C Sharp (programming language)

C#

Paradigm	Multi-paradigm: structured, imperative, object-oriented, event-driven, task-driven, functional, generic, reflective, concurrent
Family	C
Designed by	Microsoft
Developer	Microsoft
First appeared	2000; 24 years ago (2000)[1]
Stable release	9.0[2] / November 10, 2020; 3 years ago (2020-11-10)
Typing discipline	Static, dynamic,[3] strong, safe, nominative, partially inferred
Platform	Common Language Infrastructure
License	Roslyn compiler: MIT/X11[4] .NET Core CLR: MIT/X11[5] Mono compiler: dual GPLv3 and MIT/X11 DotGNU: dual GPL and LGPL
Filename extensions	.cs, .csx
Website	docs.microsoft.com/en-us/dotnet/csharp/
Major implementations
Visual C#, .NET Framework, Mono, .NET Core, DotGNU (discontinued), Universal Windows Platform
Dialects
Cω, Polyphonic C#, Enhanced C#
Influenced by
C++,[6] Cω, Eiffel, F#,[lower-alpha 1] Haskell, Icon, J#, J++, Java,[6] ML, Modula-3, Object Pascal,[7] VB
Influenced
Chapel,[8] Clojure,[9] Crystal,[10] D, J#, Dart,[11] F#, Hack, Java,[12][13] Kotlin, Nemerle, Oxygene, Rust, Swift,[14] Vala, TypeScript
C Sharp Programming at Wikibooks

C# (/si ʃɑːrp/ see sharp)^{[lower-alpha 2]} is a general-purpose, multi-paradigm programming language. C# encompasses static typing, strong typing, lexically scoped, imperative, declarative, functional, generic, object-oriented (class-based), and component-oriented programming disciplines.^[15]

C# was developed around 2000 by Microsoft as part of its .NET initiative and later approved as an international standard by Ecma (ECMA-334) in 2002 and ISO (ISO/IEC 23270) in 2003. It was designed by Anders Hejlsberg, and its development team is currently led by Mads Torgersen, being one of the programming languages designed for the Common Language Infrastructure (CLI). The most recent version is 9.0, which was released in 2020 in .NET 5.0 and included in Visual Studio 2019 version 16.8.^[16][17]

Mono is a free and open-source project to develop a cross-platform compiler and runtime environment (i.e. virtual machine) for the language.

Design goals

The Ecma standard lists these design goals for C#:^[15]

• The 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.
• Portability is very important for source code and programmers, especially those 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.

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).^[18][19] 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".^[20] 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#.

Hejlsberg is C#'s principal designer and lead architect at Microsoft, and was previously involved with the design of Turbo Pascal, Embarcadero Delphi (formerly CodeGear Delphi, Inprise Delphi and Borland Delphi), and Visual J++. In interviews and technical papers he has stated that flaws^[21] 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# language itself.

James Gosling, who created the Java programming language in 1994, and Bill Joy, a co-founder of Sun Microsystems, the originator of Java, called C# an "imitation" of Java; Gosling further said that "[C# is] sort of Java with reliability, productivity and security deleted."^[22][23] 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,"^[24] "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."^[25] In July 2000, Hejlsberg said that C# is "not a Java clone" and is "much closer to C++" in its design.^[26]

Since the release of C# 2.0 in November 2005, the C# and Java languages have evolved on increasingly divergent trajectories, becoming two quite different languages. One of the first major departures came with the addition of generics to both languages, with vastly different implementations. C# makes use of reification to provide "first-class" generic objects that can be used like any other class, with code generation performed at class-load time.^[27] Furthermore, C# has added several major features to accommodate functional-style programming, culminating in the LINQ extensions released with C# 3.0 and its supporting framework of lambda expressions, extension methods, and anonymous types.^[28] These features enable C# programmers to use functional programming techniques, such as closures, when it is advantageous to their application. The LINQ extensions and the functional imports help developers reduce the amount of boilerplate code that is included in common tasks like querying a database, parsing an xml file, or searching through a data structure, shifting the emphasis onto the actual program logic to help improve readability and maintainability.^[29]

C# used to have a mascot called Andy (named after Anders Hejlsberg). It was retired on January 29, 2004.^[30]

C# was originally submitted to the ISO subcommittee JTC 1/SC 22 for review,^[31] under ISO/IEC 23270:2003,^[32] was withdrawn and was then approved under ISO/IEC 23270:2006.^[33] The 23270:2006 is withdrawn under 23270:2018 and approved with this version.^[34]

Name

Microsoft first used the name C# in 1988 for a variant of the C language designed for incremental compilation.^[35] That project was not completed but the name lives on.

