Microsoft has a version of .NET now available only for Windows, called the Shared Source CLI (Rotor). Microsoft's shared source license may be insufficient for the needs of the community (it explicitly forbids commercial use). The Mono project has many of the same goals as the Portable.NET project.
The Mono runtime contains a just-in-time compilation (JIT) engine for a number of processors: x86, SPARC, PowerPC, ARM, S390 (in 32-bit and 64-bit mode), and x86_64, IA64 and SPARC for 64-bit modes. The runtime will perform a just-in-time compilation to the machine's native code which is cached as the application runs. It is also possible to precache the native image before execution. For other supported systems not listed, an interpreter performs each byte code one by one without compiling the image to native code. In almost every condition the JIT method will outperform the interpreted method.
Internally at Ximian there was much discussion about building tools to increase productivity: making it possible to create more applications in less time and therefore reduce time and cost of development. After a feasibility study, which clearly stated that it was possible to build the technology, Ximian reassigned staff from other projects and created the Mono team. Lacking the resources to build a full .NET replacement on their own, they formed the Mono open source project, which was announced on July 19 2001 at the O'Reilly conference.
The logo of Mono features a monkey, mono being the Spanish word for monkey. The name relates to the monkeys and apes that are a central theme within Ximian, such as the GNOME Project foot, the Bonobo project (a OLE/COM like library used in GNOME created by Ximian) which is a species of ape related to chimpanzees and humans, the Novell Evolution (formerly Ximian Evolution) Project, and the Ximian logo itself. The Mono FAQ claims that the origin of the name is "We like monkeys." For what it may be worth, "Ximian" is near-homophone of simian.
An open source implementation of Silverlight, called Moonlight, is now underway and is included in Mono 1.9. The Silverlight 2.0 .NET classes are implemented as part of the Olive module. Moonlight shipped as part of Mono 1.2.6 release.
The core components include the C# compiler, the virtual machine, and the base class libraries. These components are based on the Ecma-334 and Ecma-335 standards, allowing Mono to provide a standards compliant, free and open source CLI virtual machine.
The Mono/Linux/GNOME development stack provide tools for application development while leveraging existing GNOME and Free and Open Source libraries. These include: Gtk# for GUI development, Mozilla libraries for working with the Gecko rendering engine, Unix integration libraries, database connectivity libraries, a security stack, and the XML schema language RelaxNG. Gtk# allows Mono applications to integrate into the Gnome desktop as native applications. The database libraries provide connectivity to MySQL, SQLite, PostgreSQL, Firebird, Open Database Connectivity (ODBC), Microsoft SQL Server (MSSQL), Oracle, the object-relational database db4o, and many others. The Mono project tracks developing database components at its website
The Microsoft compatibility stack provides a pathway for porting Windows .NET applications to Linux. This group of components include ADO.NET, ASP.NET, and Windows.Forms, among others. As these components are not covered by ECMA standards, some of them remain subject to patent fears and concerns.
Assemblies are the physical packaging of the class libraries. These are .dll files, just as (but not to be confused with) Win32 shared libraries. Examples of assemblies are mscorlib.dll, System.dll, System.Data.dll and Accessibility.dll. Namespaces are often distributed among several assemblies and one assembly can be composed of several files.
The Common Language Specification (CLS) is specified in chapter 6 of ECMA-335 and defines the interface to the CLI, such as conventions like the underlying types for Enum. The Mono compiler generates an image that conforms to the CLS. This is the Common Intermediate Language. The Mono runtime takes this image and runs it. The ECMA standard formally defines a library that conforms to the CLS as a framework.
The MonoDoc browser provides access to API documentation and code samples. The documentation browser uses wiki-style content management, allowing developers to edit and improve the documentation.
The base technologies submitted to the ECMA, and therefore also the Unix/Gnome-specific parts, may be non-problematic. The concerns primarily relate to technologies developed by Microsoft on top of the .NET Framework, such as ASP.NET, ADO.NET and Windows Forms, i.e. parts composing Mono’s Windows compatibility stack. These technologies are today not fully implemented in Mono and not required for developing Mono-applications. Not providing patented capabilities would weaken interoperability, but it would still be possible to provide the free software / open source software community with good development tools, which is the primary reason for developing Mono. This has been summed up by Richard Stallman:
Mono is a free implementation of Microsoft's language C#. Microsoft has declared itself our enemy and we know that Microsoft is getting patents on some features of C#. So I think it's dangerous to use C#, and it may be dangerous to use Mono. There’s nothing wrong with Mono. Mono is a free implementation of a language that users use. It's good to provide free implementations. We should have free implementations of every language. But, depending on it is dangerous, and we better not do that.
On November 2 2006, Microsoft and Novell announced a joint agreement whereby Microsoft agreed to not sue Novell’s customers for patent infringement. According to Mono project leader Miguel de Icaza, this agreement extends to Mono but only for Novell developers and customers. It was criticized by the free software community because it violates the principles of giving equal rights to all users of a particular program (see Novell and their Patent Agreement with Microsoft).
The following are programs that use the Mono API and C#.