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|Developer(s)||Xamarin (formerly by Novell, originally by Ximian), and the Mono community|
|Initial release||June 30, 2004|
|Stable release||2.10.8 / December 19, 2011|
|Preview release||2.11 / March 22, 2012|
|License||MIT, LGPLv2 and GPLv2, or dual license|
Mono is a free and open source project led by Xamarin (formerly by Novell and originally by Ximian) to create an Ecma standard compliant .NET Framework-compatible set of tools including, among others, a C# compiler and a Common Language Runtime.
The stated purpose of Mono is not only to be able to run Microsoft .NET applications cross-platform, but also to bring better development tools to Linux developers. Mono can be run on many operating systems including Android, BSD, iOS, Linux, OS X, Windows, Solaris, Unix, and some for game consoles such as PlayStation 3, Wii, and Xbox 360.
|2004-06-30||1.0||C# 1.0 support|
|2006-11-09||1.2||C# 2.0 support|
|2008-10-06||2.0||Mono's APIs are now in par with .NET 2.0. Introduces the C# 3.0 and Visual Basic 8 compilers. New Mono-specific APIs: Mono.Cecil, Mono.Cairo and Mono.Posix. Gtk# 2.12 is released. The Gendarme verification tool and Mono Linker are introduced.|
|2009-01-13||2.2||Mono switches its JIT engine to a new internal representation  that gives it a performance boost and introduces SIMD support in the Mono.Simd  Mono.Simd namespace.
Mono introduces Full Ahead of Time compilation that allows developers to create full static applications and debuts the C# Compiler as a Service  and the C# Interactive Shell  (C# REPL)
|2009-03-30||2.4||This release mostly polishes all the features that shipped in 2.2 and became the foundation for the Long-Term support of Mono in SUSE Linux.|
|2009-12-15||2.6||The Mono runtime is now able to use LLVM as a code generation backend and this release introduces Mono co-routines, the Mono Soft Debugger and the CoreCLR security system required for Moonlight and other Web-based plugins.
On the class library System.IO.Packaging, WCF client, WCF server, LINQ to SQL debut. The Interactive shell supports auto-completion and the LINQ to SQL supports multiple database backends. The xbuild build system is introduced.
|2010-09-22||2.8||Defaults to .NET 4.0 profile, C# 4.0 support, new generational garbage collector, includes Parallel Extensions, WCF Routing, CodeContracts, ASP.NET 4.0, drops the 1.0 profile support; the LLVM engine tuned to support 99.9% of all generated code, runtime selectable llvm and gc; incorporates Dynamic Language Runtime, MEF, ASP.NET MVC2, OData Client open source code from Microsoft;. Will become release 3.0|
When Microsoft first announced their .NET Framework in June 2000 it was described as "a new platform based on Internet standards", and in December of that year the underlying Common Language Infrastructure was published as an open standard, "ECMA-335" - opening up the potential for independent implementations. Miguel de Icaza of Ximian believed that .NET had the potential to increase programmer productivity and began investigating whether a Linux version was feasible. Recognizing that their small team could not expect to build and support a full product, they launched the Mono open source project, on July 19, 2001 at the O'Reilly conference.
After three years development, Mono 1.0 was released on June 30, 2004. Mono evolved from its initial focus of a developer platform for Linux desktop applications to supporting a wide range of architectures and operating systems - including embedded systems.
On May 16, Miguel de Icaza announced in his blog that Mono would continue to be supported by Xamarin, a company he founded after being laid off from Novell. The original Mono team had also moved to the new company. Xamarin plans to keep working on Mono and had planned to rewrite the commercial .NET stacks for iOS and Android from scratch because Novell still owned MonoTouch and Mono for Android at the time. After this announcement, the future of the project was questioned, MonoTouch and Mono for Android being in direct competition with the existing commercial offerings now owned by Attachmate, and considering that the Xamarin team would have difficulties proving that they did not use technologies they formerly developed when they were employed by Novell for the same work. However, in July 2011, Novell, now a subsidiary of Attachmate, and Xamarin, announced that it granted a perpetual license to Xamarin for Mono, MonoTouch and Mono for Android, which officially took stewardship of the project.
Mono's current version is 2.10.8 (as of March 2012[update]). This version provides the core API of the .NET Framework and support for Visual Basic.NET and C# versions 2.0, 3.0, and 4.0. LINQ to Objects, XML, and SQL are part of the distribution. C# 4.0 is now the default mode of operation for the C# compiler. Windows Forms 2.0 is also supported, but not actively developed, and as such its support on Mono is incomplete.
