关于动态加载控件时， Page.IsValid 不正确的问题。

Question

上代码先
LoadControl.csnamespace ASPNET20
{
    public partial class LoadControl : System.Web.UI.Page
    {
        private List<String> _steps;
        private Control _currentControl;
        public int StepIndex
        {
            get
            {
                if(ViewState["StepIndex"]!=null)
                {
                    return (int) ViewState["StepIndex"];
                }
                    return 0;
            }
            set
            {
                ViewState["StepIndex"] = value;
            }
        }        protected void Page_PreRender()
        {
            this.btnPrevious.Enabled = StepIndex > 0 ? true : false;
            this.btnNext.Enabled = StepIndex < _steps.Count - 1 ? true : false;
        }        protected void Page_Load(object sender, EventArgs e)
        {
            _steps = new List<string>
                     {
                         "~/UserControls/Step1.ascx",
                         "~/UserControls/Step2.ascx",
                         "~/UserControls/Step3.ascx"
                     };
            
            LoadSteps();
        }        protected void LoadSteps()
        {
            _currentControl = Page.LoadControl(_steps[StepIndex]);
           // _currentControl.ID = "xxx";
            this.plhSteps.Controls.Clear();
            if (_currentControl != null)
            {
                this.plhSteps.Controls.Add(_currentControl);
                ((IWizardStep) _currentControl).LoadStep();
            }
        }        protected void btnPrevious_Click(object sender, EventArgs e)
        {
            if (StepIndex >0)
            {
                StepIndex--;
                LoadSteps();
            }
        }        protected void btnNext_Click(object sender, EventArgs e)
        {
            if (((IWizardStep)_currentControl).NextStep() && StepIndex<_steps.Count -1)
            {
                StepIndex++;
                LoadSteps();
            }
        }    }
}Step1.ascx        public void LoadStep()
        {
             if(Session["FirstName"] != null)
             {
                 txtFirstName.Text = (string) Session["FirstName"];
             }
             if (Session["LastName"] != null)
             {
                 txtLastName.Text = (string)Session["LastName"];
             }
        }        public bool NextStep()
        {
            if(Page.IsValid)
            {
                Session["FirstName"] = txtFirstName.Text;
                Session["LastName"] = txtLastName.Text;
                return true;
            }
            return false;
        }Step2.ascx         public void LoadStep()
        {
            if(Session["FavoriteColor"] != null)
            {
                this.txtFavColor.Text = Session["FavoriteColor"].ToString();
            }
        }
        public bool NextStep()
        {
            if(Page.IsValid)
            {
                Session["FavoriteColor"] = txtFavColor.Text;
                return true;
            }
            return false;
        }Step3.ascx         public void LoadStep()
        {
            if (Session["FirstName"] != null )
            {
                txtFirstName.Text = Session["FirstName"].ToString();
            }
            if (Session ["LastName"] != null)
            {
                txtLastName.Text = Session["LastName"].ToString();
            }
            if (Session ["FavoriteColor"]!=null)
            {
                txtFavColor.Text = Session["FavoriteColor"].ToString();
            }
        }
        public bool NextStep()
        {
            return false;
        }
请注意 // _currentControl.ID = "xxx";
问题是  
如果没有这一句的话  在 Step2 中，即使输入了color,  Page.IsValid 依然等于 false;  显示错误信息。
如果有着句话， Step2中， 输入color , Page.IsValid == true,