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C# is an object-oriented programming language developed by Microsoft as part of the .NET initiative and later approved as a standard by ECMA (ECMA-334) and ISO (ISO/IEC 23270). Anders Hejlsberg, the Designer of Delphi, leads development of the C# language, which has an object-oriented syntax based on C++ and includes influences from aspects of several other programming languages (most notably Delphi and Java) with a particular emphasis on simplification.
By design, C# is the programming language that most directly reflects the underlying Common Language Infrastructure (CLI). Most of C#'s intrinsic types correspond to value-types implemented by the CLI framework. However, the C# 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 (CLR), 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; in practice, all existing C# implementations target CIL.
C# differs from C and C++ as much as it resembles Java, including:
- There are no global variables or functions. All methods and members must be declared within classes. It is possible, however, to use static methods/variables within public classes instead of global variables/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 type, bool. Statements that take conditions, such as while and if, require an expression of a 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 references, which cannot be made invalid. 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 cannot dereference them.
- Managed memory cannot be explicitly freed, but is automatically garbage collected. Garbage collection addresses memory leaks. C# also provides direct support for deterministic finalization with the using statement (supporting the Resource Acquisition Is Initialization idiom).
- 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, avoid dependency hell 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 and conversion from a derived type to a base type. This is enforced at compile-time, during JIT, and, in some cases, at runtime. There are no implicit conversions between booleans and integers and between enumeration members and integers (except 0, which can be implicitly converted to an enumerated type), and any user-defined conversion must be explicitly marked as explicit or implicit, unlike C++ copy constructors (which are implicit by default) and conversion operators (which are always implicit).
- Enumeration members are placed in their own namespace.
- Accessors called properties can be used to modify an object with syntax that resembles C++ member field access. In C++, declaring a member public enables both reading and writing to that member, and accessor methods must be used if more fine-grained control is needed. In C#, properties allow control over member access and data validation.
- Full type reflection and discovery is available.
- C# currently (as of 3 June 2008) has 77 reserved words.
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