.NET Framework Components

The .NET Framework is an integral Windows component that supports building and running the next generation of applications and XML Web services. The .NET Framework is designed to fulfill the following objectives: 1. Provide a consistent object-oriented programming environment whether object code is stored and executed locally, executed locally but Internet-distributed, or executed remotely. 2. Provide a code execution environment that minimizes software deployment and versioning conflicts. 3. Provide a code execution environment that guarantees safe execution of code, including code created by an unknown or semi-trusted third party. 4. Provide a code execution environment that eliminates the performance problems of scripted or interpreted environments. 5. Make the developer experience consistent across widely varying types of applications, such as Windows-based applications and Web-based applications. 6. Build all communication on industry standards to ensure that code based on the .NET Framework can integrate with any other code.
The .NET Framework has two main components:
1. The common language runtime
2. The .NET Framework class library
Principal design features
Interoperability
Because interaction between new and older applications is commonly required, the .NET Framework provides means to access functionality that is implemented in programs that execute outside the .NET environment. Access to COMcomponents is provided in the System.Runtime.InteropServices and System.EnterpriseServices namespaces of the framework; access to other functionality is provided using the P/Invoke feature.
Common Runtime Engine
The Common Language Runtime (CLR) is the virtual machine component of the .NET framework. All .NET programs execute under the supervision of the CLR, guaranteeing certain properties and behaviors in the areas of memory management, security, and exception handling.
Language Independence
The .NET Framework introduces a Common Type System, or CTS. The CTS specification defines all possible datatypes and programmingconstructs supported by the CLR and how they may or may not interact with each other. Because of this feature, the .NET Framework supports the exchange of instances of types between programs written in any of the .NET languages. This is discussed in more detail in Microsoft .NET Languages.
Base Class Library
The Base Class Library (BCL), part of the Framework Class Library (FCL), is a library of functionality available to all languages using the .NET Framework. The BCL provides classes which encapsulate a number of common functions, including file reading and writing, graphic rendering, database interaction and XML document manipulation.
Simplified Deployment
Installation of computer software must be carefully managed to ensure that it does not interfere with previously installed software, and that it conforms to increasingly stringent[citation needed] security requirements. The .NET framework includes design features and tools that help address these requirements.
Security
The design is meant to address some of the vulnerabilities, such as buffer overflows, that have been exploited by malicious software. Additionally, .NET provides a common security model for all applications.
Portability
The design of the .NET Framework allows it to theoretically be platform agnostic, and thus cross-platform compatible. That is, a program written to use the framework should run without change on any type of system for which the framework is implemented. Microsoft's commercial implementations of the framework cover Windows, Windows CE, and the Xbox 360. In addition, Microsoft submits the specifications for the Common Language Infrastructure (which includes the core class libraries, Common Type System, and the Common Intermediate Language), the C# language, and the C++/CLI language to both ECMA and the ISO, making them available as open standards. This makes it possible for third parties to create compatible implementations of the framework and its languages on other platforms.

Architecture
The core aspects of the .NET framework lie within the Common Language Infrastructure, or CLI. The purpose of the CLI is to provide a language-neutral platform for application development and execution, including functions for exception handling, garbage collection, security, and interoperability. Microsoft's implementation of the CLI is called the Common Language Runtime or CLR. The CLR is composed of four primary parts:
Common Type System (CTS)
Common Language Specification (CLS)
Metadata
Virtual Execution System (VES)

Common Type System(CTS)
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, 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).
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.
Type categories
The common type system supports two general categories of types:
Value types
Value types directly contain their data, and instances of value types are either allocated on the stack or allocated in line in a structure. Value types can be built-in (implemented by the runtime), user-defined, or enumerations.
Reference types
Reference types 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.
Common Language Specification
The Common Language Specification is a set of base rules to which any language targeting the CLI(Common Language Infrastructure) should conform in order to interoperate with other CLS-compliant languages. The CLS rules define a subset of the Common Type System.
.NET metadata
.NET metadata, in the Microsoft.NET framework, refers to certain data structures embedded within the Common Intermediate Language code that describes the high-level structure of the code. Metadata describes all classes and class members that are defined in the assembly, and the classes and class members that the current assembly will call from another assembly. The metadata for a method contains the complete description of the method, including the class (and the assembly that contains the class), the return type and all of the Method parameters.
A .NET language compiler will generate the metadata and store this in the assemblie containing the CIL. When the CLR executes CIL it will check to make sure that the metadata of the called method is the same as the metadata that is stored in the calling method. This ensures that a method can only be called with exactly the right number of parameters and exactly the right parameter types.
Attributes
Developers can add metadata to their code through attributes. There are two types of attributes, custom and pseudo custom attributes, and to the developer these have the same syntax. Attributes in code are messages to the compiler to generate metadata. In CIL, metadata such as inheritance modifiers, scope modifiers, and almost anything that isn't either opcodes or streams, are also referred to as attributes.
A custom attribute is a regular class that inherits from the Attribute class. A custom attribute can be used on any method, property, class or entire assembly with the syntax: [Attribute name(optional parameter, optional name=value pairs)] as in: [Custom]
[Custom(1)]
[Custom(1, comment="yes")]
Custom attributes are used by the .NET Framework extensively. Windows Communication Framework uses attributes to define service contracts, ASP.NET uses these to expose methods as web services, LINQ to SQL uses them to define the mapping of classes to the underlying relational schema,Visual Studio uses them to group together properties of an object, the class developer indicates the category for the object's class by applying the [Category] custom attribute. Custom attributes are interpreted by application code and not the CLR.When the compiler sees a custom attribute it will generate custom metadata that is not recognised by the CLR. The developer has to provide code to read the metadata and act on it. As an example, the attribute shown in the example can be handled by the code:class CustomAttribute : Attribute
{
int ParamNumber = 0;
string Comment = "";

