C Sharp (programming language)

C# (pronounced "see sharp") is a multi-paradigm programming language encompassing imperative, functional, generic, object-oriented (class-based), and component-oriented programming disciplines. It was developed by Microsoft within the .NET initiative and later approved as a standard by Ecma (ECMA-334) and ISO (ISO/IEC 23270). C# is one of the programming languages designed for the Common Language Infrastructure.

C# is intended to be a simple, modern, general-purpose, object-oriented programming language. Its development team is led by Anders Hejlsberg, the designer of Borland's Turbo Pascal, who has said that its object-oriented syntax is based on C++ and other languages. The most recent version is C# 3.0, which was released in conjunction with the .NET Framework 3.5 in 2007. The next proposed version, 4.0, is in development.

Design goals

The Ecma standard lists these design goals for C#:

• C# language is intended to be a simple, modern, general-purpose, object-oriented programming language.
• The language, and implementations thereof, should provide support for software engineering principles such as strong type checking, array bounds checking, detection of attempts to use uninitialized variables, and automatic garbage collection. Software robustness, durability, and programmer productivity are important.
• The language is intended for use in developing software components suitable for deployment in distributed environments.
• Source code portability is very important, as is programmer portability, especially for those programmers already familiar with C and C++.
• Support for internationalization is very important.
• C# is intended to be suitable for writing applications for both hosted and embedded systems, ranging from the very large that use sophisticated operating systems, down to the very small having dedicated functions.
• Although C# applications are intended to be economical with regard to memory and processing power requirements, the language was not intended to compete directly on performance and size with C or assembly language.

Language name

File:C sharp music.svg

C-sharp musical note

The name "C sharp" was inspired from musical notation where a sharp indicates that the written note should be made a half-step higher in pitch. This is similar to the language name of C++, where "++" indicates that a variable should be incremented by 1.

Due to technical limitations of display (standard fonts, browsers, etc.) and the fact that the sharp symbol ([[Sharp (music)|Template:Music]], U+266F, MUSIC SHARP SIGN) is not present on the standard keyboard, the number sign (#, U+0023, NUMBER SIGN) was chosen to represent the sharp symbol in the written name of the programming language. This convention is reflected in the ECMA-334 C# Language Specification. However, when it is practical to do so (for example, in advertising or in box art), Microsoft uses the intended musical symbol.

The "sharp" suffix has been used by a number of other .NET languages that are variants of existing languages, including J# (a .NET language also designed by Microsoft which is derived from Java 1.1), A# (from Ada), and the functional F#. The original implementation of Eiffel for .NET was called Eiffel#, a name since retired since the full Eiffel language is now supported. The suffix has also been used for libraries, such as Gtk# (a .NET wrapper for GTK+ and other GNOME libraries), Cocoa# (a wrapper for Cocoa) and Qt# (a .NET language binding for the Qt toolkit).

History

During the development of the .NET Framework, the class libraries were originally written using a managed code compiler system called Simple Managed C (SMC). In January 1999, Anders Hejlsberg formed a team to build a new language at the time called Cool, which stood for "C-like Object Oriented Language". Microsoft had considered keeping the name "Cool" as the final name of the language, but chose not to do so for trademark reasons. By the time the .NET project was publicly announced at the July 2000 Professional Developers Conference, the language had been renamed C#, and the class libraries and ASP.NET runtime had been ported to C#.

C#'s principal designer and lead architect at Microsoft is Anders Hejlsberg, who was previously involved with the design of Turbo Pascal, Embarcadero Delphi (formerly CodeGear Delphi and Borland Delphi), and Visual J++. In interviews and technical papers he has stated that flaws in most major programming languages (e.g. C++, Java, Delphi, and Smalltalk) drove the fundamentals of the Common Language Runtime (CLR), which, in turn, drove the design of the C# programming language itself.

