Attribute (computing)

<h2 id="rules-and-typing">Rules and typing</h2>
<ul><li>Rules: Each named attribute has an associated set of rules called <a href="/facts/Operation_(mathematics)/Yplv602Q">operations</a>: For example, one doesn't sum characters or manipulate and process an <a href="/facts/Integer/PUwwrawx">integer</a> <a href="/facts/Array_data_structure/BKGvF8AM">array</a> the same way as an image object. Neither does one process text as if it was type of floating point (<a href="/facts/Decimal_number/7omfVNIM">decimal numbers</a>).</li>
<li>Data types: It follows that an object definition can be extended by imposing <a href="/facts/Data_type/fcx5hgVL">data typing</a> which can consist of a representation format, a default value, and legal operations (rules) and restrictions (e.g. "division by zero is not to be tolerated") are all potentially involved in defining an attribute, or conversely one may view them as attributes of that object's type.</li></ul>
<p>Picture file formats (for example <a href="/facts/JPEG/HQ8QS3uB">JPEG</a>, <a href="/facts/PNG/gj0685PR">PNG</a> and <a href="/facts/Bitmap/Fr80ySnT">BMP</a>) are not decoded using the same operations (however similar the images look — these are all graphics data formats). Similarly, a programming language does not use the same operations to evaluate a floating point typed number and typed long integers.
</p><p>For example, in computer graphics, line objects can have attributes such as thickness (with real values), color (with descriptive values such as brown or green or values defined in a certain color model, such as RGB), dashing attributes, etc. A circle object can be defined in similar attributes plus an origin and radius.
In reference to computer systems, attributes are defined particularly for read or write attributes for specific read or write.
</p>
<h2 id="attribute-usage">Attribute usage</h2>
<p>If the element in question could be considered a property (CUSTOMER_NAME) of another entity (let's say CUSTOMER), the element can have zero or more attributes (properties) of its own (CUSTOMER_NAME is of TYPE = "KINDOFTEXT").
</p>
<h3>C#</h3>
<p>In the <a href="/facts/C_Sharp_(programming_language)/UC2gxeyb">C# programming language</a>, attributes are <a href="/facts/Metadata/qt35z79C">metadata</a> attached to a field or a block of code like <a href="/facts/Assembly_(programming)/D9rbllH5">assemblies</a>, <a href="/facts/Member_variable/BAtCyPVd">members</a> and <a href="/facts/Data_type/fcx5hgVL">types</a>, and are equivalent to <a href="/facts/Java_annotation/bEff4u2b">annotations in Java</a>. Attributes are accessible to both the compiler and programmatically through <a href="/facts/Reflection_(computer_programming)/Nj8stFfp">reflection</a>. In contrast, properties, in C# terminology, are members of a class which syntactically are used like instance (or class) variables, but are implemented as a pair of getter/setter functions. (In the absence of a setter, properties are read-only.)
</p><p>Users of the language see many examples where attributes are used to address cross-cutting concerns and other mechanistic or platform uses. This creates the false impression that this is their sole intended purpose.
</p><p>Their specific use as metadata is left to the developer and can cover a wide range of types of information about any given application, classes and members that is not instance-specific. The decision to expose any given attribute as a property is also left to the developer as is the decision to use them as part of a larger application framework.
</p><p>Attributes are implemented as classes that are derived from System.Attribute. They are often used by the <a href="/facts/Common_Language_Runtime/4Y0cUdkd">CLR</a> services, like <a href="/facts/Component_Object_Model/2DtIAufW">COM</a> interoperability, <a href="/facts/Remoting/uMMb9CQw">remoting</a>, <a href="/facts/Serialisation/T00auFvv">serialisation</a> and can be queried at runtime.
</p><p>The example shows how attributes are defined in C#:
</p>
[Obsolete("Use class C1 instead", IsError = true)]  // causes compiler message saying
public class C {...}                                // that C is obsolete

public class ObsoleteAttribute : Attribute {         // class name ends with "Attribute"
    public string Message { get; }                   // but can be used as "Obsolete"
    public bool IsError { get; set; }
    public ObsoleteAttribute() {...}
    public ObsoleteAttribute(string msg) {...}
    public ObsoleteAttribute(string msg, bool error) {...}}

[Obsolete]
[Obsolete("This is obsolete")]
[Obsolete("This is obsolete", false)]
[Obsolete("This is obsolete", IsError = false)]

