A tour of C# - Overview _ Microsoft Learn.pdf

A tour of the C# language
Article • 05/04/2023
C# (pronounced "See Sharp") is a modern, object-oriented, and type-safe programming
language. C# enables developers to build many types of secure and robust applications
that run in .NET. C# has its roots in the C family of languages and will be immediately
familiar to C, C++, Java, and JavaScript programmers. This tour provides an overview of
the major components of the language in C# 11 and earlier. If you want to explore the
language through interactive examples, try the introduction to C# tutorials.
C# is an object-oriented, component-oriented programming language. C# provides
language constructs to directly support these concepts, making C# a natural language
in which to create and use software components. Since its origin, C# has added features
to support new workloads and emerging software design practices. At its core, C# is an
object-oriented language. You define types and their behavior.
Several C# features help create robust and durable applications. Garbage collection
automatically reclaims memory occupied by unreachable unused objects. Nullable types
guard against variables that don't refer to allocated objects. Exception handling provides
a structured and extensible approach to error detection and recovery. Lambda
expressions support functional programming techniques. Language Integrated Query
(LINQ) syntax creates a common pattern for working with data from any source.
Language support for asynchronous operations provides syntax for building distributed
systems. C# has a unified type system. All C# types, including primitive types such as
int and double , inherit from a single root object type. All types share a set of
common operations. Values of any type can be stored, transported, and operated upon
in a consistent manner. Furthermore, C# supports both user-defined reference types
and value types. C# allows dynamic allocation of objects and in-line storage of
lightweight structures. C# supports generic methods and types, which provide increased
type safety and performance. C# provides iterators, which enable implementers of
collection classes to define custom behaviors for client code.
C# emphasizes versioning to ensure programs and libraries can evolve over time in a
compatible manner. Aspects of C#'s design that were directly influenced by versioning
considerations include the separate virtual and override modifiers, the rules for
method overload resolution, and support for explicit interface member declarations.
.NET architecture

C# programs run on .NET, a virtual execution system called the common language
runtime (CLR) and a set of class libraries. The CLR is the implementation by Microsoft of
the common language infrastructure (CLI), an international standard. The CLI is the basis
for creating execution and development environments in which languages and libraries
work together seamlessly.
Source code written in C# is compiled into an intermediate language (IL) that conforms
to the CLI specification. The IL code and resources, such as bitmaps and strings, are
stored in an assembly, typically with an extension of .dll. An assembly contains a
manifest that provides information about the assembly's types, version, and culture.
When the C# program is executed, the assembly is loaded into the CLR. The CLR
performs Just-In-Time (JIT) compilation to convert the IL code to native machine
instructions. The CLR provides other services related to automatic garbage collection,
exception handling, and resource management. Code that's executed by the CLR is
sometimes referred to as "managed code." "Unmanaged code," is compiled into native
machine language that targets a specific platform.
Language interoperability is a key feature of .NET. IL code produced by the C# compiler
conforms to the Common Type Specification (CTS). IL code generated from C# can
interact with code that was generated from the .NET versions of F#, Visual Basic, C++.
There are more than 20 other CTS-compliant languages. A single assembly may contain
multiple modules written in different .NET languages. The types can reference each
other as if they were written in the same language.
In addition to the runtime services, .NET also includes extensive libraries. These libraries
support many different workloads. They're organized into namespaces that provide a
wide variety of useful functionality. The libraries include everything from file input and
output to string manipulation to XML parsing, to web application frameworks to
Windows Forms controls. The typical C# application uses the .NET class library
extensively to handle common "plumbing" chores.
For more information about .NET, see Overview of .NET.
The "Hello, World" program is traditionally used to introduce a programming language.
Here it is in C#:
C#
Hello world
using System;

The "Hello, World" program starts with a using directive that references the System
namespace. Namespaces provide a hierarchical means of organizing C# programs and
libraries. Namespaces contain types and other namespaces—for example, the System
namespace contains a number of types, such as the Console class referenced in the
program, and many other namespaces, such as IO and Collections . A using
directive that references a given namespace enables unqualified use of the types that
are members of that namespace. Because of the using directive, the program can use
Console.WriteLine as shorthand for System.Console.WriteLine .
The Hello class declared by the "Hello, World" program has a single member, the
method named Main . The Main method is declared with the static modifier. While
instance methods can reference a particular enclosing object instance using the
keyword this , static methods operate without reference to a particular object. By
convention, a static method named Main serves as the entry point of a C# program.
The line starting with // is a single line comment. C# single line comments start with //
continue to the end of the current line. C# also supports multi-line comments. Multi-line
comments start with /* and end with */ . The output of the program is produced by
the WriteLine method of the Console class in the System namespace. This class is
provided by the standard class libraries, which, by default, are automatically referenced
by the compiler.
You use the .NET SDK to build your own "Hello, World" program. Once you install the
SDK, you run dotnet new console to create a basic "Hello, World" program that you
can modify. For more information, see the Hello, World tutorial in the .NET Get started
section.
A type defines the structure and behavior of any data in C#. The declaration of a type
may include its members, base type, interfaces it implements, and operations permitted
for that type. A variable is a label that refers to an instance of a specific type.
class Hello
{
static void Main()
{
// This line prints "Hello, World"
Console.WriteLine("Hello, World");
}
}
Types and variables