James Gosling, who created the Java programming language in 1994, and Billy Joy, a co-founder of Sun Microsystems, the proprietor of Java, called C# an "imitation" of Java; Gosling further claimed that "[C# is] sort of Java with reliability, productivity and security deleted." Klaus Kreft and Angelika Langer (authors of a C++ streams book) stated in a blog post that "Java and C# are almost identical programming languages. Boring repetition that lacks innovation," "Hardly anybody will claim that Java or C# are revolutionary programming languages that changed the way we write programs," and "C# borrowed a lot from Java - and vice versa. Now that C# supports boxing and unboxing, we'll have a very similar feature in Java."

Versions

In the course of its development, the C# language has gone through several versions:

Version	Language Specification	Microsoft Compiler
C# 1.0	December 2001	January 2002
C# 2.0	December 2002	November 2005
C# 3.0	June 2005	November 2006
C# 4.0	June 2006	Intended release March 2010 (Expected delayed)

Features

See also: C Sharp syntax

Note: The following description is based on the language standard and other documents listed in the external links section.

By design, C# is the programming language that most directly reflects the underlying Common Language Infrastructure (CLI). Most of its intrinsic types correspond to value-types implemented by the CLI framework. However, the language specification does not state the code generation requirements of the compiler: that is, it does not state that a C# compiler must target a Common Language Runtime, or generate Common Intermediate Language (CIL), or generate any other specific format. Theoretically, a C# compiler could generate machine code like traditional compilers of C++ or FORTRAN.

Some notable distinguishing features of C# are:

• There are no global variables or functions. All methods and members must be declared within classes. Static members of public classes can substitute for global variables and functions.
• Local variables cannot shadow variables of the enclosing block, unlike C and C++. Variable shadowing is often considered confusing by C++ texts.
• C# supports a strict Boolean datatype, bool. Statements that take conditions, such as while and if, require an expression of a type that implements the true operator, such as the boolean type. While C++ also has a boolean type, it can be freely converted to and from integers, and expressions such as if(a) require only that a is convertible to bool, allowing a to be an int, or a pointer. C# disallows this "integer meaning true or false" approach on the grounds that forcing programmers to use expressions that return exactly bool can prevent certain types of programming mistakes such as if (a = b) (use of = instead of ==).
• In C#, memory address pointers can only be used within blocks specifically marked as unsafe, and programs with unsafe code need appropriate permissions to run. Most object access is done through safe object references, which always either point to a "live" object or have the well-defined null value; it is impossible to obtain a reference to a "dead" object (one which has been garbage collected), or to a random block of memory. An unsafe pointer can point to an instance of a value-type, array, string, or a block of memory allocated on a stack. Code that is not marked as unsafe can still store and manipulate pointers through the System.IntPtr type, but it cannot dereference them.
• Managed memory cannot be explicitly freed; instead, it is automatically garbage collected. Garbage collection addresses the problem of memory leaks by freeing the programmer of responsibility for releasing memory which is no longer needed.
• In addition to the try...catch construct to handle exceptions, C# has a try...finally construct to guarantee execution of the code in the finally block.
• Multiple inheritance is not supported, although a class can implement any number of interfaces. This was a design decision by the language's lead architect to avoid complication and simplify architectural requirements throughout CLI.
• C# is more typesafe than C++. The only implicit conversions by default are those which are considered safe, such as widening of integers. This is enforced at compile-time, during JIT, and, in some cases, at runtime. There are no implicit conversions between booleans and integers, nor between enumeration members and integers (except for literal 0, which can be implicitly converted to any enumerated type). Any user-defined conversion must be explicitly marked as explicit or implicit, unlike C++ copy constructors and conversion operators, which are both implicit by default.
• Enumeration members are placed in their own scope.
• C# provides properties as syntactic sugar for a common pattern in which a pair of methods, accessor (getter) and mutator (setter) encapsulate operations on a single attribute of a class.
• Full type reflection and discovery is available.
• C# currently (as of 3 June 2008) has 77 reserved words.

