I often find myself in a situation where I have a user input form that I want to reuse, but I don't know how many times I'll be reusing it. For example, let's say you want to let users on your site upload images, but you don't know how many images they'll want to upload. Giving the user an "Add" button can let them dynamically add as many upload boxes to the page as they want.
That's fine if you have a single control that you want to add dynamically, but what if you have several controls in a complex layout? In that case, the easiest way to handle this is by wrapping the controls you want to add into a single user control, and then dynamically adding the user control to your page. I'll show you how to do that, how to remove user controls, and also how to access the values of the controls within the user control.
Using the code
First, let's create a user control and populate it with several controls for our "template". The user control content looks like this:
<asp:TextBox ID="tbx1" runat="server" />
<asp:DropDownList ID="ddl1" runat="server">
<asp:ListItem Text="Dropdown 1" />
<asp:ListItem Text="Dropdown 2" />
<asp:ListItem Text="Dropdown 3" />
<asp:CheckBoxList ID="cbx1" runat="server">
<asp:ListItem Text="Checkbox 1" />
<asp:ListItem Text="Checkbox 2" />
<asp:ListItem Text="Checkbox 3" />
<asp:Button ID="btnRemove" runat="server" Text="Remove" />
Now, add an event handler to the code-behind on the user control so we can tell when the Delete button was clicked:
Partial Class WebUserControl
Public Event RemoveUserControl As EventHandler
Protected Friend Sub btnRemove_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles btnRemove.Click
RaiseEvent RemoveUserControl(sender, e)
Now that we have the controls we want to display, we'll add the user control to another page dynamically.
There are three important things we have to do in the page load. First, we need to determine what control fired the postback. We need to make sure the "Add" button was clicked.
This needs to be done in the page load event, instead of in the
onclick event for the "Add" button, because of the way the ASP.NET page lifecycle works. If we try to add any controls in the
onclick event for the Add button (instead of the page load), those values will be blown away when the page posts back.
If the "Add" button was clicked, the next thing to do is increment the count of the user controls to add to the page, so we know how many controls to display. I store this value in a
Literal control across postbacks.
We also need to attach an event handler to take care of the 'Delete' button on the user control when it gets clicked. This will remove the control from the
Panel and decrement the number of controls on the page.
Finally, once all the setup is taken care of, we can dynamically load all the requested user controls into a
Protected Sub Page_Load(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles Me.Load
Private Sub AddAndRemoveDynamicControls()
'Determine which control fired the postback event.
Dim c As Control = GetPostBackControl(Page)
If Not IsNothing(c) Then
'If the add button was clicked, increase
'the count to let the page know we want
'to display an additional user control
If c.ID.ToString = "btnAdd" Then
ltlCount.Text = Convert.ToInt16(ltlCount.Text) + 1
'Be sure everything in the placeholder control is cleared out
Dim ControlID As Integer = 0
'Since these are dynamic user controls, re-add them every time the page loads.
For i As Integer = 0 To (Convert.ToInt16(ltlCount.Text) - 1)
Dim DynamicUserControl As WebUserControl = _
'If this particular control id has been deleted
'from the page, DO NOT use it again. If we do, it will
'pick up the viewstate data from the old item that
'had this control id, instead of generating
'a completely new control. Instead, increment
'the control ID so we're guaranteed to get a "new"
'control that doesn't have any lingering information in the viewstate.
While InDeletedList("uc" & ControlID) = True
ControlID += 1
'Note that if the item has not been deleted from the page,
'we DO want it to use the same control id
'as it used before, so it will automatically maintain
'the viewstate information of the user control
DynamicUserControl.ID = "uc" & ControlID
'Add an event handler to this control to raise
'an event when the delete button is clicked
'on the user control
AddHandler DynamicUserControl.RemoveUserControl, _
'Finally, add the user control to the panel
'Increment the control id for the next round through the loop
ControlID += 1
Private Function InDeletedList(ByVal ControlID As String) As Boolean
'Determine if the passed in user control ID
'has been stored in the list of controls that
'were previously deleted off the page
Dim DeletedList() As String = ltlRemoved.Text.Split("|")
For i As Integer = 0 To DeletedList.GetLength(0) - 1
If ControlID.ToLower = DeletedList(i).ToLower Then
Sub HandleRemoveUserControl(ByVal sender As Object, ByVal e As EventArgs)
'This handles delete event fired from the user control
'Get the user control that fired this event, and remove it
Dim DynamicUserControl As WebUserControl = sender.parent
'Keep a pipe delimited list of which user controls
'were removed. This will increase the
'viewstate size if the user keeps removing
'dynamic controls, but under normal use
'this is such a small increase in size that it shouldn't be an issue.
ltlRemoved.Text &= DynamicUserControl.ID & "|"
'Also, now that we've removed a user control decrement
'the count of total user controls on the page
ltlCount.Text = Convert.ToInt16(ltlCount.Text) - 1
Protected Sub btnAdd_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles btnAdd.Click
'Handled in page load
There's one more issue here. How do we get at the values stored in the user controls? It's actually fairly straightforward. We loop through all the controls in the
PlaceHolder, then look at the name of the control to determine if this is one of the user controls we added to the
If it is, we can use
FindControl to access the values stored in textboxes, dropdowns, checkboxes, or anything else we added to the user control. Here, I'm getting those values and then writing them out to the screen:
Protected Sub btnDisplayValues_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles btnDisplayValues.Click
ltlValues.Text = ""
For Each c As Control In ph1.Controls
'Find the specific user control that we added to this
'placeholder, and then get the selected values
'for the dropdownlist, checkbox, and textbox and print them to the screen.
If c.GetType.Name.ToLower = "webusercontrol_ascx" Then
Dim uc As UserControl = CType(c, UserControl)
Dim tbx1 As TextBox = uc.FindControl("tbx1")
Dim ddl1 As DropDownList = uc.FindControl("ddl1")
Dim cbx1 As CheckBoxList = uc.FindControl("cbx1")
Dim sb As New System.Text.StringBuilder
sb.Append("Textbox value: " & tbx1.Text & "<br />")
sb.Append("Dropdown value: " & ddl1.SelectedValue & "<br />")
sb.AppendLine("Checkbox values: ")
For Each li As ListItem In cbx1.Items
If li.Selected = True Then
sb.Append(li.Value & "<br />")
ltlValues.Text &= sb.ToString
I hope this has given you another way to think about dynamically adding and removing controls. Enjoy!