public RecentAttribute() { }
public RecentAttribute(int num) { paramNumber = num; }

public String comment
{
set { Comment = value; }
}
}
Name of the class is mapped to the attribute name. The Visual C# compiler automatically adds the string "attribute" at the end of any attribute name. Consequently, every class implementing the handling of an attribute must end with this string but usage of this string is optional when using the attribute. Using the attribute invokes the constructor of the class. Overloaded constructors are supported. Name-Value pairs are mapped to properties, the name denotes the name of the property and the value supplied is set by the property.
A pseudo-custom attribute is used just like regular custom attributes but they do not have a custom handler; rather the compiler has intrinsic awareness of the attributes and handles the code marked with such attributes differently. Attributes such as Serializable and Obsolete are implemented as pseudo-custom attributes. Pseudo-custom attributes should never be used by ILASM, as it has adequate syntax to describe the metadata.
Metadata storage
Assemblies contain tables of metadata. These tables are described by the CIL specification. The metadata tables will have zero or more entries and the position of an entry determines its index. When CIL code uses metadata it does so through a metadata token. This is a 32-bit value where the top 8 bits identify the appropriate metadata table, and the remaining 24 bits give the index of the metadata in the table. The Framework SDK contains a sample called metainfo that will list the metadata tables in an assembly, however, this information is rarely of use to a developer. Metadata in an assembly may be viewed using the ILDASM tool provided by the .NET Framework SDK.
Reflection
Reflection is the API used to read .NET metadata. The reflection API provides a logical view of metadata rather than the literal view provided by tools like metainfo. Reflection in version 1.1 of the .NET framework can be used to inspect the descriptions of classes and their members, and invoke methods. However, it does not allow runtime access to the CIL for a method. Version 2.0 of the framework allows the CIL for a method to be obtained.
Other Metadata Tools
Besides the System.Reflection namespace, other tools are also available that can be used to handle metadata. The Microsoft .NET Framework ships a CLR metadata manipulation library that is implemented in native code. Third party tools to retrieve and manipulate metadata include PostSharp and Mono Cecil can also be used.

