protected override object LoadPageStateFromPersistenceMedium()
{
    var viewStateID = (string)((Pair)base.LoadPageStateFromPersistenceMedium()).Second;
    var stateStr = (string)Cache[viewStateID];
    if (stateStr == null)
    {
        var fn = Path.Combine(this.Request.PhysicalApplicationPath, @"App_Data/ViewState/" + viewStateID);
        stateStr = File.ReadAllText(fn);
    }
    return new ObjectStateFormatter().Deserialize(stateStr);
}

protected override void SavePageStateToPersistenceMedium(object state)
{
    var value = new ObjectStateFormatter().Serialize(state);
    var viewStateID = (DateTime.Now.Ticks + (long)this.GetHashCode()).ToString(); //产生离散的id号码
    var fn = Path.Combine(this.Request.PhysicalApplicationPath, @"App_Data/ViewState/" + viewStateID);
    ThreadPool.QueueUserWorkItem(obj => File.WriteAllText(fn, value));
    Cache.Insert(viewStateID, value);
    base.SavePageStateToPersistenceMedium(viewStateID);
}

protected void Application_Start(object sender, EventArgs e)
{
    var dir = new DirectoryInfo(this.Server.MapPath("~/App_Data/ViewState/"));
    if (!dir.Exists)
        dir.Create();
    else
    {
        var nt = DateTime.Now.AddHours(-1);
        dir.GetFiles().ForEach(f =>
        {
            if (f.CreationTime < nt)
                f.Delete();
        });
    }
}