Common Type system (CTS)

C# has a unified type system. This unified type system is called Common Type System (CTS).

A unified type system implies that all types, including primitives such as integers, are subclasses of the System.Object class. For example, every type inherits a ToString() method. For performance reasons, primitive types (and value types in general) are internally allocated on the stack.

Categories of datatypes

CTS separates datatypes into two categories:

1. Value types
2. Reference types

Value types are plain aggregations of data. Instances of value types do not have referential identity nor a referential comparison semantics - equality and inequality comparisons for value types compare the actual data values within the instances, unless the corresponding operators are overloaded. Value types are derived from System.ValueType, always have a default value, and can always be created and copied. Some other limitations on value types are that they cannot derive from each other (but can implement interfaces) and cannot have an explicit default (parameterless) constructor. Examples of value types are some primitive types, such as int (a signed 32-bit integer), float (a 32-bit IEEE floating-point number), char (a 16-bit Unicode code unit), and System.DateTime (identifies a specific point in time with nanosecond precision). Other examples are enum (enumerations) and struct(user defined structures).

In contrast, reference types have the notion of referential identity - each instance of a reference type is inherently distinct from every other instance, even if the data within both instances is the same. This is reflected in default equality and inequality comparisons for reference types, which test for referential rather than structural equality, unless the corresponding operators are overloaded (such as the case for System.String). In general, it is not always possible to create an instance of a reference type, nor to copy an existing instance, or perform a value comparison on two existing instances, though specific reference types can provide such services by exposing a public constructor or implementing a corresponding interface (such as ICloneable or IComparable). Examples of reference types are object (the ultimate base class for all other C# classes), System.String (a string of Unicode characters), and System.Array (a base class for all C# arrays).

Both type categories are extensible with user-defined types.

Boxing and unboxing

Boxing is the operation of converting a value of a value type into a value of a corresponding reference type. Boxing in C# is implicit.

Unboxing is the operation of converting a value of a reference type (previously boxed) into a value of a value type. Unboxing in C# requires an explicit type cast.

Example:

int foo = 42;         // Value type.
object bar = foo;     // foo is boxed to bar.
int foo2 = (int)bar;  // Unboxed back to value type.

Preprocessor

C# features "preprocessor directives" (though it does not have an actual preprocessor) based on the C preprocessor that allow programmers to define symbols but not macros. Conditionals such as #if, #endif, and #else are also provided. Directives such as #region give hints to editors for code folding.

public class Foo
{
    #region Procedures
    public void IntBar(int firstParam) {}  
    public void StrBar(string firstParam) {}  
    public void BoolBar(bool firstParam) {}  
    #endregion

    #region Constructors
    public Foo() {}
    public Foo(int firstParam) {}
    #endregion
}

Code comments

C# utilizes a double forward slash (//) to indicate the rest of the line is a comment.

public class Foo
{
    // a comment
    public static void Bar(int firstParam) {}  //Also a comment
}

Multi-line comments can be indicated by a starting forward slash/asterisk (/*) and ending asterisk/forward slash (*/).

public class Foo
{
    /* A Multi-Line
       comment  */
    public static void Bar(int firstParam) {}  
}

XML documentation system

C#'s documentation system is similar to Java's Javadoc, but based on XML. Two methods of documentation are currently supported by the C# compiler.

Single-line documentation comments, such as those commonly found in Visual Studio generated code, are indicated on a line beginning with ///.

public class Foo
{
    /// <summary>A summary of the method.</summary>
    /// <param name="firstParam">A description of the parameter.</param>
    /// <remarks>Remarks about the method.</remarks>
    public static void Bar(int firstParam) {}
}

Multi-line documentation comments, while defined in the version 1.0 language specification, were not supported until the .NET 1.1 release. These comments are designated by a starting forward slash/asterisk/asterisk (/**) and ending asterisk/forward slash (*/).

public class Foo
{
    /** <summary>A summary of the method.</summary>
     *  <param name="firstParam">A description of the parameter.</param>
     *  <remarks>Remarks about the method.</remarks> */
    public static void Bar(int firstParam) {}
}

Note there are some stringent criteria regarding white space and XML documentation when using the forward slash/asterisk/asterisk (/**) technique.

