The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
, the Common Type System (CTS) is a standard that specifies how Type definitions and specific values of Types are represented in
computer memory. It is intended to allow programs written in different programming languages to easily share information. As used in programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s, a Type can be described as a definition of a set of values (for example, "all integers between 0 and 10"), and the allowable operations on those values (for example, addition and subtraction).
The specification for the CTS is contained in Ecma
ECMA
Ecma or ECMA may refer to:* Ecma International, an international standards organization for Information Communication Technology and Consumer Electronics* Engineering College Magazines Associated, a group of student-run engineering-based publications...
standard 335, "Common Language Infrastructure (CLI) Partitions I to VI." The CLI
Common Language Infrastructure
The Common Language Infrastructure is an open specification developed by Microsoft and standardized by ISO and ECMA that describes the executable code and runtime environment that form the core of the Microsoft .NET Framework and the free and open source implementations Mono and Portable.NET...
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
is an implementation of the standard.
Functions of the Common Type System
To establish a framework that helps enable cross-language integration, type safety, and high performance code execution .
To provide an object-oriented model that supports the complete implementation of many programming languages.
To define rules that languages must follow, which helps ensure that objects written in different languages can interact with each other.
The CTS also defines the rules that ensures that the data types of objects written in various languages are able to interact with each other.
The CTS also specifies the rules for type visibility and access to the members of a type, i.e. the CTS establishes the rules by which assemblies form scope for a type, and the Common Language Runtime enforces the visibility rules.
The CTS defines the rules governing type inheritance, virtual methods and object lifetime.
Languages supported by .NET can implement all or some common data types
Rounding a numerical value means replacing it by another value that is approximately equal but has a shorter, simpler, or more explicit representation; for example, replacing $23.4476 with $23.45, or the fraction 312/937 with 1/3, or the expression √2 with 1.414.Rounding is often done on purpose to...
fractional values, the halfway-to-even ("banker's") method is used by default, throughout the Framework. Since version 2, "Symmetric Arithmetic Rounding" (round halves away from zero) is also available by programmer's option.
it is used to communicate with other languages
Type categories
The common type system supports two general categories of types:
Value types : Value type
Value type
In computer science, the term value type is commonly used to refer to one of two kinds of data types: Types of values or Types of objects with deep copy semantics.- Types of Values :...
s directly contain their data, and instances of value types are either allocated on the stack
Call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
or allocated inline in a structure. Value types can be built-in (implemented by the runtime), user-defined, or enumerations.
Reference types : Reference type
Reference type
In programming language theory, a reference type is a data type that can only be accessed by references. Unlike objects of value types, objects of reference types cannot be directly embedded into composite objects and are always dynamically allocated...
s store a reference to the value's memory address, and are allocated on the heap. Reference types can be self-describing types, pointer types, or interface types. The type of a reference type can be determined from values of self-describing types. Self-describing types are further split into arrays and class types. The class types are user-defined classes, boxed value types, and delegates.
Visual Basic .NET , is an object-oriented computer programming language that can be viewed as an evolution of the classic Visual Basic , which is implemented on the .NET Framework...
shows the difference between reference types and value types:
Imports System
Class Class1
Public Value As Integer = 0
End Class 'Class1
Class Test
Shared Sub Main
Dim val1 As Integer = 0
Dim val2 As Integer = val1
val2 = 123
Dim ref1 As New Class1
Dim ref2 As Class1 = ref1
ref2.Value = 123
Console.WriteLine("Values: {0}, {1}", val1, val2)
Console.WriteLine("Refs: {0}, {1}", ref1.Value, ref2.Value)
End Sub 'Main
End Class 'Test
The output of the above example
Values: 0, 123
Refs: 123, 123
Boxing
Converting value types to reference types is also known as boxing. As can be seen in the example below, it is not necessary to tell the compiler an Int32 is boxed to an object, because it takes care of this itself.
Int32 x = 10;
object o = x ; // Implicit boxing
Console.WriteLine("The Object o = {0}",o); // prints out 10
However, an Int32 can always be explicitly boxed like this:
Int32 x = 10;
object o = (object) x; // Explicit boxing
Console.WriteLine("The object o = {0}",o); // prints out 10
Unboxing
The following example intends to show how to unbox a reference type back to a value type. First an Int32 is boxed to an object, and then it is unboxed again. Note that unboxing requires explicit cast.
Int32 x = 5;
object o1 = x; // Implicit Boxing
x = (int)o1; // Explicit Unboxing