<p>Positional parameters like first parameter of type string above are parameters of the attribute's constructor. Name parameters like the Boolean parameter in the example are a property of the attribute and should be a constant value.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>
</p><p>Attributes should be contrasted against XML documentation that also defines metadata, but is not included in the compiled assembly and therefore cannot be accessed programmatically.
</p>
<h3>HTML & JavaScript</h3>
<p>Display the checked attribute and property of a checkbox as it changes.
</p>
<!doctype html>
<html lang="en">
<head>
<body>
<input name="food" type="meal" id="meal">
  <meta charset="utf-8">
  <title>attr demo</title>
  <style>
  p { border;1px solid black;
font-family; arial, sans-serif;
text-align center;
  }
  b {
    color: blue;
  }
  </style>
  <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>
 
<input id="check1" type="checkbox" checked="checked">
<label for="check1">Check me</label>
<p></p>
 
<script>
$( "input" )
  .change(function() {
    var $input = $( this );
    $( "p" ).html( ".attr( 'checked' ): <b>" + $input.attr( "checked" ) + "</b><br>" +
      ".prop( 'checked' ): <b>" + $input.prop( "checked" ) + "</b><br>" +
      ".is( ':checked' ): <b>" + $input.is( ":checked" ) + "</b>" );
  })
  .change();
</script>
</body>
</html>

<h4>before click</h4>
.attr( 'checked' ): checked
.prop( 'checked' ): false
.is( ':checked' ):  false

<h4>after click</h4>
.attr( 'checked' ):   checked
.prop( 'checked' ):    true
.is( ':checked' ):     true

<h3>Multi-valued databases</h3>
<p>On many post-relational or <a href="/facts/MultiValue/csrAQ5Pe">multi-valued</a> databases systems, relative to SQL, tables are files, rows are items, and columns are attributes. Both in the database and code, attribute is synonymous with property and variable although attributes can be further defined to contain values and subvalues.
</p><p>The first of these databases was the <a href="/facts/Pick_operating_system/LSXLknzu">Pick operating system</a>. Two current platforms include <a href="/facts/Rocket_U2/lfoqc5m6"> Rocket U2's Universe</a> and <a href="/facts/InterSystems/rwll8eY6">InterSystems' Caché</a>.
</p>
<h3>XML</h3>
<p>In <a href="/facts/XML/clNgUfWk">XML</a>, an attribute is a markup construct consisting of a name/value pair that exists within a <a href="/facts/Start-tag/clNgUfWk">start-tag</a> or <a href="/facts/XML/clNgUfWk">empty-element tag</a>. Markup languages, such as <a href="/facts/HTML/sSc3fT78">HTML</a> and <a href="/facts/XML/clNgUfWk">XML</a>, use attributes to describe data and the formatting of data.
</p>
<p class="note">Further information: <a href="/facts/HTML_attribute/0zRQefTK">HTML attribute</a></p>
<p>A good example is the process of XML assigning values to properties (elements). Note that the element's value is found before the (separate) end tag, not in the element itself. The element itself may have a number of attributes set (NAME = "IAMAPROPERTY").
</p><p>If the element in question could be considered a property (CUSTOMER_NAME) of another entity (let's say CUSTOMER), the element can have zero or more attributes (properties) of its own (CUSTOMER_NAME is of TYPE = "KINDOFTEXT").
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Attribute_(knowledge_representation)/EIRll6sn">Attribute (knowledge representation)</a></li>
<li><a href="/facts/Attribute%E2%80%93value_pair/buweLY96">Attribute–value pair</a></li>
<li><a href="/facts/File_attribute/eVga2BAu">File attribute</a></li>
<li><a href="/facts/Extended_file_attributes/1wmerAyK">Extended file attributes</a></li>
<li><a href="/facts/Field_(computer_science)/jt622EDb">Field (computer science)</a></li>
<li><a href="/facts/Property_(programming)/MRIIAw5g">Property (programming)</a></li>
<li><a href="/facts/Java_annotation/bEff4u2b">Java annotation</a></li>
<li><a href="/facts/Metadata/qt35z79C">Metadata</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Mössenböck, Hanspeter (2002-03-25). "Advanced C#: Variable Number of Parameters" (PDF). Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik. p. 44. Retrieved 2011-08-08. <a href="http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/Part2.pdf" target="_blank">http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/Part2.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Attribute (computing) open-in-new

Attribute (computing)