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In the Microsoft .NET framework
Software framework

A software framework, in computer programming, is an abstraction in which common code providing generic functionality can be selectively overridden or specialized by user code providing specific functionality....
, an assembly is a partially compiled
Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language . The most common reason for wanting to transform source code is to create an executable program....
 code library for use in deployment, versioning and security. In the Microsoft Windows
Microsoft Windows

Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces ....
 implementation of .NET, an assembly is a PE (portable executable
Portable Executable

The Portable Executable format is a file format for executables, object file, and Dynamic-link librarys, used in 32-bit and 64-bit versions of Microsoft Windows operating systems....
) file for Windows GUI on Intel x86. There are two types: process assemblies (EXE) and library assemblies (DLL). A process assembly represents a process which will use classes
Class (computer science)

In object-oriented programming, a class is a programming language construct that is used as a blueprint to create Object s. This blueprint includes Attribute s and Method s that the created objects all share....
 defined in library assemblies. In version 1.1 of the CLR
Common Language Runtime

The Common Language Runtime is a core component of Microsoft .NET Framework initiative. It is Microsoft's implementation of the Common Language Infrastructure standard, which defines an execution environment for program code....
, classes can only be exported from library assemblies; in version 2.0 this restriction is relaxed.






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In the Microsoft .NET framework
Software framework

A software framework, in computer programming, is an abstraction in which common code providing generic functionality can be selectively overridden or specialized by user code providing specific functionality....
, an assembly is a partially compiled
Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language . The most common reason for wanting to transform source code is to create an executable program....
 code library for use in deployment, versioning and security. In the Microsoft Windows
Microsoft Windows

Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces ....
 implementation of .NET, an assembly is a PE (portable executable
Portable Executable

The Portable Executable format is a file format for executables, object file, and Dynamic-link librarys, used in 32-bit and 64-bit versions of Microsoft Windows operating systems....
) file for Windows GUI on Intel x86. There are two types: process assemblies (EXE) and library assemblies (DLL). A process assembly represents a process which will use classes
Class (computer science)

In object-oriented programming, a class is a programming language construct that is used as a blueprint to create Object s. This blueprint includes Attribute s and Method s that the created objects all share....
 defined in library assemblies. In version 1.1 of the CLR
Common Language Runtime

The Common Language Runtime is a core component of Microsoft .NET Framework initiative. It is Microsoft's implementation of the Common Language Infrastructure standard, which defines an execution environment for program code....
, classes can only be exported from library assemblies; in version 2.0 this restriction is relaxed. The compiler will have a switch to determine if the assembly is a process or library and will set a flag in the PE file. .NET does not use the extension to determine if the file is a process or library. This means that a library may have either .dll or .exe as its extension. .NET assemblies contain code in CIL
Common Intermediate Language

Common Intermediate Language is the lowest-level human-readable programming language in the Common Language Infrastructure and in the .NET Framework....
, which is generated from .NET languages, which is then compiled into machine language at runtime
Runtime

In computer science, runtime or run time describes the operation of a computer program, the duration of its execution, from beginning to termination ....
 by the CLR just-in-time compiler.

An assembly can consist of one or more files. Code files are called modules. An assembly can contain more than one code module and since it is possible to use different languages to create code modules it is technically possible to use several different languages to create an assembly. Visual Studio however does not support using different languages in one assembly.

Assembly names

The name of an assembly consists of four parts:

  1. The short name. On Windows this is the name of the PE file without the extension.
  2. The culture. This is an RFC 1766 identifier of the locale for the assembly. In general, library and process assemblies should be culture neutral; the culture should only be used for satellite assemblies.
  3. The version. This is a dotted number made up of 4 values — major, minor, build and revision. The version is only used if the assembly has a strong name (see below).
  4. A public key token. This is a 64-bit
    Bit

    A bit is a binary numeral system numerical digit, taking a value of either 0 or 1. Binary digits are a basic unit of information Computer data storage and transmission in digital computing and digital information theory....
     hash
    Cryptographic hash function

    A cryptographic hash function is a algorithm that takes an arbitrary block of data and returns a fixed-size bit string, the hash value, such that an accidental or intentional change to the data will almost certainly change the hash value....
     of the public key which corresponds to the private key used to sign the assembly. A signed assembly is said to have a strong name.


The public key token is used to make the assembly name unique. Thus, two strong named assemblies can have the same PE file name and yet .NET will recognize them as different assemblies. The Windows file system
File system

In computing, a file system is a method for store and organize computer files and the data they contain to make it easy to find and access them....
 (FAT32
File Allocation Table

File Allocation Table or FAT is a computer file system architecture now widely used on most computer systems and most memory cards, such as those used with digital cameras....
 and NTFS
NTFS

NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
) only recognizes the PE file name, so two assemblies with the same PE file name (but different culture, version or public key token) cannot exist in the same Windows folder. To solve this issue .NET introduces something called the GAC (Global Assembly Cache
Global Assembly Cache

The Global Assembly Cache or GAC is a machine-wide .NET assemblies cache for Microsoft's Common Language Runtime Platform . The approach of having a specially controlled central repository addresses the shared library concept and helps to avoid pitfalls of other solutions that lead to drawbacks like DLL hell....
) which is treated as a single folder by the .NET CLR, but is actually implemented using nested NTFS (or FAT32) folders.

