Asp.net MVC里的TempData是一个整体

var a = TempData["a"];

var alsoA = TempData["a"] ;

//true

Debug.Assert(Object.ReferenceEqual(a, alsoA));

=================

objects within TempData will only be removed till the end of current request, if during which period you havn't set something new/old with the key. Sounds simple, but to me, it's not.

And here is a much simpler model, every piece of data got removed when ever you try go get it:

    /// <summary>
    /// TempData is a whole, all it's value exits in the whole request life time.
    /// So here is Flash, any item will  be removed whenever you try to get it, much simpler model.
    /// </summary>
    public class Flash
    {
        private readonly HttpSessionState session;

        public static Flash Instance
        {
            get
            {
                return new Flash(HttpContext.Current);
            }
        }

        private Flash(HttpContext context)
        {
            session = context.Session;
        }

        public object this[string key]
        {
            get
            {
                var sessionKey = GetSessionKey(key);
                var obj = session[sessionKey];
                session.Remove(sessionKey);
                return obj;
            }
            set
            {
                var sessionKey = GetSessionKey(key);
                session[sessionKey] = value;
            }
        }

        private static string GetSessionKey(string key)
        {
            return "_Flash_Data_{0}".Form(key);
        }
    }