Question

particularly when overriding Asp.net MVC ValidationAttribute, is it possible to know class name which has field marked with attribute ? (AttributeUsage is AttributeTargets.Property)

public class UniqueAttribute: ValidationAttribute

        public override bool IsValid(object value)
        {
            // how to know which class has this attribute?
        }

Answer 1

Given an instance of an attribute it is not possible to determine what class, field, method, etc … originated this attribute. Primarily because it doesn’t need to be attached to one. All attributes can be created just like a normal object and hence not be attached to anything

var instance = new UniqueAttribute();

You may need to use a different constructor but one must exist or it couldn’t be applied to a member in the first place.

It is possible to go the other way though. Given a member or type to find out if it has a given attribute applied to it. For example, assuming there is a single instance of UnqiueAttribute attached to the type SomeType this code will retrieve it.

var type = typeof(SomeType);
var attrib = (UniqueAtrtibute)type.GetCustomAttributes(typeof(UnqiueAttribute),true)[0];