Reflection of object properties

i have this code

 public class ParameterOrderInFunction : Attribute
    {
        public int ParameterOrder { get; set; }
        public ParameterOrderInFunction(int parameterOrder)
        {
            this.ParameterOrder = parameterOrder;
        }
    }


    public interface IGetKeyParameters
    {

    }

    public class Person: IGetKeyParameters
    {

        [ParameterOrderInFunction(4)]
        public string Age { get; set; }
        public string Name { get; set; }
        [ParameterOrderInFunction(3)]
        public string Address { get; set; }
        [ParameterOrderInFunction(2)]
        public string Language { get; set; }

        [ParameterOrderInFunction(1)]
        public string City { get; set; }

        public string Country { get; set; }        
    }


    class Program
    {
        static void Main(string[] args)
        {

            Person person = new Person();

            person.Address = "my address";
            person.Age = "32";
            person.City = "my city";
            person.Country = "my country";            

            Test t = new Test();
            string result = t.GetParameter(person);
            //string result = person.GetParameter();

            Console.ReadKey();

        }      
    }

    public class Test
    {
        public string GetParameter(IGetKeyParameters obj)
        {
            string[] objectProperties = obj.GetType()
               .GetProperties()
               .Where(p => Attribute.IsDefined(p, typeof(ParameterOrderInFunction)))
                 .Select(p => new
                 {
                     Attribute = (ParameterOrderInFunction)Attribute.GetCustomAttribute(p, typeof(ParameterOrderInFunction), true),
                     PropertyValue = p.GetValue(this) == null ? string.Empty : p.GetValue(this).ToString()
                 })
               .OrderBy(p => p.Attribute.ParameterOrder)
               .Select(p => p.PropertyValue)
               .ToArray();
            string keyParameters = string.Join(string.Empty, objectProperties);
            return keyParameters;

        }
    }

What i am trying to do is to get properties values as one string with some order .

it work fine if i put the function GetParameter inside the Person class. however, i want to use the function GetParameter with other class as well, so i create empty interface. Now i want that every object that is of type IGetKeyParameters can use the function. but i am getting exception in the line:

PropertyValue = p.GetValue(this) == null ? string.Empty : p.GetValue(this).ToString()