To prevent spoofing attack
Spoofing attack

In the context of network security, a spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage....
s, where a cracker
Black hat

A black hat is the villain or bad guy, especially in a Western in which such a character would wear a black hat in contrast to the hero white hat....
 would try to pass off an assembly appearing as something else, the assembly is signed with a private key. The developer of the intended assembly keeps the private key secret, so a cracker cannot have access to it nor simply guess it. Thus the cracker cannot make his assembly impersonate something else, lacking the possibility to correctly sign it after the change. Signing the assembly involves taking a hash of important parts of the assembly and then encrypting
Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key ....
 the hash with the private key. The signed hash is stored in the assembly along with the public key. The public key will decrypt the signed hash. When the CLR loads a strongly named assembly it will generate a hash from the assembly and then compare this with the decrypted hash. If the comparison succeeds then it means that the public key in the file (and hence the public key token) is associated with the private key used to sign the assembly. This will mean that the public key in the assembly is the public key of the assembly publisher and hence a spoofing attack is thwarted.

Assemblies versions

.NET assemblies can have version information, allowing them to eliminate most conflicts between applications caused by shared assemblies. However, this does not eliminate all possible versioning conflicts between assemblies.

Assemblies and .NET security

.NET Code Access Security
Code Access Security

Code Access Security , in the Microsoft .NET framework, is Microsoft's solution to prevent untrusted code from performing privileged actions. When the Common Language Runtime loads an .NET assemblies it will obtain #Evidence for the assembly and use this to identify the #Code group that the assembly belongs to....
 is based on assemblies and evidence
Code Access Security

Code Access Security , in the Microsoft .NET framework, is Microsoft's solution to prevent untrusted code from performing privileged actions. When the Common Language Runtime loads an .NET assemblies it will obtain #Evidence for the assembly and use this to identify the #Code group that the assembly belongs to....
. Evidence can be anything deduced from the assembly, but typically it is created from the source of the assembly — whether the assembly was downloaded from the Internet, an intranet
Intranet

An intranet is a private computer networking that uses Internet technologies to securely share any part of an organization's information or operational systems with its employees....
, or installed on the local machine (if the assembly is downloaded from another machine it will be stored in a sandboxed
Sandbox (computer security)

In computer security, a sandbox is a security mechanism for safely running programs. It is often used to execute untested code, or untrusted programs from unverified third-parties, suppliers and untrusted users....
 location within the GAC and hence is not treated as being installed locally). Permissions are applied to entire assemblies, and an assembly can specify the minimum permissions it requires through custom attributes (see .NET metadata
.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....
). When the assembly is loaded the CLR will use the evidence for the assembly to create a permission set of one or more code access permissions. The CLR will then check to make sure that this permission set contains the required permissions specified by the assembly.

.NET code can perform a code access security demand. This means that the code will perform some privileged action only if all of the assemblies of all of the methods
Method (computer science)

In object-oriented programming, a method is a subroutine that is exclusively associated either with a class or with an object . Like a procedure in procedural programming languages, a method usually consists of a sequence of statement to perform an action, a set of input parameter to customize those actions, and possibly an output value...
 in the call stack
Call stack

In computer science, a call stack is a dynamic Stack data structure that stores information about the active subroutines of a computer program....
 have the specified permission. If one assembly does not have the permission a security exception is thrown.

The .NET code can also perform Linked Demand for getting the permission from the call stack. In this case the CLR will look for only one method in the call stack in the TOP position has the specified permission. Here the stack walk-through is bound to one method in the call stack by which the CLR assumes that all the other methods in the CALL STACK have the specified permission.

Satellite assemblies

In general, assemblies should contain culture-neutral resources. If you want to localize your assembly (for example use different strings
String (computer science)

In computer programming and some branches of mathematics, a string is an ordered sequence of symbols. These symbols are chosen from a predetermined set or alphabet....
 for different locales) you should use satellite assemblies — special, resource-only assemblies. Satellites are not loaded by Fusion and so they should not contain code. As the name suggests, a satellite is associated with an assembly called the main assembly. That assembly (say, lib.dll) will contain the neutral resources (which Microsoft says is International English
English language

English is a West Germanic language that originated in Anglo-Saxon England and has lingua franca status in many parts of the world as a result of the military, economic, scientific, political and cultural influence of the British Empire in the 18th, 19th and early 20th centuries and that of the United States from the mid 20th century onwa...
, but implies to be US English). Each satellite has the name of the associated library appended with .resources (for example lib.resources.dll). The satellite is given a non-neutral culture name, but since this is ignored by existing Windows file systems (FAT32 and NTFS) this would mean that there could be several files with the same PE name in one folder. Since this is not possible, satellites must be stored in subfolders under the application folder. For example, a satellite with the UK English resources will have a .NET name of "lib.resources Version=0.0.0.0 Culture=en-GB PublicKeyToken=null", a PE file name of lib.resources.dll, and will be stored in a subfolder called en-GB.

Satellites are loaded by a .NET class called System.Resources.ResourceManager. The developer has to provide the name of the resource and information about the main assembly (with the neutral resources). The ResourceManager class will read the locale of the machine and use this information and the name of the main assembly to get the name of the satellite and the name of the subfolder that contains it. ResourceManager can then load the satellite and obtain the localized resource.

Referencing assemblies


One can reference an executable code library by using the /reference flag of the C# compiler .

Delay-signing of an assembly


The shared assemblies need to give a strong name for uniquely identifying the assembly which might be shared among the applications. The strong naming consists of the public key token, culture, version and PE file name. If an assembly is likely to be used for the development purpose which is a shared assembly, the strong naming procedure contains only public key generation. The private key is not generated at that time. It is generated only when the assembly is deployed.

Language of an assembly

The assembly is built up with the CIL code. CIL code is a form of .NET assembly language (it is not assembly language
Assembly language

An assembly language is a low-level language for programming computers. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture....
!). The framework internally converts the high level language code into assembly code. If we have a program that prints "Hello world", the equivalent CIL code for the method is:

.method private hidebysig static void Main(string[] args) cil managed // end of method Class1Main

So the assembly code loads the String first into stack. Then it calls the Writeline function and stores the address where the control should return after the function is over.

See also




External links