Name

Microsoft first used the name C# in 1988 for a variant of the C language designed for incremental compilation.^[37] That project was not completed, and the name was later reused.

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.^[38] 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++.^[39]

Due to technical limitations of display (standard fonts, browsers, etc.) and the fact that the sharp symbol (U+266F ♯ MUSIC SHARP SIGN (♯)) is not present on most keyboard layouts, the number sign (U+0023 # NUMBER SIGN (#)) was chosen to approximate the sharp symbol in the written name of the programming language.^[40] This convention is reflected in the ECMA-334 C# Language Specification.^[17]

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#.^[41] The original implementation of Eiffel for .NET was called Eiffel#,^[42] 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

Portability

By design, C# is the programming language that most directly reflects the underlying Common Language Infrastructure (CLI).^[68] 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. Some C# compilers can also generate machine code like traditional compilers of C++ or Fortran.^[69][70]

Typing

C# supports strongly, implicitly typed variable declarations with the keyword var,^[16]: 470 and implicitly typed arrays with the keyword new[] followed by a collection initializer.^{[16]: 80 [20]: 58}

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,^{[16]: 144 [20]: 23} 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 is supported through .NET APIs, which enable scenarios such as type metadata inspection and dynamic method invocation.
• Expression trees^[71] represent 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 trees introduce some homoiconicity to the language.
• Attributes are metadata that can be attached to types, members, or entire assemblies, equivalent to annotations in Java. Attributes are accessible both to the compiler and to code through reflection. Many of native attributes duplicate the functionality of GCC's and VisualC++'s platform-dependent preprocessor directives.
• System.Reflection.Emit namespace,^[72] which contains classes that emit metadata and CIL (types, assemblies, etc) at runtime.
• The .NET Compiler Platform (Roslyn) provides API access to language compilation services, allowing for the compilation of C# code from within .NET applications. It exposes APIs for syntactic (lexical) analysis of code, semantic analysis, dynamic compilation to CIL, and code emission.^[73]
• Source generators,^[74] a feature of the Roslyn C# compiler, enable compile time metaprogramming. During the compilation process, developers can inspect the code being compiled with the compiler's API and pass additional generated C# source code to be compiled.

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.^[75]

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.^{[16]: 103–105 [20]: 202–203}

The type dynamic allows for run-time method binding, allowing for JavaScript-like method calls and run-time object composition.^{[16]: 114–118}

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,^[76] 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.^[77]

Memory access

In C#, memory address pointers can only be used within blocks specifically marked as unsafe,^[78] 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 objects subject to garbage collections such as class instances, arrays or strings. 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 handling to be put in place for particular circumstances as needed.^[79]

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.^[80]

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.^[81]

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.^[82]

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.^[83] 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.^[84]

LINQ has two syntaxes: query syntax and method syntax. However, the compiler always converts the query syntax to method syntax at compile time.^[85]

using System.Linq;

var numbers = new int[] { 5, 10, 8, 3, 6, 12 };

// Query syntax (SELECT num FROM numbers WHERE num % 2 = 0 ORDER BY num)
var numQuery1 =
        from num in numbers
        where num % 2 == 0
        orderby num
        select num;

// Method syntax
var numQuery2 = 
        numbers
        .Where(num => num % 2 == 0)
        .OrderBy(n => n);

Functional programming

Though primarily an imperative language, C# always adds functional features over time,^[86][87] for example:

• Functions as first-class citizen – C# 1.0 delegates^[88]
• Higher-order functions – C# 1.0 together with delegates
• Anonymous functions – C# 2 anonymous delegates and C# 3 lambdas expressions^[89]
• Closures – C# 2 together with anonymous delegates and C# 3 together with lambdas expressions^[89]
• Type inference – C# 3 with implicitly typed local variables var and C# 9 target-typed new expressions new()
• List comprehension – C# 3 LINQ
• Tuples – .NET Framework 4.0 but it becomes popular when C# 7.0 introduced a new tuple type with language support^[90]
• Nested functions – C# 7.0^[90]
• Pattern matching – C# 7.0^[90]
• Immutability – C# 7.2 readonly struct C# 9 record types^[91] and Init only setters^[92]
• Type classes – C# 12 roles/extensions (in development^[93])

Categories of data types

CTS separates data types into two categories:^[94]

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 because they already have an implicit one which initializes all contained data to the type-dependent default value (0, null, or alike). 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.^[94] 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.^[94] 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).^[95]

Example:

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

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:^[97]

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;

class Program
{
    static void Main()
    {
        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()

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.^[98] 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. (Note, however, that short programs can be written using Top Level Statements introduced in C# 9, as mentioned earlier.)

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 Windows 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".