Question

When I click on a button and add a control to a Placeholder inside an UpdatePanel, everything is fine, except when I click really quickly several times and then get an error message like the following:

'Sys.WebForms.PageRequestManagerServerErrorException: Sys.WebForms.PageRequestManagerServerErrorException: Multiple controls with the same ID 'VehicleRegistrationEnhancedTextBox3_Label' were found. FindControl requires that controls have unique IDs.' when calling method: [nsIDOMEventListener::handleEvent]
[Break On This Error]   

I save an integer in a hidden field, to create unique Ids, when I click really fast, the method executes several times, but the count value has not yet updated. I tried using the C# lock keyword, but that did nothing.

The code is below:

protected void AddVehicleButton_Click(object sender, EventArgs e)
        {
            lock (this)
            {
                int count = Convert.ToInt32(VehicleRegistrationCountHiddenField.Value);

                var TBId = "VehicleRegistrationEnhancedTextBox" + count;
                IList<Panel> oldPanels = (IList<Panel>)Session["VehiclePanels"] ?? new List<Panel>();

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

                //RemoveSpan
                Literal span = new Literal() { Text = "<span class=\"RemoveVehicleRegistration\">X</span>" };

                //Crop
                Control uc = LoadControl("~/Controls/ImageUploadAndCrop/ImageUploadAndCrop.ascx");
                uc.ID = "VehicleRegistrationImageUploadAndCrop" + count;

                //Vehicle Registration
                Label vehicleRegistration = new Label
                {
                    ID = TBId + "_Label",
                    AssociatedControlID = TBId,
                    Text = "Vehicle Registration:"
                };
                EnhancedTextBox vehicleTypeTextBox = new EnhancedTextBox
                {
                    ID = TBId,
                    Required = true,
                    RequiredErrorText = "Vehicle Registration is a required field."
                };

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

                newPanel.Controls.Add(hr);
                newPanel.Controls.Add(span);
                newPanel.Controls.Add(vehicleRegistration);
                newPanel.Controls.Add(uc);
                newPanel.Controls.Add(vehicleTypeTextBox);

                AddVehiclePlaceholder.Controls.Add(newPanel);

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

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

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

Answer 1

The problem is you are kicking of several postbacks with the same request, then the last response is what’s updating your update panel div, causing your issue. One possible solution is to include some javascript to disable your button on click.

<asp:Button ID="myUpdatePanelPostBackButton" runat="server" OnClientClick="this.disabled=true; return true;" Text="Submit" />

Assuming your button is within your UpdatePanel, when the update panel comes back, the button will be re-enabled.