Mono is a project led by Novell (formerly by Ximian) to create an Ecma standard compliant .NET compatible set of tools, including among others a C# compiler and a Common Language Runtime. Mono can be run on Linux, BSD, UNIX, Mac OS X, Solaris and Windows operating systems.
Microsoft has a version of .NET available for FreeBSD, Windows and Mac OS X 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.
Miguel de Icaza became interested in .NET technology as soon as the .NET documents came out in December 2000. After looking at the byte code interpreter, he realized that there were no specifications for the metadata. In February 2001 de Icaza asked for the missing information on the metadata file format in the .NET mailing lists and at the same time started to work on a C# compiler written in C#, as an exercise in C#. In April 2001 ECMA published the missing file format, and at GUADEC (April 6 – April 8, 2001) de Icaza demonstrated the features of his compiler (which by then was able to parse itself).
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 humanpower 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.
Almost three years later, on June 30, 2004 Mono 1.0 was released.
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.
Current Status and Roadmap
Mono’s current version is 1.9 (as of March 14, 2008). This version provides the core API of the .NET Framework as well as support for C# 2.0 and Visual Basic.NET and C# 3.0. LINQ to objects and XML is part of the distribution, but not LING to SQL C# 3.0 is now the default mode of operation for the C# compiler. While Windows Forms 1.1 is supported, Windows Forms 2.0 implementation is not complete. It is now possible to build and use the DLR with Mono.
Implementation of .NET Framework 3.0 is under development under an experimental Mono subproject called Olive, but the availability of a Mono framework supporting .NET 3.0 is still not planned yet.
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.
Mono consists of three groups of components:
1. Core components
2. Mono/Linux/GNOME development stack
3. Microsoft compatibility stack.
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.
The class library provides a comprehensive set of facilities for application development. They are primarily written in C#, but thanks to the Common Language Specification they can be used by any .NET language. The class library is structured into namespaces, and deployed in shared libraries known as assemblies. When we speak of the .NET framework, we are primarily referring to this class library.
Namespaces and assemblies
Namespaces are a mechanism for logically grouping similar classes into a hierarchical structure. This prevents naming conflicts. The structure is implemented using dot-separated words, where the most common top-level namespace is System, such as System.IO and System.Net (a complete list can be found in Mono Documentation). There are other top-level namespaces as well, such as Accessibility and Windows. A user can define a namespace by placing elements inside a namespace block.
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.
Common Language Infrastructure and Common Language Specification
The Common Language Infrastructure (CLI), or more commonly known as the Common Language Runtime, is implemented by the Mono executable. The runtime is used to execute compiled .NET applications. The common language infrastructure is defined by the ECMA standard ECMA-335. To run an application, you must invoke the runtime with the relevant parameters.
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.
Managed and unmanaged code
Within a native .NET/Mono application, all code is managed; that is, it is governed by the CLI’s style of memory management and thread safety. Other .NET or Mono applications can use legacy code, which is referred to as unmanaged, by using the System.Runtime.InteropServices libraries to create C# bindings. Many libraries which ship with Mono use this feature of the CLI, such as Gtk#.
There exist lots of projects related to Mono that extend Mono and allows developers to use Mono in their development environment. These projects include:
* Cocoa#  wrappers around the native Mac OS X toolkit (Cocoa).
* Gecko#, bindings for embedding the layout engine used in Mozilla (Gecko).
* Gtk#, C# wrappers around the underlying GTK+ and GNOME libraries, written in C.
* Tao, a collection of graphics and gaming bindings (OpenGL, SDL, Glut, Cg).
* MoMA, Mono Migration Analyzer (MoMA) tool.
MonoDevelop is a free GNOME integrated development environment primarily designed for C# and other .NET languages such as Nemerle, Boo, and Java (via IKVM.NET). MonoDevelop was originally a port of SharpDevelop to Gtk#, but it has since evolved to meet the needs of Mono developers. The IDE includes class management, built-in help, code completion, Stetic (a GUI designer), project support, and an integrated debugger.
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
Mono is dual licensed by Novell, similar to other products such as Qt and the Mozilla Application Suite. Mono’s C# compiler and tools are released under the GNU General Public License (GPL), the runtime libraries under the GNU Lesser General Public License (LGPL) and the class libraries under the MIT License. These are all free software and open-source licenses and hence Mono is free and open-source software. Developers contributing source code to Mono upstream must agree to distribute their code to Novell under a license that allows Novell to relicense the code under other licensing terms. This practice is similar to copyright assignment agreements used in other free and open source software projects (used in many GNU projects and by MySQL), however, this method allows the developer to retain copyright of the original work while still preserving Novell’s ability to commercially license Mono for specific customers that require different licensing terms than what is provided in general release versions (such as running Mono in embedded firmware environments).
Mono and Microsoft’s patents
Mono’s implementation of those components of the .NET stack not submitted to the ECMA for standardization has been the source of patent violation concerns for much of the life of the project. In particular, discussion has taken place about whether Microsoft could destroy the Mono project through patent suits.
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).
Software developed with Mono
The following are programs that use the Mono API and C#.
* Banshee music management and playback software for GNOME
* Beagle is a desktop search tool.
* Blam! is an RSS-news aggregator especially good for reading Planet-feeds like Planet Gnome.
* Diva video editing application for GNOME.
* Gnome Do desktop application launching software (similar to QuickSilver).
* F-Spot is a photo management program.
* iFolder 3 (by Novell) lets you share files across multiple computers and with other users through peer-to-peer or Novell’s groupware server products.
* libsecondlife is an open source implementation of the Second Life networking protocol written in C# that will run under Mono or .NET.
* MonoDevelop is an IDE for creating Mono applications. It was originally a port of SharpDevelop to Gtk#, but is today developed on its own. A Windows installer is not provided, but it can be built and run on Windows.
* Muine is a music player with a user interface designed to be intuitive, developed by Jorn Baayen who also worked on Rhythmbox.
* Second Life, the virtual world created by Linden Lab, will in the future be compiling all of the scripts in their own Linden Scripting Language (LSL) to CIL. They will use an embedded Mono runtime to execute the CIL.
* Tomboy is a desktop note-taking application which uses a wiki-like linking system.
* Unity is a game engine using Mono for game scripting.
* MonoTorrent (previously named bitsharp) a bittorrent library written with mono
Source : wikipedia