Attributes in C#

 Attributes are a method of using metadata with code such as methods, assemblies, structures, enumerators, types. properties etc. The attributes are specified by using declarative tags that are created using square brackets placed before the required element.

Attribute Uses

Some of the major uses of attributes are given as follows:

1. Attributes can be used to describe the COM properties of methods, classes, interfaces etc.
2. The WebMethod attribute in web services can be used to mark methods with the help of attributes.
3. Unmanaged code can be called using attributes with the DLLImport Attribute class.
4. The assembly can be described in terms of version, trademark, title, description etc. using attributes.
5. Attributes can be used to describe the way to map between class members and XML codes. This is for XML serialization.
6. Attributes can be used to specify the different characteristics that are used to manage the security.
7. The class members that need to be serialized for persistence are described using attributes.

Attribute Syntax

The syntax for attribute specification is given as follows:

[ attributeName (parameter_1, parameter_2…..parameter_n) ]
Elements

In the above syntax, the attributeName specifies the class name of the attribute which is followed by zero or more parameters. All of this data is specified in square brackets.

Predefined Attributes

There are a number of attributes in the System.Attribute base class library. Some of them are given as follows:

Source: MSDN

A program that demonstrates one of the predefined attributes i.e. the Obsolete attribute in C# is given as follows:

Source Code: Program to implement Obsolete attribute in C#

using System;
namespace AttributesDemo
{
   class Example
   {
       static void Main(string[] args)
       {
           Console.WriteLine("Program for Attributes");
           DemoMethod();
       }
       [Obsolete("Don't use the DemoMethod()",false)]
       public static void DemoMethod()
       {
           Console.WriteLine("Inside DemoMethod()");  
       }
   }
}

The output of the above program is as follows:

main.cs(10,13): warning CS0618: `AttributesDemo.Example.DemoMethod()' is obsolete: `Don't use the DemoMethod()'

Program for Attributes
Inside DemoMethod()

Now let us understand the above program.

The Main() function calls the DemoMethod(). The message in the Obsolete attribute is "Don't use the DemoMethod()". Also false is displayed after that. This means that the compiler issues a warning. The code snippet for this is given as follows:

static void Main(string[] args)
       {
           Console.WriteLine("Program for Attributes");
           DemoMethod();
       }
       [Obsolete("Don't use the DemoMethod()",false)]
       public static void DemoMethod()
       {
           Console.WriteLine("Inside DemoMethod()");  
       }