C-sharp musical note

The name "C sharp" was inspired by the musical notation whereby a sharp symbol indicates that the written note should be made a semitone higher in pitch.^[36] This is similar to the language name of C++, where "++" indicates that a variable should be incremented by 1 after being evaluated. The sharp symbol also resembles a ligature of four "+" symbols (in a two-by-two grid), further implying that the language is an increment of C++.^[37]

Due to technical limitations of display (standard fonts, browsers, etc.) and the fact that the sharp symbol (U+266F ♯ MUSIC SHARP SIGN (HTML ♯)) is not present on most keyboard layouts, the number sign (U+0023 # NUMBER SIGN (HTML #)) was chosen to approximate the sharp symbol in the written name of the programming language.^[38] This convention is reflected in the ECMA-334 C# Language Specification.^[15]

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 that is derived from Java 1.1), A# (from Ada), and the functional programming language F#.^[39] The original implementation of Eiffel for .NET was called Eiffel#,^[40] a name 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) and Cocoa# (a wrapper for Cocoa).

Versions

Syntax

The core syntax of the C# language is similar to that of other C-style languages such as C, C++ and Java, particularly:

• Semicolons are used to denote the end of a statement.
• Curly brackets are used to group statements. Statements are commonly grouped into methods (functions), methods into classes, and classes into namespaces.
• Variables are assigned using an equals sign, but compared using two consecutive equals signs.
• Square brackets are used with arrays, both to declare them and to get a value at a given index in one of them.

Distinguishing features

Some notable features of C# that distinguish it from C, C++, and Java where noted, are:

Portability

By design, C# is the programming language that most directly reflects the underlying Common Language Infrastructure (CLI).^[57] 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.

Typing

C# supports strongly, implicitly typed variable declarations with the keyword var, and implicitly typed arrays with the keyword new[] followed by a collection initializer.

C# supports a strict Boolean data type, 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 assignment = instead of equality ==).

C# is more type safe than C++. The only implicit conversions by default are those that are considered safe, such as widening of integers. This is enforced at compile-time, during JIT, and, in some cases, at runtime. No implicit conversions occur 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.

C# has explicit support for covariance and contravariance in generic types, unlike C++ which has some degree of support for contravariance simply through the semantics of return types on virtual methods.

Enumeration members are placed in their own scope.

The C# language does not allow for 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++.

Metaprogramming

Metaprogramming can be achieved in several ways:

• Reflection using framework API
• Expression tree^[58] language feature represents code as an abstract syntax tree, where each node is an expression that can be inspected or executed. This enables dynamic modification of executable code at runtime. Expression tree introduced some homoiconicity to the language.
• Attribute language feature are metadata attached to a field or a block of code like assemblies, members and types, and are equivalent to annotations in Java. Attributes are accessible to both the compiler and programmatically through reflection. Many of these attributes duplicate the functionality of GCC's and VisualC++'s platform-dependent preprocessor directives.
• Source generators,^[59] feature of Roslyn C# compiler, enable compile time metaprogramming. During the compilation process, developers can inspect the code being compiled (using compiler API) and generate new C# source files that can be added to the compilation.

Methods and functions

A method in C# is a member of a class that can be invoked as a function (a sequence of instructions), rather than the mere value-holding capability of a class property. As in other syntactically similar languages, such as C++ and ANSI C, the signature of a method is a declaration comprising in order: any optional accessibility keywords (such as private), the explicit specification of its return type (such as int, or the keyword void if no value is returned), the name of the method, and finally, a parenthesized sequence of comma-separated parameter specifications, each consisting of a parameter's type, its formal name and optionally, a default value to be used whenever none is provided. Certain specific kinds of methods, such as those that simply get or set a class property by return value or assignment, do not require a full signature, but in the general case, the definition of a class includes the full signature declaration of its methods.

Like C++, and unlike Java, C# programmers must use the scope modifier keyword virtual to allow methods to be overridden by subclasses.^[60]

Extension methods in C# allow programmers to use static methods as if they were methods from a class's method table, allowing programmers to add methods to an object that they feel should exist on that object and its derivatives.

The type dynamic allows for run-time method binding, allowing for JavaScript-like method calls and run-time object composition.

C# has support for strongly-typed function pointers via the keyword delegate. Like the Qt framework's pseudo-C++ signal and slot, C# has semantics specifically surrounding publish-subscribe style events, though C# uses delegates to do so.

