14 Attributes

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Attributes


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Attributes can be defined on various items within a

program.

It allows for defining declarative tags placed in certain

places in the source code to specify additional

information. this information can be retrieved by

reflection.

Custom attributes are the ones defined by the user.

ofcourse there is no special benefits that you get from

compiler out of them because compiler doesnt know

about them.

however these will be used as metadata in the compiled

assembly when they are applied to the program element.


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you could use these for documentation or may be the

reflection classes can use the meta-data to make some

decisions at runtime.

When compiler looks at an attribute attached to an

element, it first appends Attribute to the attribute name (if

it does not already have Attribute ending its name). it

then looks for the Attribute class in all the namespace in

the search path. Attributes classes are public classes derived from

System.Attribute that have at least one public

constructor.


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Common Default Attributes

System.SerializableAttribute [Serializable]

Allows your class to be serializable to disk or over a

network.

System.NonSerializedAttribute [NonSerialized]

Allows certain members to be nonserialized so that data

wont be saved to disk or over a network. Similar to the

transient keyword in Java.

System.Web.Services.WebServiceAttribute

Allows you to specify a name [WebService] and

description for a Web service.


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System.Web.Services.WebMethodAttribute

Marks a method to be [WebMethod] exposed as part of aWeb service.

System.AttributeUsageAttribute

Defines the usage parameters [AttributeUsage] forcustom attributes.

System.ObsoleteAttribute [Obsolete]

Marks a specific section of code as obsolete.


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Example

using System;

[Serializable]// OR [Serializable, WebService]

class SerialaizableClass

{//Class definition here

}

[Obsolete("Use ConcatStrings instead.")]

public static string ConcatTwoStrings(string

StringOne,string StringTwo)


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Creating Attribute-CustomAttributes

using System;[AttributeUsage(AttributeTargets.All)]

public class HelpAttribute : System.Attribute

{ public readonly string Url;

private string topic;

// Topic is a named parameter

public string Topic{

get{return topic;}

set{topic = value;} }

// url is a positional parameter

public HelpAttribute(string url)

{ this.Url = url; }


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AttributeTargetsmultiple targets can be given separated by |

Target Description

All

Supported by the .NET Compact

Framework. default

Attribute can be applied to any

application element.

AssemblySupported by the .NET Compact

Framework.

Attribute can be applied to anassembly.

Class


Framework.

Attribute can be applied to a class.

Constructor


Framework.

Attribute can be applied to a

constructor.


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Delegate


Framework.


delegate.

Enum


Framework.

Attribute can be applied to an

enumeration.

Event


Framework.

Attribute can be applied to an

event.

Field


Framework.

Attribute can be applied to a field.

InterfaceSupported by the .NET Compact

Framework.

Attribute can be applied to aninterface.

Method


Framework.


method.


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Module


Framework.

Attribute can be applied to a module.

Note Module refers to a portable

executable file (.dll or .exe) and nota Visual Basic standard module.

Parameter


Framework.


parameter.

PropertySupported by the .NET Compact

Framework.

Attribute can be applied to aproperty.

ReturnValue


Framework.

Attribute can be applied to a return

value.

Struct


Framework.


structure; that is, a value type.


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Using Attributes

Style 1: using Positional parameter

HelpAttribute("http://localhost/MyClassInfo")]

class MyClass {

//class definition

}Style 2: using Positional parameter with Named

parameter

[Help(http://localhost/MyClassInfo,

Topic=MyTopic)]class MyClass

{

//class definition

}


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AllowMultiple

A Boolean value that indicates whether multiple

attributes can be specified for one program element.

The default value for this parameter is False.

Example;

using System;

[AttributeUsage(AttributeTargets.Assembly

| AttributeTargets.Class

| AttributeTargets.Method,

AllowMultiple=true)]

public class AAAttribute : System.Attribute{

}


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Accessing Attributes Through

Reflection

class MainClass {

public static void Main()

{ System.Reflection.MemberInfo info =

typeof(MyClass);object[] attributes = info.GetCustomAttributes(true);

for (int i = 0; i < attributes.Length; i ++)

{

System.Console.WriteLine(attributes[i]);

}

} }Specifies to search the attribute in

inheritance heirarchy.

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