This code block:

/**
 * <summary>
 * A summary of the method.</summary>*/

produces a different XML comment from this code block:

/**
 * <summary>
   A summary of the method.</summary>*/

Syntax for documentation comments and their XML markup is defined in a non-normative annex of the ECMA C# standard. The same standard also defines rules for processing of such comments, and their transformation to a plain XML document with precise rules for mapping of CLI identifiers to their related documentation elements. This allows any C# IDE or other development tool to find documentation for any symbol in the code in a certain well-defined way.

Libraries

The C# specification details a minimum set of types and class libraries that the compiler expects to have available. In practice, C# is most often used with some implementation of the Common Language Infrastructure (CLI), which is standardized as ECMA-335 Common Language Infrastructure (CLI).

"Hello, world" example

The following is a very simple C# program, a version of the classic "Hello, world" example:

using System;
class ExampleClass
{
    static void Main()
    {
        Console.WriteLine("Hello, world!");
    }
}

The effect is to write the following text to the output console:

Hello, world!

Each line has a purpose:

using System;

The above line of code tells the compiler to use 'System' as a candidate prefix for types used in the source code. In this case, when the compiler sees use of the 'Console' type later in the source code, it tries to find a type named 'Console', first in the current assembly, followed by all referenced assemblies. In this case the compiler fails to find such a type, since the name of the type is actually 'System.Console'. The compiler then attempts to find a type named 'System.Console' by using the 'System' prefix from the using statement, and this time it succeeds. The using statement allows the programmer to state all candidate prefixes to use during compilation instead of always using full type names.

class ExampleClass

Above is a class definition. Everything between the following pair of braces describes ExampleClass.

static void Main()

This declares the class member method where the program begins execution. The .NET runtime calls the Main method. (Note: Main may also be called from elsewhere, e.g. from the code Main() in another method of ExampleClass.) The static keyword makes the method accessible without an instance of ExampleClass. Each console application's Main entry point must be declared static. Otherwise, the program would require an instance, but any instance would require a program. To avoid that irresolvable circular dependency, C# compilers processing console applications (like above) report an error if there is no static Main method. The void keyword declares that Main has no return value (see also side effect).

Console.WriteLine("Hello, world!");

This line writes the output. Console is a static class in the System namespace. It provides an interface to the standard input, output, and error streams for console applications. The program calls the Console method WriteLine, which displays on the console a line with the argument, the string "Hello, world!".

Standardization

In August, 2000, Microsoft Corporation, Hewlett-Packard and Intel Corporation co-sponsored the submission of specifications for C# as well as the Common Language Infrastructure (CLI) to the standards organization ECMA International. In December 2001, ECMA released ECMA-334 C# Language Specification. C# became an ISO standard in 2003 (ISO/IEC 23270:2006 - Information technology—Programming languages—C#). ECMA had previously adopted equivalent specifications as the 2nd edition of C#, in December 2002.

In June 2005, ECMA approved edition 3 of the C# specification, and updated ECMA-334. Additions included partial classes, anonymous methods, nullable types, and generics (similar to C++ templates).

In July 2005, ECMA submitted the standards and related TRs to ISO/IEC JTC 1 via the latter's Fast-Track process. This process usually takes 6–9 months.