C# offers Java-like synchronized method calls, via the attribute [MethodImpl(MethodImplOptions.Synchronized)], and has support for mutually-exclusive locks via the keyword lock.

Property

C# supports classes with properties. The properties can be simple accessor functions with a backing field, or implement getter and setter functions.

Since C# 3.0 the syntactic sugar of auto-implemented properties is available,^[61] where the accessor (getter) and mutator (setter) encapsulate operations on a single attribute of a class.

Namespace

A C# namespace provides the same level of code isolation as a Java package or a C++

namespace

, with very similar rules and features to a package. Namespaces can be imported with the "using" syntax.^[62]

Memory access

In C#, memory address pointers can only be used within blocks specifically marked as unsafe,^[63] 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 that has been garbage collected), or to a random block of memory. An unsafe pointer can point to an instance of an 'unmanaged' value type that does not contain any references to garbage-collected objects, array, string, or a block of stack-allocated memory. 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 that is no longer needed in most cases. Code that retains references to objects longer than is required can still experience higher memory usage than necessary, however once the final reference to an object is released the memory is available for garbage collection.

Exception

A range of standard exceptions are available to programmers. Methods in standard libraries regularly throw system exceptions in some circumstances and the range of exceptions thrown is normally documented. Custom exception classes can be defined for classes allowing specific handling to be put in place for particular circumstances as needed.^[64]

Checked exceptions are not present in C# (in contrast to Java). This has been a conscious decision based on the issues of scalability and versionability.^[65]

Polymorphism

Unlike C++, C# does not support multiple inheritance, although a class can implement any number of "interfaces" (fully abstract classes). This was a design decision by the language's lead architect to avoid complications and to simplify architectural requirements throughout CLI.

When implementing multiple interfaces that contain a method with the same name and taking parameters of the same type in the same order (i.e. the same signature), similar to Java, C# allows both a single method to cover all interfaces and if necessary specific methods for each interface.

However, unlike Java, C# supports operator overloading.^[66] Only the most commonly overloaded operators in C++ may be overloaded in C#.

Language Integrated Query (LINQ)

C# has the ability to utilize LINQ through the .NET Framework. A developer can query a variety of data sources, provided IEnumerable<T> interface is implemented on the object. This includes XML documents, an ADO.NET dataset, and SQL databases.^[67]

Using LINQ in C# brings advantages like Intellisense support, strong filtering capabilities, type safety with compile error checking ability, and consistency for querying data over a variety of sources.^[68] There are several different language structures that can be utilized with C# and LINQ and they are query expressions, lambda expressions, anonymous types, implicitly typed variables, extension methods, and object initializers.^[69]

Functional programming

Though primarily an imperative language, C# 2.0 offered limited support for functional programming through first-class functions and closures in the form of anonymous delegates.^[70] C# 3.0 expanded support for functional programming with the introduction of a lightweight syntax for lambda expressions,^[71] extension methods (an affordance for modules), and a list comprehension syntax in the form of a "query comprehension" language. C# 7.0 adds features typically found in functional languages like tuples, local functions and pattern matching.^[72] C# 9.0 introduces record feature^[73] which is primarily built for better supporting immutable data models.

Common type system

C# has a unified type system. This unified type system is called Common Type System (CTS).^[74]

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.

Categories of data types

CTS separates data types into two categories:^[74]

1. Reference types
2. Value types

Instances of value types neither have referential identity nor 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 all 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, meaning that 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

). Some operations are not always possible, such as creating an instance of a reference type, copying an existing instance, or performing 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-type object into a value of a corresponding reference type.^[74] 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.^[74] Unboxing in C# requires an explicit type cast. A boxed object of type T can only be unboxed to a T (or a nullable T).^[75]

Example:

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

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).

In addition to the standard CLI specifications, there are many commercial and community class libraries that build on top of the .NET Framework libraries to provide additional functionality.^[76]

C# can make calls to any library included in the List of .NET libraries and frameworks.

Examples

Hello World

The following is a very simple C# program, a version of the classic "Hello world" example using the top-level statements feature introduced in C# 9:^[77]

using System;

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

For code written as C# 8 or lower, the entry point logic of a program must be written in a Main method inside a type:

using System;

// A version of the classic "Hello World" program
class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello, world!");
    }
}

This code will display this text in the console window:

Hello, world!

Each line has a purpose:

using System;

The above line imports all types in the System namespace. For example, the Console class used later in the source code is defined in the System namespace, meaning it can be used without supplying the full name of the type (which includes the namespace).