There are two kinds of types in C#: value types and reference types. Variables of value
types directly contain their data. Variables of reference types store references to their
data, the latter being known as objects. With reference types, it's possible for two
variables to reference the same object and possible for operations on one variable to
affect the object referenced by the other variable. With value types, the variables each
have their own copy of the data, and it isn't possible for operations on one to affect the
other (except for ref and out parameter variables).
An identifier is a variable name. An identifier is a sequence of unicode characters
without any whitespace. An identifier may be a C# reserved word, if it's prefixed by @ .
Using a reserved word as an identifier can be useful when interacting with other
languages.
C#'s value types are further divided into simple types, enum types, struct types, nullable
value types, and tuple value types. C#'s reference types are further divided into class
types, interface types, array types, and delegate types.
The following outline provides an overview of C#'s type system.
Value types
Simple types
Signed integral: sbyte , short , int , long
Unsigned integral: byte , ushort , uint , ulong
Unicode characters: char , which represents a UTF-16 code unit
IEEE binary floating-point: float , double
High-precision decimal floating-point: decimal
Boolean: bool , which represents Boolean values—values that are either
true or false
Enum types
User-defined types of the form enum E {...} . An enum type is a distinct
type with named constants. Every enum type has an underlying type, which
must be one of the eight integral types. The set of values of an enum type is
the same as the set of values of the underlying type.
Struct types
User-defined types of the form struct S {...}
Nullable value types
Extensions of all other value types with a null value
Tuple value types
User-defined types of the form (T1, T2, ...)
Reference types
Class types

Ultimate base class of all other types: object
Unicode strings: string , which represents a sequence of UTF-16 code units
User-defined types of the form class C {...}
Interface types
User-defined types of the form interface I {...}
Array types
Single-dimensional, multi-dimensional, and jagged. For example: int[] ,
int[,] , and int[][]
Delegate types
User-defined types of the form delegate int D(...)
C# programs use type declarations to create new types. A type declaration specifies the
name and the members of the new type. Six of C#'s categories of types are user-
definable: class types, struct types, interface types, enum types, delegate types, and
tuple value types. You can also declare record types, either record struct , or record
class . Record types have compiler-synthesized members. You use records primarily for
storing values, with minimal associated behavior.
A class type defines a data structure that contains data members (fields) and
function members (methods, properties, and others). Class types support single
inheritance and polymorphism, mechanisms whereby derived classes can extend
and specialize base classes.
A struct type is similar to a class type in that it represents a structure with data
members and function members. However, unlike classes, structs are value types
and don't typically require heap allocation. Struct types don't support user-
specified inheritance, and all struct types implicitly inherit from type object .
An interface type defines a contract as a named set of public members. A
class or struct that implements an interface must provide implementations
of the interface's members. An interface may inherit from multiple base
interfaces, and a class or struct may implement multiple interfaces.
A delegate type represents references to methods with a particular parameter list
and return type. Delegates make it possible to treat methods as entities that can
be assigned to variables and passed as parameters. Delegates are analogous to
function types provided by functional languages. They're also similar to the
concept of function pointers found in some other languages. Unlike function
pointers, delegates are object-oriented and type-safe.
The class , struct , interface , and delegate types all support generics, whereby
they can be parameterized with other types.