Mono's aim is to achieve full support for the features in .NET 4.0 except Windows Presentation Foundation (WPF) (which the Mono team do not plan to support due to the amount of work it would need), Entity Framework and Windows Workflow Foundation (WF), limited Windows Communication Foundation (WCF). Some missing parts of the .NET Framework are under development in an experimental Mono subproject called Olive.
The Mono project has also created a Visual Basic .NET compiler and a runtime designed for running VB.NET applications. It is currently being developed by Rolf Bjarne Kvinge.
An open source implementation of Silverlight, called Moonlight, has been included since Mono 1.9. Moonlight 1.0, which supports the Silverlight 1.0 APIs, was released January 20, 2009. Moonlight 2.0 supports Silverlight 2.0 and some features of Silverlight 3.0. A preview release of Moonlight 3.0 was announced in February 2010 and contains updates to Silverlight 3 support.
The Moonlight project was abandoned on May 29th, 2012. According to Miguel, two factors sealed the fate of the project: Microsoft added "artificial restrictions" that "...made it useless for desktop programming...", and the technology had not gained enough traction on the web.
Mono consists of three groups of components:
The core components include the C# compiler, the virtual machine for the Common Language Infrastructure and the core 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. Microsoft issued a statement that covers both standards under their Community Promise license.
The Mono/Linux/GNOME development stack provide tools for application development while using the existing GNOME and free and open source libraries. These include: Gtk# for graphical user interface (GUI) development, Mozilla libraries for working with the Gecko rendering engine, Unix integration libraries (Mono.Posix), 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 the object-relational database db4o, Firebird, Microsoft SQL Server (MSSQL), MySQL, Open Database Connectivity (ODBC), Oracle, PostgreSQL, SQLite, 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 GNU/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 major components of Mono include:
The Mono runtime contains a code execution engine that translates ECMA CIL byte codes into native code and supports a number of processors: ARM, MIPS (in 32-bit mode only), SPARC, PowerPC, S390 (in 64-bit mode), x86, x86-64 and IA-64 for 64-bit modes.
The code generator is exposed in three modes:
Starting with Mono 2.6, it is possible to configure Mono to use the LLVM as the code generation engine instead of Mono's own code generation engine. This is useful for high performance computing loads and other situations where the execution performance is more important than the startup performance.
Starting with the Mono 2.7 preview, it is no longer necessary to pick one engine over the other at configuration time. The code generation engine can be selected at startup by using the
--nollvm command line arguments, and it defaults to the fast Mono code generation engine.
As of Mono 2.8, the Mono runtime ships with two garbage collectors, a generational collector and the Boehm conservative collector.
The current default garbage collector (the Boehm-Demers-Weiser Conservative Garbage Collector) has significant limitations compared to commercial garbage-collected runtimes like the Java Virtual Machine or the .NET framework's runtime, such as a conservative garbage collection can exhibit memory leaks on certain class of applications, this can make it unsuitable for long-running server applications.
As of October 2010[update], a new generational collector called the Simple Generational GC (SGen-GC) is part of Mono. Just like the LLVM code generation engine is now selectable at startup, users can elect to use the new SGen garbage collector by passing the
--gc=sgen switch to the Mono runtime at startup. This new garbage collector has many advantages over a traditional conservative scanner. It uses generational garbage collection where new objects are allocated from a nursery, during the garbage collection cycle, all objects that survived are migrated to an older generation memory pool. The idea being that many objects are transient and can quickly be collected and only a handful of objects are long-term objects that live for the entire life of the application. To improve performance this collector assigns memory pools to each thread to let threads allocate new memory blocks without having to coordinate with other threads. Migration of objects from the nursery to the old generation is done by copying the data from the nursery to the old generation pool and updating any live pointers that point to the data to point to the new location. This can be expensive for large objects, so Mono's SGen uses a separate pool of memory for large objects (Large Object Section) and uses a mark-and-sweep algorithm for those objects.
Currently SGen treats the stack and registers conservatively and pins any objects that could be referenced by any of these roots. The upcoming version of Mono scans the managed stack precisely reducing the number of pinned objects.
The class library provides a comprehensive set of facilities for application development. They are primarily written in C#, but due 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. Speaking of the .NET framework is primarily referring to this class library.
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. 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 like (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 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. 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.
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#.
Mono has innovated in some areas with new extensions to the core C# and CLI specifications:
In addition, Mono is available on a variety of operating systems and architectures.
Several projects extend Mono and allow developers to use it in their development environment. These projects include:
Microsoft has a version of .NET 2.0 now available only for Windows XP, 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).
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), although it also supports languages such as C, C++, Python, Java, and Vala. 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.
