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 sameAttribute
class. A custom attribute can be used on any method, property, class or entire assembly with the syntax: 'AttributeName''(optional ''parameter'', optional ''name=value'' pairs)/code> as in:
ustom ustom(1) ustom(1, Comment="yes")
Custom attributes are used by CLI extensively. Windows Communication Framework
The Windows Communication Foundation (WCF), previously known as Indigo, is a free and open-source runtime and a set of APIs in the .NET Framework for building connected, service-oriented applications.
.NET Core 1.0, released 2016, did not sup ...
uses attributes to define service contracts, ASP.NET
ASP.NET is an open-source, server-side web-application framework designed for web development to produce dynamic web pages. It was developed by Microsoft to allow programmers to build dynamic web sites, applications and services. The name s ...
uses these to expose methods as web services, LINQ to SQL
Language Integrated Query (LINQ, pronounced "link") is a Microsoft .NET Framework component that adds native data Query language, querying capabilities to List of CLI languages, .NET languages, originally released as a major part of .NET Framework ...
uses them to define the mapping of classes to the underlying relational schema, Visual Studio
Visual Studio is an integrated development environment (IDE) from Microsoft. It is used to develop computer programs including web site, websites, web apps, web services and mobile apps. Visual Studio uses Microsoft software development platfor ...
uses them to group together properties
Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property.
Property may also refer to:
Mathematics
* Property (mathematics)
Philosophy and science
* Property (philosophy), in philosophy and ...
of an object, the class developer indicates the category for the object's class by applying the ategory/code> 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
The 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 attribute class name should end with this string, but it is legal to define an attribute without the Attribute
-suffix. When affixing an attribute to an item, the compiler will look for both the literal name and the name with Attribute
added to the end, i. e. if you were to write ustom/code> the compiler would look for both Custom
and CustomAttribute
. If both exist, the compiler fails. The attribute can be prefixed with "@
" if you don't want to risk ambiguity, so writing Custom
Custom, customary, or consuetudinary may refer to:
Traditions, laws, and religion
* Convention (norm), a set of agreed, stipulated or generally accepted rules, norms, standards or criteria, often taking the form of a custom
* Norm (social), a r ...
/code> will not match CustomAttribute
. 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.
Sometimes there is ambiguity concerning to what you are affixing the attribute. Consider the following code:
range
Range may refer to:
Geography
* Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra)
** Mountain range, a group of mountains bordered by lowlands
* Range, a term used to i ...
public int ExampleMethod(string input)
What has been marked as orange? Is it the ExampleMethod
, its return value, or perhaps the entire assembly? In this case, the compiler will default, and treat the attribute as being affixed to the method. If this is not what was intended, or if the author wishes to clarify their code, an ''attribute target'' may be specified. Writing eturn: Orange/code> will mark the return value as orange, ssembly: Orange/code> will mark the entire assembly. The valid targets are assembly
, field
, event
, method
, module
, param
, property
, return
and type
.
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
ILAsm (IL Assembler) generates a portable executable (PE) file from a text representation of Common Intermediate Language (CIL) code. It is not to be confused with NGEN (Native Image Generator), which compiles Common Intermediate Language code int ...
, 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
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
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 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
The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...
SDK.
In the CIL standard, metadata is defined in ILAsm (assembly language) form, an on-disk representation form for storage, and a form that is embedded into assemblies of the Portable Executable
The Portable Executable (PE) format is a file format for executables, object code, DLLs and others used in 32-bit and 64-bit versions of Windows operating systems. The PE format is a data structure that encapsulates the information necessary fo ...
(PE, .exe or .dll) format. The PE form is based on the on-disk form.
Reflection
Reflection is the API
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standa ...
used to read CLI 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 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
In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
. Third-party tools to retrieve and manipulate metadata includ
PostSharp
an
Mono Cecil
can also be used.
See also
* Java annotation
In the Java computer programming language, an annotation is a form of syntactic metadata that can be added to Java source code. Classes, methods, variables, parameters and Java packages may be annotated. Like Javadoc tags, Java annotations can ...
References
{{Common Language Infrastructure
Common Language Infrastructure