反射中自定义特点类为什么找不到

反射中自定义特性类为什么找不到？
刚学习c# 再看反射，写了反射的demo
遇到了点小问题。不知道哪里不对。就是找不到对应的属性！

代码如下：

using System;
using System.Reflection;

namespace DemoClass
{
    [LastModified("2015/1/24","MyTest")]
    public class DemoClass
    {
        static void Main(string[] args)
        {
            Type clsType = typeof(DemoClass);
            Assembly assy = clsType.Assembly;
            String assyName = assy.GetName().Name;
            bool isdef = Attribute.IsDefined(assy,
                typeof(LastModifiedAttribute));
            if (isdef)
            {
                // Affirm that the attribute is defined.
                Console.WriteLine("The AssemblyDescription attribute " +
                    "is defined for assembly {0}.", assyName);
                // Get the description attribute itself.
                LastModifiedAttribute adAttr =
                    (LastModifiedAttribute)Attribute.GetCustomAttribute(
                    assy, typeof(LastModifiedAttribute));
                // Display the description.
                if (adAttr != null)
                    Console.WriteLine("The DateModified is \"{0}\".",
                        adAttr.DateModified);
                else
                    Console.WriteLine("The DateModified could not " +
                        "be retrieved.");
            }
            else
                Console.WriteLine("The LastModifiedAttribute attribute is not " +
                    "defined for assembly {0}.", assyName);
            Console.ReadKey();
        }
    }

    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = false)]
    public class LastModifiedAttribute : Attribute
    {
        private readonly DateTime dateModified;
        public DateTime DateModified { get { return dateModified; } }

        private readonly string changes;
        public string Changes { get { return changes; } }

        public string Issues { get; set; }
        public LastModifiedAttribute(string dateModified, string changes)
        {
            this.dateModified = DateTime.Parse(dateModified);
            this.changes = changes;
        }
    }
}

------解决思路----------------------
你这是把attribute定义在class上，但是从assembly上取它，所以找不到。
或者从class上取，或者是把attribute定义在assembly上才对。
------解决思路----------------------
IsDefined 有许多个版本, 楼主用错了, 应该如下:
bool isdef = Attribute.IsDefined(clsType, typeof(LastModifiedAttribute));