ASP.Net向占位符控件添加控件失败

Question

我在一个按钮的点击事件中为占位符添加了一组控件。我可以添加一次控件，但第二次我收到此消息。我正在使用Ajax Toolkit来制作一些自定义控件。

  

发生了System.InvalidOperationException     消息=在PreRender之后可能无法注册Extender控件。     来源= System.Web.Extensions程序     堆栈跟踪：          在System.Web.UI.ScriptControlManager.RegisterExtenderControl [TExtenderControl]（TExtenderControl extenderControl，Control targetControl）。

代码如下：

        /// <summary>
        /// Handles the Click event of the AddWitnessButton control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="System.EventArgs"/> instance containing the event data.</param>
        /// <remarks></remarks>
        protected void AddWitnessButton_Click(object sender, EventArgs e)
        {

            int count = Convert.ToInt32(WitnessCountHiddenField.Value);
            var fullNameTextBoxId = "FullNameEnhancedTextBox" + count;
            var mobileFilteredTextBoxId = "MobileFilteredTextBox" + count;
            IList<Panel> oldPanels = (IList<Panel>)Session["WitnessPanels"] ?? new List<Panel>();

            //Container
            Panel panel = new Panel();

            //Seperator
            Literal hr = new Literal { Text = "<HR/>" };

            //Full Name
            Label fullNameLabel = new Label
            {
                ID = fullNameTextBoxId + "_Label",
                AssociatedControlID = fullNameTextBoxId,
                Text = "Full Name:"
            };
            EnhancedTextBox fullNameEnhancedTextBox = new EnhancedTextBox
            {
                ID = fullNameTextBoxId,
                Required = true,
                RequiredErrorText = "Full Name is a required field."
            };

            //Mobile
            Label mobileLabel = new Label
            {
                ID = mobileFilteredTextBoxId + "_Label",
                AssociatedControlID = mobileFilteredTextBoxId,
                Text = "Mobile:"
            };
            FilteredTextBox mobileFilteredTextBox = new FilteredTextBox
            {
                ID = mobileFilteredTextBoxId,
                FilterMode = FilterModes.ValidChars,
                ValidChars = "0123456789+()",
                Required = true,
                RequiredErrorText = "Mobile is a required field."
            };

            //Readd previously added panels
            foreach (var addedPanel in oldPanels)
            {
                AddWitnessPlaceHolder.Controls.Add(addedPanel);
            }

            //Add new controls to the form
            Panel newPanel = new Panel();

            newPanel.Controls.Add(hr);
            newPanel.Controls.Add(fullNameLabel);
            newPanel.Controls.Add(fullNameEnhancedTextBox);
            newPanel.Controls.Add(mobileLabel);
            newPanel.Controls.Add(mobileFilteredTextBox);

            AddWitnessPlaceHolder.Controls.Add(newPanel);

            //Increment the ID count
            count++;
            WitnessCountHiddenField.Value = count.ToString();

            //Save the panel to the Session.
            oldPanels.Add(newPanel);
            Session["WitnessPanels"] = oldPanels;

            //Go back to the same wizard step.
            ShowStep2HiddenField.Value = "false";
            ShowStep3HiddenField.Value = "true";
        }

 protected void Page_PreRender(object sender, EventArgs e)
        {
            if(IsPostBack)
            {
                //Readd previously added panels
                var vehicleControls = (IList<Panel>)Session["VehiclePanels"] ?? new List<Panel>();

                if (vehicleControls.Any())
                {
                    foreach (var addedPanel in vehicleControls)
                    {
                        AddVehiclePlaceholder.Controls.Add(addedPanel);
                    }

                }

                var witnessControls = (IList<Panel>)Session["WitnessPanels"] ?? new List<Panel>();

                if (witnessControls.Any())
                {
                    foreach (var addedPanel in witnessControls)
                    {
                        AddWitnessPlaceHolder.Controls.Add(addedPanel);

                    }

                }
            }
        }