Assemblies
The intermediate CLI code is housed in .NET assemblies. As mandated by specification, assemblies are stored in the Portable Executable (PE) format, common on the Windows platform for all DLL and EXE files. The assembly consists of one or more files, one of which must contain the manifest, which has the meta-data for the assembly. The complete name of an assembly (not to be confused with the filename on disk) contains its simple text name, version number, culture, and public key token. The public key token is a unique hash generated when the assembly is compiled, thus two assemblies with the same public key token are guaranteed to be identical from the point of view of the framework. A private key can also be specified known only to the creator of the assembly and can be used for strong naming and to guarantee that the assembly is from the same author when a new version of the assembly is compiled (required to add an assembly to the Global Assembly Cache).
Metadata
All CLI is Self-Describing through .NET meta-data. The CLR checks on meta-data to ensure that the correct method is called. Meta-data is usually generated by language compilers but developers can create their own meta-data through custom attributes. Meta-data contains information about the assembly, and is also used to implement the reflective programming capabilities of .NET Framework.
Security
.NET has its own security mechanism with two general features: Code Access Security (CAS), and validation and verification. Code Access Security is based on evidence that is associated with a specific assembly. Typically the evidence is the source of the assembly (whether it is installed on the local machine or has been downloaded from the intranet or Internet). Code Access Security uses evidence to determine the permissions granted to the code. Other code can demand that calling code is granted a specified permission. The demand causes the CLR to perform a call stack walk: every assembly of each method in the call stack is checked for the required permission; if any assembly is not granted the permission a security exception is thrown.
When an assembly is loaded the CLR performs various tests. Two such tests are validation and verification. During validation the CLR checks that the assembly contains valid metadata and CIL, and whether the internal tables are correct. Verification is not so exact. The verification mechanism checks to see if the code does anything that is 'unsafe'. The algorithm used is quite conservative; hence occasionally code that is 'safe' does not pass. Unsafe code will only be executed if the assembly has the 'skip verification' permission, which generally means code that is installed on the local machine.
.NET Framework uses appdomains as a mechanism for isolating code running in a process. Appdomains can be created and code loaded into or unloaded from them independent of other appdomains. This helps increase the fault tolerance of the application, as faults or crashes in one appdomain do not affect rest of the application. Appdomains can also be configured independently with different security privileges. This can help increase the security of the application by isolating potentially unsafe code. The developer, however, has to split the application into subdomains; it is not done by the CLR.
Class library
Microsoft.NET Framework includes a set of standard class libraries. The class library is organized in a hierarchy of namespaces. Most of the built in APIs are part of either System.* or Microsoft.* namespaces. It encapsulates a large number of common functions, such as file reading and writing, graphic rendering, database interaction, and XML document manipulation, among others. The .NET class libraries are available to all .NET languages. The .NET Framework class library is divided into two parts: the Base Class Library and the Framework Class Library.
The Base Class Library (BCL) includes a small subset of the entire class library and is the core set of classes that serve as the basic API of the Common Language Runtime. The classes in mscorlib.dll and some of the classes in System.dll and System.core.dll are considered to be a part of the BCL. The BCL classes are available in both .NET Framework as well as its alternative implementations including .NET Compact Framework,Microsoft Silverlight and Mono.
The Framework Class Library (FCL) is a superset of the BCL classes and refers to the entire class library that ships with .NET Framework. It includes an expanded set of libraries, including WinForms, ADO.NET, ASP.NET, Language Integrated Query, Windows Presentation Foundation, Windows Communication Foundation among others. The FCL is much larger in scope than standard libraries for languages like C++, and comparable in scope to the standard libraries of Java.

Memory management
The .NET Framework CLR frees the developer from the burden of managing memory (allocating and freeing up when done); instead it does the memory management itself. To this end, the memory allocated to instantiations of .NET types (objects) is done contiguously from the managed heap, a pool of memory managed by the CLR. As long as there exists a reference to an object, which might be either a direct reference to an object or via a graph of objects, the object is considered to be in use by the CLR. When there is no reference to an object, and it cannot be reached or used, it becomes garbage. However, it still holds on to the memory allocated to it. .NET Framework includes a garbage collector which runs periodically, on a separate thread from the application's thread, that enumerates all the unusable objects and reclaims the memory allocated to them.
The .NET Garbage Collector (GC) is a non-deterministic, compacting, mark-and-sweep garbage collector. The GC runs only when a certain amount of memory has been used or there is enough pressure for memory on the system. Since it is not guaranteed when the conditions to reclaim memory are reached, the GC runs are non-deterministic. Each .NET application has a set of roots, which are pointers to objects on the managed heap (managed objects). These include references to static objects and objects defined as local variables or method parameters currently in scope, as well as objects referred to by CPU registers. When the GC runs, it pauses the application, and for each object referred to in the root, it recursively enumerates all the objects reachable from the root objects and marks them as reachable. It uses .NET meta-data and to discover the objects encapsulated by an object, and then recursively walk them. It then enumerates all the objects on the heap (which were initially allocated contiguously) using reflection. All objects not marked as reachable are garbage. This is the mark phase. Since the memory held by garbage is not of any consequence, it is considered free space. However, this leaves chunks of free space between objects which were initially contiguous. The objects are then compacted together, by using memcpy to copy them over to the free space to make them contiguous again. Any reference to an object invalidated by moving the object is updated to reflect the new location by the GC. The application is resumed after the garbage collection is over.
The GC used by .NET Framework is actually generational. Objects are assigned a generation; newly created objects belong to Generation 0. The objects that survive a garbage collection are tagged as Generation 1, and the Generation 1 objects that survive another collection are Generation 2 objects. The .NET Framework uses up to Generation 2 objects. Higher generation objects are garbage collected less frequently than lower generation objects. This helps increase the efficiency of garbage collection, as older objects tend to have a larger lifetime than newer objects. Thus, by removing older (and thus more likely to survive a collection) objects from the scope of a collection run, fewer objects need to be checked and compacted.

Versions

Microsoft started development on the .NET Framework in the late 1990s originally under the name of Next Generation Windows Services (NGWS). By late 2000 the first beta versions of .NET 1.0 were released.