Code
　 [Description("状态"), DefaultValue(true)]
        public bool EnableShow
        {
            get
            {
                object o = ViewState["EnableShow"];
                if ((o != null) && (o.ToString().ToLower() == "false"))
                {
                    return false;
                }
                else
                {
                    return true;
                }
            }
            set
            {
                ViewState["EnableShow"] = value;
            }

         }

Code
 [Description("状态"), DefaultValue(true)]
    public bool EnableShow {
        get {
            object o = ViewState["EnableShow"];
            return o == null ? true : (bool)o;//注意这里的 true 表示默认情形
        }
        set {
            ViewState["EnableShow"] = value;
        }
    }

代码示例
using System;
using System.ComponentModel;

namespace Lucifer.CSharp.Sample
{
    class Program
    {
        static void Main()
        {
            TestClass test = new TestClass();

            AttributeCollection attributes =
                TypeDescriptor.
                GetProperties(test)["EnableShow"].
                Attributes;

            DefaultValueAttribute attribute =
                (DefaultValueAttribute)attributes
                [typeof(DefaultValueAttribute)];

            Console.WriteLine(
                "The default value is: " + 
                attribute.Value.ToString());
            Console.WriteLine(
                "The default value is: " + 
                test.EnableShow.ToString());
        }
    }

    class TestClass
    {
        [DefaultValue(true)]
        public bool EnableShow { get; set; }
    }
}