Attribute basics

Learn to add metadata with [Attribute].

What is an attribute?

An attribute adds metadata to code that can be read at compile time or runtime. It is written in square brackets before an element such as a class, method or property: [Serializable] class Sample { }.

Attributes do not change behavior directly; the compiler, runtime and reflection read them to decide behavior. For example [Serializable] marks a class as serializable; [Obsolete] marks a method as deprecated.

Attribute syntax

Attributes are written in square brackets before the target element. Example: [Obsolete("Use NewMethod")] void OldMethod() { }. Here Obsolete is the attribute name and "Use NewMethod" is a constructor argument.

The attribute class name ends with Attribute (e.g. ObsoleteAttribute); but you can omit the suffix when applying. Multiple attributes are comma-separated or on separate lines. Some take parameters, some don't.

Exercise Apply Serializable and check

Add the [Serializable] attribute to the Sample class below. In Main, get the type with Type t = typeof(Sample); and print the result with Console.WriteLine(t.IsSerializable);.

Show example solution

Write [Serializable] above the Sample class. In Main add Type t = typeof(Sample); and Console.WriteLine(t.IsSerializable);.
Example solution:
using System;

[Serializable]
class Sample { }

class Program
{
    static void Main()
    {
        Type t = typeof(Sample);
        Console.WriteLine(t.IsSerializable);
    }
}

Test yourself

What does an attribute do?
- Runs code faster
- Hides a method
- Adds metadata to code
- Makes a variable constant
Show answer

Adds metadata to code — Correct. An attribute adds metadata to code and can be read via reflection.

Complete the attribute usage

[____] class MyClass { }

Show answer

[Serializable] class MyClass { }

Match the attribute to its target

[Serializable]

Class
[Obsolete]

Deprecated method
[Conditional]

Conditional method
[XmlAttribute]

Property

Next lesson: Custom attributes

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