C#

class Program
{
    static void Main(string[] args)
    {            
        ExecTest(typeof(ITest));
        ExecTest(typeof(Test));
        Console.ReadLine();            
    }

    static void ExecTest(Type type)
    {
        PropertyInfo prop = type.GetProperty("PropertyName");
        if (prop == null)
            throw new InvalidOperationException("invalid type - must have PropertyName property");
        object[] attrs = prop.GetCustomAttributes(typeof(DefaultSettingValueAttribute), true);
        Console.WriteLine("Number of DefaultSettingValueAttribute's on '{1}': {0}", attrs.Length,type.Name);
    }


    public class Test : ITest
    {
        #region ITest Members

        public string PropertyName
        {
            get
            {
                throw new NotImplementedException();
            }
            set
            {
                throw new NotImplementedException();
            }
        }

        #endregion
    }

    public interface ITest
    {
        [DefaultSettingValue("test")]
        string PropertyName { get; set; }
    }
}