What Are Attributes in .NET?

In .NET, attributes are special classes used to add metadata (extra information) to your code elements such as:

Classes
Methods
Properties
Fields
Parameters
Assemblies

They provide additional information that can be read at compile time or runtime using reflection.

Simple Definition

An attribute in .NET is a declarative tag that you attach to code elements to describe behavior or configuration.

Why Are Attributes Used?

Attributes are used to:

Control behavior of frameworks (ASP.NET MVC, Web API)
Define validation rules
Configure serialization
Define authorization rules
Mark obsolete methods
Provide metadata for tools and libraries

Basic Syntax

Attributes are written inside square brackets [ ].

[Obsolete("Use NewMethod instead")]
public void OldMethod()
{
}

Here, [Obsolete] is a built-in attribute.

Example 1: Built-in Attribute

1. Obsolete Attribute

[Obsolete("This method is deprecated.")]
public void OldMethod()
{
}

When someone calls this method, the compiler gives a warning.

Example 2: Validation Attributes (ASP.NET MVC)

public class UserModel
{
    [Required]
    [StringLength(100)]
    public string Name { get; set; }

    [Range(18, 60)]
    public int Age { get; set; }
}

These attributes are used by model validation in ASP.NET MVC.

Example 3: Custom Attribute

You can create your own attribute.

Step 1: Create Custom Attribute Class

using System;

[AttributeUsage(AttributeTargets.Method)]
public class LogAttribute : Attribute
{
    public string Message { get; }

    public LogAttribute(string message)
    {
        Message = message;
    }
}

Step 2: Use Custom Attribute

[Log("This method performs calculation")]
public void Calculate()
{
}

AttributeUsage

The AttributeUsage attribute defines where your custom attribute can be applied.

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]

Common targets:

Class
Method
Property
Field
Parameter
Assembly

Reading Attributes Using Reflection

Attributes can be read at runtime using reflection.

var method = typeof(MyClass).GetMethod("Calculate");
var attribute = (LogAttribute)Attribute.GetCustomAttribute(method, typeof(LogAttribute));

if (attribute != null)
{
    Console.WriteLine(attribute.Message);
}

Types of Attributes in .NET

1. Predefined Attributes

Examples:

Obsolete
Serializable
Conditional
AttributeUsage

2. Validation Attributes

Used in ASP.NET MVC:

Required
Range
StringLength
Compare
EmailAddress

3. Custom Attributes

Developer-defined attributes for:

Logging
Auditing
Permissions
Mapping
Metadata handling

Real-World Use Cases (Especially for You – ASP.NET MVC)

Since you work with ASP.NET MVC 5, attributes are commonly used for:

Authorization

[Authorize]
public ActionResult Dashboard()
{
    return View();
}

Custom Authorization Attribute

public class CustomAuthorizeAttribute : AuthorizeAttribute
{
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        return httpContext.User.Identity.IsAuthenticated;
    }
}

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