To the surprise of most developers, .NET assemblies can be developed and executed on non- Microsoft operating systems—Mac OS X, various Linux distributions, Solaris, as well as the Apple iPhone device (via the MonoTouch API). To understand how this is possible, you need to come to terms with yet another abbreviation in the .NET universe: CLI (Common Language Infrastructure).
When Microsoft released the C# programming language and the .NET platform, they also crafted a set of formal documents that described the syntax and semantics of the C# and CIL languages, the .NET assembly format, core .NET namespaces, and the mechanics of a hypothetical .NET runtime engine (known as the Virtual Execution System, or VES).
Better yet, these documents have been submitted to (and ratified by) Ecma International as official international standards (http://www.ecma-international.org). The specifications of interest are
ECMA-334: The C# Language Specification
ECMA-335: The Common Language Infrastructure (CLI)
The importance of these documents becomes clear when you understand that they enable third parties to build distributions of the .NET platform for any number of operating systems and/or processors. ECMA-335 is the more “meaty” of the two specifications, so much so that is has been broken into six partitions, as shown in the following table.
Table 1-4. Partitions of the CLI
| Partitions of ECMA-335 | Meaning in Life |
|---|---|
| Partition I: Concepts and Architecture | Describes the overall architecture of the CLI, including the rules of the CTS and CLS, and the mechanics of the .NET runtime engine. |
| Partition II: Metadata Definition and Semantics | Describes the details of .NET metadata and the assembly format. |
| Partition III: CIL Instruction Set | Describes the syntax and semantics of CIL code. |
| Partition IV: Profiles and Libraries | Gives a high-level overview of the minimal and complete class libraries that must be supported by a .NET distribution. |
| Partition V | Debug Interchange Format |
| Partition VI: Annexes | Provides a collection of odds-and-ends details such as class library design guidelines and the implementation details of a CIL compiler. |
Be aware that Partition IV (Profiles and Libraries) defines only a minimal set of namespaces that represent the core services expected by a CLI distribution (e.g., collections, console I/O, file I/O, threading, reflection, network access, core security needs, XML data manipulation). The CLI does not define namespaces that facilitate web development (ASP.NET), database access (ADO.NET), or desktop graphical user interface (GUI) application development (Windows Forms/Windows Presentation Foundation).
The good news, however, is that the mainstream .NET distributions extend the CLI libraries with Microsoft-compatible equivalents of ASP.NET, ADO.NET, and Windows Forms in order to provide fullfeatured, production-level development platforms. To date, there are two major implementations of the CLI (beyond Microsoft’s Windows-specific offering). Although this text focuses on the creation of .NET applications using Microsoft’s .NET distribution, Table 1-5 provides information regarding the Mono and Portable .NET projects.
Table 1-5. Open Source .NET Distributions
| Distribution | Meaning in Life |
|---|---|
| http://www.mono-project.com | The Mono project is an open source distribution of the CLI that targets various Linux distributions (e.g., openSUSE, Fedora) as well as Windows and Mac OS X / iPhone devices. |
| http://www.dotgnu.org | Portable.NET is another open source distribution of the CLI that runs on numerous operating systems. Portable.NET aims to target as many operating systems as possible (e.g., Windows, AIX, BeOS, Mac OS X, Solaris, all major Linux distributions). |
Both Mono and Portable.NET provide an ECMA-compliant C# compiler, .NET runtime engine, code samples, documentation, as well as numerous development tools that are functionally equivalent to the tools that ship with Microsoft’s .NET Framework 4.0 SDK. Furthermore, Mono and Portable.NET collectively ship with a VB .NET, Java, and C complier.
Note Coverage of creating cross-platform .NET applications using Mono can be found in Appendix B.