specify a path to a libgdiplus.

  • --enable-minimal=LIST

    • Use this feature to specify optional runtime
      components that you might not want to include. This
      is only useful for developers embedding Mono that
      require a subset of Mono functionality.

    • The list is a comma-separated list of components that
      should be removed, these are:

      • aot:
        Disables support for the Ahead of Time compilation.

      • attach:
        Support for the Mono.Management assembly and the
        VMAttach API (allowing code to be injected into
        a target VM)

      • com:
        Disables COM support.

      • debug:
        Drop debugging support.

      • decimal:
        Disables support for System.Decimal.

      • full_messages:
        By default Mono comes with a full table
        of messages for error codes. This feature
        turns off uncommon error messages and reduces
        the runtime size.

      • generics:
        Generics support. Disabling this will not
        allow Mono to run any 2.0 libraries or
        code that contains generics.

      • jit:
        Removes the JIT engine from the build, this reduces
        the executable size, and requires that all code
        executed by the virtual machine be compiled with
        Full AOT before execution.

      • large_code:
        Disables support for large assemblies.

      • logging:
        Disables support for debug logging.

      • pinvoke:
        Support for Platform Invocation services,
        disabling this will drop support for any
        libraries using DllImport.

      • portability:
        Removes support for MONO_IOMAP, the environment
        variables for simplifying porting applications that
        are case-insensitive and that mix the Unix and Windows path separators.

      • profiler:
        Disables support for the default profiler.

      • reflection_emit:
        Drop System.Reflection.Emit support

      • reflection_emit_save:
        Drop support for saving dynamically created
        assemblies (AssemblyBuilderAccess.Save) in

      • shadow_copy:
        Disables support for AppDomain’s shadow copies
        (you can disable this if you do not plan on
        using appdomains).

      • simd:
        Disables support for the Mono.SIMD intrinsics

      • ssa:
        Disables compilation for the SSA optimization
        framework, and the various SSA-based optimizations.

  • --enable-llvm

    • This enables the use of LLVM as a code generation engine
      for Mono. The LLVM code generator and optimizer will be
      used instead of Mono’s built-in code generator for both
      Just in Time and Ahead of Time compilations.

    • See for the
      full details and up-to-date information on this feature.

    • You will need to have an LLVM built that Mono can link

  • --enable-big-arrays - Enable use of arrays with indexes larger
    than Int32.MaxValue.

    • By default Mono has the same limitation as .NET on
      Win32 and Win64 and limits array indexes to 32-bit
      values (even on 64-bit systems).

    • In certain scenarios where large arrays are required,
      you can pass this flag and Mono will be built to
      support 64-bit arrays.

    • This is not the default as it breaks the C embedding
      ABI that we have exposed through the Mono development

  • --enable-parallel-mark

    • Use this option to enable the garbage collector to use
      multiple CPUs to do its work. This helps performance
      on multi-CPU machines as the work is divided across CPUS.

    • This option is not currently the default on OSX
      as it runs into issues there.

    • This option only applies to the Boehm GC.

  • --enable-dtrace

    • On Solaris and MacOS X builds a version of the Mono
      runtime that contains DTrace probes and can
      participate in the system profiling using DTrace.
  • --disable-dev-random

    • Mono uses /dev/random to obtain good random data for
      any source that requires random numbers. If your
      system does not support this, you might want to
      disable it.

    • There are a number of runtime options to control this
      also, see the man page.

  • --with-csc=roslyn,mcs,default

    • Use this option to configure which C# compiler to use. By default
      the configure script will pick Roslyn, except on platforms where
      Roslyn does not work (Big Endian systems) where it will pick mcs.

      If you specify “mcs”, then Mono’s C# compiler will be used. This
      also allows for a complete bootstrap of Mono’s core compiler and
      core libraries from source.

If you specify “roslyn”, then Roslyn’s C# compiler will be used.
This currently uses Roslyn binaries.

  • --enable-nacl

    • This configures the Mono compiler to generate code
      suitable to be used by Google’s Native Client:

    • Currently this is used with Mono’s AOT engine as
      Native Client does not support JIT engines yet.

  • --enable-wasm

    • Use this option to configure mono to run on WebAssembly. It will
      set both host and target to the WebAssembly triplet. This overrides
      the values passed to --host or --target and ignored what config.sub guesses.

      This is a workaround to enable usage of old automake versions that don’t
      recognize the wasm triplet.

Working With Submodules

Mono references several external git submodules, for example
a fork of Microsoft’s reference source code that has been altered
to be suitable for use with the Mono runtime.

This section describes how to use it.

An initial clone should be done recursively so all submodules will also be
cloned in a single pass:

$ git clone --recursive [email protected]:mono/mono

Once cloned, submodules can be updated to pull down the latest changes.
This can also be done after an initial non-recursive clone:

$ git submodule update --init --recursive

To pull external changes into a submodule:

$ cd <submodule>
$ git pull origin <branch>
$ cd <top-level>
$ git add <submodule>
$ git commit

By default, submodules are detached because they point to a specific commit.
Use git checkout to move back to a branch before making changes:

$ cd <submodule>
$ git checkout <branch>
# work as normal; the submodule is a normal repo
$ git commit/push new changes to the repo (submodule)

$ cd <top-level>
$ git add <submodule> # this will record the new commits to the submodule
$ git commit

To switch the repo of a submodule (this should not be a common or normal thing
to do at all), first edit .gitmodules to point to the new location, then:

$ git submodule sync -- <path of the submodule>
$ git submodule update --recursive
$ git checkout <desired new hash or branch>

The desired output diff is a change in .gitmodules to reflect the
change in the remote URL, and a change in / where you see
the desired change in the commit hash.


See the LICENSE file for licensing information, and the PATENTS.TXT
file for information about Microsoft’s patent grant.

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The use of trademarks and logos for Mono can be found here.