Sometimes, the actual class, function, or type isn't enough to describe all the behavior you want. For this reason, attributes exist: they are metadata you can attach to your code. They don’t do anything by themselves — they just describe something, and it’s up to other code (like frameworks, libraries, or your own tools) to read them and react accordingly.

Defining your own attribute

All attributes are just classes that inherit from System.Attribute:

public class MyTagAttribute : Attribute
{
	public string Note { get; }
	public MyTagAttribute(string note)
	{
		Note = note;
	}
}

Using an attribute

You use an attribute by applying it to something else. The things you can apply it to are:

• Classes
• Structs
• Methods
• Properties
• Parameters
• Even assemblies

[MyTag("Just a test")]
public class MyClass {}

But even if you've applied it, it still doesn't do anything. In order for it to actually do something you need some code that is able to read these attributes and make decisions based on them. Since attributes are also code, what you end up with is basically "code reading code". The term for this is Reflection.