C# supports single-dimensional and multi-dimensional arrays of any type. Unlike the
types listed above, array types don't have to be declared before they can be used.
Instead, array types are constructed by following a type name with square brackets. For
example, int[] is a single-dimensional array of int , int[,] is a two-dimensional
array of int , and int[][] is a single-dimensional array of single-dimensional arrays, or
a "jagged" array, of int .
Nullable types don't require a separate definition. For each non-nullable type T , there's
a corresponding nullable type T? , which can hold an additional value, null . For
instance, int? is a type that can hold any 32-bit integer or the value null , and
string? is a type that can hold any string or the value null .
C#'s type system is unified such that a value of any type can be treated as an object .
Every type in C# directly or indirectly derives from the object class type, and object is
the ultimate base class of all types. Values of reference types are treated as objects
simply by viewing the values as type object . Values of value types are treated as
objects by performing boxing and unboxing operations. In the following example, an
int value is converted to object and back again to int .
C#
When a value of a value type is assigned to an object reference, a "box" is allocated to
hold the value. That box is an instance of a reference type, and the value is copied into
that box. Conversely, when an object reference is cast to a value type, a check is made
that the referenced object is a box of the correct value type. If the check succeeds, the
value in the box is copied to the value type.
C#'s unified type system effectively means that value types are treated as object
references "on demand." Because of the unification, general-purpose libraries that use
type object can be used with all types that derive from object , including both
reference types and value types.
There are several kinds of variables in C#, including fields, array elements, local
variables, and parameters. Variables represent storage locations. Every variable has a
type that determines what values can be stored in the variable, as shown below.
Non-nullable value type
A value of that exact type
int i = 123;
object o = i; // Boxing
int j = (int)o; // Unboxing

Nullable value type
A null value or a value of that exact type
object
A null reference, a reference to an object of any reference type, or a reference
to a boxed value of any value type
Class type
A null reference, a reference to an instance of that class type, or a reference to
an instance of a class derived from that class type
Interface type
A null reference, a reference to an instance of a class type that implements
that interface type, or a reference to a boxed value of a value type that
implements that interface type
Array type
A null reference, a reference to an instance of that array type, or a reference
to an instance of a compatible array type
Delegate type
A null reference or a reference to an instance of a compatible delegate type
The key organizational concepts in C# are programs, namespaces, types, members, and
assemblies. Programs declare types, which contain members and can be organized into
namespaces. Classes, structs, and interfaces are examples of types. Fields, methods,
properties, and events are examples of members. When C# programs are compiled,
they're physically packaged into assemblies. Assemblies typically have the file extension
.exe or .dll , depending on whether they implement applications or libraries,
respectively.
As a small example, consider the below C# code:
C#
Program structure
namespace Acme.Collections;
public class Stack<T>
{
Entry _top;
public void Push(T data)
{
_top = new Entry(_top, data);
}

The fully qualified name of this class is Acme.Collections.Stack . The class contains
several members: a field named _top , two methods named Push and Pop , and a
nested class named Entry . The Entry class further contains three members: a property
named Next , a property named Data , and a constructor. The Stack is a generic class. It
has one type parameter, T that is replaced with a concrete type when it's used.
A stack is a "first in - last out" (FILO) collection. New elements are added to the top of
the stack. When an element is removed, it's removed from the top of the stack. The
previous example declares the Stack type that defines the storage and behavior for a
stack. You can declare a variable that refers to an instance of the Stack type to use that
functionality.
Assemblies contain executable code in the form of Intermediate Language (IL)
instructions, and symbolic information in the form of metadata. Before it's executed, the
Just-In-Time (JIT) compiler of .NET Common Language Runtime converts the IL code in
an assembly to processor-specific code.
Because an assembly is a self-describing unit of functionality containing both code and
metadata, there's no need for #include directives and header files in C#. The public
types and members contained in a particular assembly are made available in a C#
program simply by referencing that assembly when compiling the program. For
public T Pop()
{
if (_top == null)
{
throw new InvalidOperationException();
}
T result = _top.Data;
_top = _top.Next;
return result;
}
class Entry
{
public Entry Next { get; set; }
public T Data { get; set; }
public Entry(Entry next, T data)
{
Next = next;
Data = data;
}
}
}

example, this program uses the Acme.Collections.Stack class from the acme.dll
assembly:
C#
To compile this program, you would need to reference the assembly containing the
stack class defined in the earlier example.
C# programs can be stored in several source files. When a C# program is compiled, all
of the source files are processed together, and the source files can freely reference each
other. Conceptually, it's as if all the source files were concatenated into one large file
before being processed. Forward declarations are never needed in C# because, with few
exceptions, declaration order is insignificant. C# doesn't limit a source file to declaring
only one public type nor does it require the name of the source file to match a type
declared in the source file.
Further articles in this tour explain these organizational blocks.
class Example
{
public static void Main()
{
var s = new Acme.Collections.Stack<int>();
s.Push(1); // stack contains 1
s.Push(10); // stack contains 1, 10
s.Push(100); // stack contains 1, 10, 100
Console.WriteLine(s.Pop()); // stack contains 1, 10
Console.WriteLine(s.Pop()); // stack contains 1
Console.WriteLine(s.Pop()); // stack is empty
}
}
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