MonoTouch and Mono for Android, both developed by Xamarin, are implementations of Mono for iPhone and Android-based smart-phones. Unlike Mono itself, they are released under a commercial license only.
|2009-09-14||MonoTouch 1.0||Initial release|
|2010-04-05||MonoTouch 2.0||iPad support|
|2010-04-16||MonoTouch 3.0||iPhone 4 support|
|2011-10-12||MonoTouch 5.0||Latest version|
MonoTouch allows developers to create C# and .NET based applications that run on the iPhone. It is based on the Mono framework and developed in conjunction with Novell. Unlike Mono applications MonoTouch "Apps" are compiled down to machine code targeted specifically at the Apple iPhone. This is necessary because the iPhone kernel prevents just-in-time compilers from executing on the device.
The MonoTouch stack is made up of:
MonoDevelop is used as the primary IDE however additional links to Xcode and the iOS simulator have been written.
From April to early September 2010, the future of MonoTouch was put in doubt as Apple introduced new terms for iPhone developers that apparently prohibits them from developing in languages other than C, C++ and Objective-C, and the use of a middle layer between the iOS platform and iPhone applications. This made the future of MonoTouch, and other technologies such as UNITY, uncertain. Then, in September 2010, Apple rescinded this restriction, stating that they were relaxing the language restrictions that they had put in place earlier that year.
Mono for Android, developed by Xamarin, is a proprietary implementation of Mono for Android-based smart-phones. It was first released on April 6, 2011. Mono for Android was developed to allow developers to more easily write cross-platform applications that will run on all mobile platforms. In an interview with H-Online, Miguel de Icaza stated, "Our vision is to allow developers to reuse their engine and business logic code across all mobile platforms and swapping out the user interface code for a platform-specific API."
In August 2010, a Microsoft spokesman, Tom Hanrahan of Microsoft’s Open Source Technology Centre, stated, in reference to the lawsuit filed by Oracle against Google over Android's use of Java, that "The type of action Oracle is taking against Google over Java is not going to happen. If a .NET port to Android was through Mono it would fall under the Microsoft Community Promise Agreement."
The Mono for Android stack consists of the following components:
Mono is dual licensed by Xamarin, 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 (GPLv2 only) (starting with version 2.0 of Mono, the Mono C# compiler source code will also be available under the MIT X11 License), the runtime libraries under the GNU Lesser General Public License (LGPLv2 only) 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.
The license of the C# compiler was changed from the GPL to the MIT X11 license to allow the compiler code to be reused in a few instances where the GPL would have prevented such:
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. Yet, in June 2009 the Ubuntu Technical Board stated that it saw "no reason to exclude Mono or applications based upon it from the archive, or from the default installation set." 
The base technologies submitted to the ECMA, and therefore also the Unix/GNOME-specific parts, are claimed to be safe due to Microsoft's explicitly placing both ECMA 334 and ECMA 335 standards under the Microsoft Community Promise. The concerns primarily relate to technologies developed by Microsoft on top of the .NET Framework, such as ASP.NET, ADO.NET and Windows Forms (see non-standardized namespaces), i.e. parts composing Mono’s Windows compatibility stack. These technologies are today[when?] not fully implemented in Mono and not required for developing Mono-applications, they are simply there for developers and users who need full compatibility with the Windows system.
Should patent issues ever arise, the Mono project's stated strategy for dealing with them is as follows:
On July 6, 2009, Microsoft announced that it was placing their ECMA 334 and ECMA 335 specifications under their Community Promise pledging that they would not assert their patents against anyone implementing, distributing, or using alternative implementations of .NET. However, their position regarding the non-ECMA components like ASP.NET, ADO.NET, and Windows Forms (which are the bone of contention) remains unclarified.
The Free Software Foundation's Richard Stallman has stated in 2009-06-25 that "[...]we should discourage people from writing programs in C#. Therefore, we should not include C# implementations in the default installation of GNU/Linux distributions or in their principal ways of installing GNOME". In 2009-07-16, Brett Smith (also from the FSF) stated that "Microsoft's patents are much more dangerous: it's the only major software company that has declared itself the enemy of GNU/Linux and stated its intention to attack our community with patents.", "C# represents a unique threat to us" and "The Community Promise does nothing to change any of this".
Fedora Project Leader, Paul Frields, has stated "We do have some serious concerns about Mono and we'll continue to look at it with our legal counsel to see what if any steps are needed on our part", yet "We haven't come to a legal conclusion that is pat enough for us to make the decision to take mono out". 
A range of programs have been developed that use the Mono application programming interface (API) and C#. Some programs written for the Linux Desktop include Banshee, Beagle, Docky, F-Spot, Gbrainy, GNOME Do, MonoTorrent, Pinta, and Tomboy. A number of video games such as The Sims 3 and Second Life's scripting language, LSL (although not an official .NET language itself), OpenSimulator virtual world server, along with many games based on the Unity game engine also make use of Mono.
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