// A version of the classic "Hello World" program

This line is a comment; it describes and documents the code for the programmer(s).

class Program

Above is a class definition for the

Program

class. Everything that follows between the pair of braces describes that class.

{
   ...

}

The curly brackets demarcate the boundaries of a code block. In this first instance, they are marking the start and end of the

Program

class.

static void Main(string[] args)

This declares the class member method where the program begins execution. The .NET runtime calls the

Main

method. Unlike in Java, the

Main

method does not need the

public

keyword, which tells the compiler that the method can be called from anywhere by any class.^[78] Writing

static void Main(string[] args)

is equivalent to writing

private static void Main(string[] args)

. The static keyword makes the method accessible without an instance of

Program

. Each console application's

Main

entry point must be declared

static

otherwise the program would require an instance of

Program

, but any instance would require a program. To avoid that irresolvable circular dependency, C# compilers processing console applications (like that above) report an error if there is no

static Main

method. The

void

keyword declares that

Main

has no return value.

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!"

.

GUI

A GUI example:

using System;
using System.Windows.Forms;

class Program
{
    static void Main()
    {
        MessageBox.Show("Hello, World!");
        Console.WriteLine("Is almost the same argument!");
    }
}

This example is similar to the previous example, except that it generates a dialog box that contains the message "Hello, World!" instead of writing it to the console.

Images

Another useful library is the System.Drawing library, which is used to programmatically draw images. For example:

using System;
using System.Drawing;

public class Example
{
    public static Image img;

    static void Main()
    {
        img = Image.FromFile("Image.png");
    }
}

This will create an image that is identical to that stored in "Image.png".

Standardization and licensing

In August 2001, 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 23270:2003 - 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 (somewhat similar to C++ templates).

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

The C# language definition and the CLI are standardized under ISO and Ecma standards that provide reasonable and non-discriminatory licensing protection from patent claims.

Microsoft has agreed not to sue open source developers for violating patents in non-profit projects for the part of the framework that is covered by the OSP.^[79] Microsoft has also agreed not to enforce patents relating to Novell products against Novell's paying customers^[80] with the exception of a list of products that do not explicitly mention C#, .NET or Novell's implementation of .NET (The Mono Project).^[81] However, Novell maintains that Mono does not infringe any Microsoft patents.^[82] 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.^[83]

Implementations

Microsoft is leading the development of the open-source reference C# compilers and set of tools. The first compiler, Roslyn, compiles into intermediate language (IL), and the second one, RyuJIT,^[84] is a JIT (just-in-time) compiler, which is dynamic and does on-the-fly optimization and compiles the IL into native code for the front-end of the CPU.^[85] RyuJIT is open source and written in C++.^[86] Roslyn is entirely written in managed code (C#), has been opened up and functionality surfaced as APIs. It is thus enabling developers to create refactoring and diagnostics tools.^[4][87] Two branches of official implementation are .NET Framework (closed source, Windows 10 only since .NET 4.6.2) and .NET Core (open source, multiplatform); .NET Framework and .NET Core converged into one open source implementation .NET 5.0.^[88] At .NET Framework 4.6 a new JIT compiler replaced the former.^[84][89]

Other C# compilers (some of which include an implementation of the Common Language Infrastructure and .NET class libraries):

• 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 3.5. As of Mono 2.6, no plans exist to implement WPF; WF is planned for a later release; and there are only partial implementations of LINQ to SQL and WCF.^[90]
• The Elements tool chain from RemObjects includes RemObjects C#, which compiles C# for .NET, Java, Cocoa, Android, Windows, Linux and WebAssembly.* The DotGNU project (now discontinued) also provided 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).
• Xamarin provides tools to develop cross-platform applications for common mobile and desktop operating systems, using C# as a codebase and compiling to native code.

Mono is a common choice for game engines due to its cross-platform nature.^{[citation needed]} The Unity game engine uses Mono C# as its primary scripting language. The Godot game engine has implemented an optional Mono C# module thanks to a donation of $24,000 from Microsoft.^[91]

See also

Notes

References

Further reading

• Drayton, Peter; Albahari, Ben; Neward, Ted (2002). C# Language Pocket Reference. O'Reilly. ISBN 0-596-00429-X. https://archive.org/details/clanguagepocketr00pete. 
• Petzold, Charles (2002). Programming Microsoft Windows with C#. Microsoft Press. ISBN 0-7356-1370-2. https://archive.org/details/isbn_9780735613706. 

External links

• C# Language Specification
• C# Programming Guide
• ISO C# Language Specification
• C# Compiler Platform ("Roslyn") source code

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