obj.GetType().GetCustomAttributes

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Text;
 using System.Reflection;
 using System.Windows.Forms;

 using System.Threading;

 using System.Collections.Specialized;

 namespace Test123
 {
     public partial class Form1 : Form
     {

         public Form1()
         {
             InitializeComponent();
         }

         private void Form1_Load(object sender, EventArgs e)
         {

         }

         public class TestAttri : Attribute
         {
             public string JJ { get; set; }
         }

         [TestAttri(JJ = "")]
         public class ClassTest2 : ClassTest { }

         public abstract class ClassTest { }

         private void button3_Click(object sender, EventArgs e)
         {
             ClassTest obj = new ClassTest2();
             Type type = obj.GetType();
             TestAttri att = GetCustomAttribute<TestAttri>(type, false);
             //(TestAttri)type.GetCustomAttributes(typeof(TestAttri), false).GetValue(0);
             //或者 (TestAttri)obj.GetType().GetCustomAttributes(typeof(TestAttri), false)[0];
             att.JJ = "rich";

             //需要特别注意的地方：
             //GetCustomAttributes 方法每次都返回新的实例
             //每次获取 io 的 CustomAttributes 都是一个新的副本

             MessageBox.Show(att.JJ);

             ((TestAttri)obj.GetType().GetCustomAttributes(typeof(TestAttri), false)[]).JJ = "jigjig";
             MessageBox.Show(((TestAttri)obj.GetType().GetCustomAttributes(typeof(TestAttri), false)[]).JJ);
         }

         private static Dictionary<MemberInfo, Object> _dicCache1 = new Dictionary<MemberInfo, Object>();
         private static Dictionary<MemberInfo, Object> _dicCache2 = new Dictionary<MemberInfo, Object>();

         public static TAttribute[] GetCustomAttributes<TAttribute>(MemberInfo member, Boolean inherit)
         {
             if (member == null)
             {
                 return new TAttribute[];
             }

             // 根据是否可继承，分属两个缓存集合
             var cache = inherit ? _dicCache1 : _dicCache2;

             Object obj = null;
             if (cache.TryGetValue(member, out obj))
             {
                 return (TAttribute[])obj;
             }
             lock (cache)
             {
                 if (cache.TryGetValue(member, out obj))
                 {
                     return (TAttribute[])obj;
                 }
                 var atts = member.GetCustomAttributes(typeof(TAttribute), inherit) as TAttribute[];
                 var att = atts == null ? new TAttribute[] : atts;
                 cache[member] = att;
                 return att;
             }
         }

         public static TAttribute GetCustomAttribute<TAttribute>(MemberInfo member, Boolean inherit)
         {
             var atts = member.GetCustomAttributes<TAttribute>(inherit);
             if (atts == null || atts.Length < )
             {
                 return default(TAttribute);
             }
             return atts[];
         }

     }

 }