how to add [Something] Above a property

Hi

I have a property(s) I want to but a thingy above it like [NeedThis]

so

[NeedThis]
public stirng MyProperty {get;set;}

then i need a method which gets all the properties with this.

I know in entity framework its call dataannotations, but this is not EF related.

Whats it call and how do i do it?