Criticism

Although the C# language definition and the CLI are standardized under ISO and Ecma standards which provide reasonable and non-discriminatory licensing protection from patent claims, Microsoft uses C# and the CLI in its Base Class Library (BCL) which is the foundation of its proprietary .NET framework, and which provides a variety of non-standardized classes (extended I/O, GUI, web services, etc). Some cases where Microsoft patents apply to standards used in the .NET framework are documented by Microsoft and the applicable patents are available on either RAND terms or through Microsoft's Open Specification Promise which releases patent rights to the public, but there is some concern and debate as to whether there are additional aspects patented by Microsoft which are not covered, which may deter independent implementations of the full framework. Microsoft has also agreed not to sue open source developers for violating patents in non-profit projects for the part of the framework which is covered by the OSP. Microsoft has agreed not to enforce patents relating to Novell products against Novell's paying customers with the exception of a list of products that do not explicitly mention C#, .NET or Novell's implementation of .NET (The Mono Project),. However Novell maintains that Mono does not infringe any Microsoft patents. Microsoft has also made a specific agreement not to enforce patent rights related to the Moonlight browser plugin, which depends on Mono, provided it is obtained through Novell.

In a note posted on the Free Software Foundation's news website in June 2009, Richard Stallman warned that he believes "Microsoft is probably planning to force all free C# implementations underground some day using software patents" and recommended that developers avoid taking what he described as the "gratuitous risk" associated with "depend[ing] on the free C# implementations".. The Free Software Foundation later reiterated its warnings, claiming that the extension of Microsoft Community Promise to the C# and the CLI ECMA specifications would not prevent Microsoft from harming Open-Source implementations of C#, because many specific Windows libraries included with .NET or Mono were not covered by this promise.

Implementations

The reference C# compiler is Microsoft Visual C#.

Other C# compilers exist, often including an implementation of the Common Language Infrastructure and the .NET class libraries up to .NET 2.0:

• Microsoft's Rotor project (currently called Shared Source Common Language Infrastructure) (licensed for educational and research use only) provides a shared source implementation of the CLR runtime and a C# compiler, and a subset of the required Common Language Infrastructure framework libraries in the ECMA specification (up to C# 2.0, and supported on Windows XP only).
• The Mono project provides an open source C# compiler, a complete open source implementation of the Common Language Infrastructure including the required framework libraries as they appear in the ECMA specification, and a nearly complete implementation of the Microsoft proprietary .NET class libraries up to .NET 2.0, but not specific .NET 3.0 and .NET 3.5 libraries, as of Mono 2.0.
• The DotGNU project also provides an open source C# compiler, a nearly complete implementation of the Common Language Infrastructure including the required framework libraries as they appear in the ECMA specification, and subset of some of the remaining Microsoft proprietary .NET class libraries up to .NET 2.0 (those not documented or included in the ECMA specification but included in Microsoft's standard .NET Framework distribution).
• The DotNetAnywhere Micro Framework-like Common Language Runtime is targeted at embedded systems, and supports almost all C# 2.0 specifications.

See also

• C# Syntax
• Sing#
• Mono and Microsoft’s patents
• .NET Framework Standardization and licensing
• Oxygene
• Microsoft Visual Studio, IDE for C#
• SharpDevelop, an open-source C# IDE for Windows
• MonoDevelop, an open-source C# IDE for Linux Windows and Mac OS X
• QuickSharp, an open-source, minimalist C# IDE for Windows
• Morfik C#, a C# to JavaScript compiler complete with IDE and framework for Web application development.
• Baltie, an educational IDE for children and students with little or no programming experience
• Borland Turbo C Sharp

Notes

References

• Archer, Tom (2001). Inside C#. Microsoft Press. ISBN 0-7356-1288-9. 
• C# Language Pocket Reference. O' Reilly. 2002. ISBN 0-596-00429-X. 
• Petzold, Charles (2002). Programming Microsoft Windows with C#. Microsoft Press. ISBN 0-7356-1370-2. 

External links

• C# Language (MSDN)
• C# Programming Guide (MSDN)
• C# Specification (MSDN)
• ISO C# Language Specification.
• C# Language Specification (hyperlinked)
• Microsoft Visual C# .NET