What's the simplest most elegant way to utilize a custom attribute

Question

So a little confession, I've never written an attribute class. I understand they serve the purpose of decorating classes with flags or extra functionality possibly.

Can someone give me a quick example of not just creating and applying an attribute to a class, but rather utilizing the attribute from another block of code. The only code samples I've ever seen to utilize any form of attributes was doing so with reflection, though I've always hoped there's a way of using them without reflection.

Answer 1

Attributes are always used with reflection. They are baked into the metadata of the types during compile time and the only way to read them is through reflection. Attributes are used when you want write a type and you want to associate some metadata with it which could be used by consumers of this type.

Answer 2

The simplest and most elegant way to use an attribute from another block of code is to use a property instead of an attribute.

See http://blogs.msdn.com/b/ericlippert/archive/2009/02/02/properties-vs-attributes.aspx for a discussion of the differences between properties and attributes.

Answer 3

First create your attribute

public class ImportableAttribute : Attribute
{

}

Then a class with a item that uses the Attribute

[ImportableAttribute]
public class ImportClass
{
    [ImportableAttribute]
    public string Item {get; set;}
}

Then check if that property uses that attribute. Can be done with classes to.. Of course :)

PropertyInfo property = typeof(ImportClass).GetProperty("Item");

if (property.IsDefined(typeof(ImportableAttribute),true))
{
     // do something
}

With a class:

typeof(ImportClass).IsDefined(typeof(ImportableAttribute), true);