adding controls to asp .net form from the .cs file during runtime

Question

I have a site (build with asp .net) and I want to add to it some text and pictures during runtime (I don't know the amount before).
After the user chooses the parameters, he clicks a button which invokes the following code (in the .cs file):

foreach (var car in cars)
{
  Label lbl = new Label ();
  form1.Controls.Add(lbl);
  lbl.Text = car.name;
  Image myImage = new Image ();
  form1.Controls.Add(myImage);
  myImage.ImageUrl = car.imageURL;

}

but when I run the code, I don't see the controls on the page. am I missing something?
Thanks!

At what point in the page life-cycle are you running that code? — Paulie Waulie
– Paulie Waulie, Commented Jul 4, 2011 at 14:59

ChrisLively · Accepted Answer · 2011-07-04 15:53:05Z

0

You have to assign the control to the form BEFORE setting it's properties.

For example:

foreach (var car in cars){  
  Label lbl = new Label ();
  form1.Controls.Add(lbl); // Create and immediately assign

  lbl.Text = car.name;  // now you can set some properties

  Image myImage = new Image ();
  form1.Controls.Add(myImage);  // Ditto

  myImage.ImageUrl = car.imageURL;
}

UPDATE:

I just created a brand new web application project with a page that only had a button on it; it worked as expected. I suggest you have something else going on. See below:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication1._Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />

    </div>
    </form>
</body>
</html>

and the code behind:

namespace WebApplication1 {
    public partial class _Default : System.Web.UI.Page {
        protected void Page_Load(object sender, EventArgs e) {

        }

        protected void Button1_Click(object sender, EventArgs e) {
            for (Int32 i = 0; i < 10; i++) {
                Label lbl = new Label();
                form1.Controls.Add(lbl);
                lbl.Text = "HAHAHAHAH";
            }
        }
    }
}

edited Jul 4, 2011 at 15:53

answered Jul 4, 2011 at 15:37

ChrisLively

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4 Comments

M4N Over a year ago

This is not true, it doesn't matter whether you set the properties first or after adding the control. See my sample, which works for both literal controls.

ChrisLively Over a year ago

@M4N: Actually it is true. Viewstate doesn't start tracking changes until the control is added to the hierarchy. So, if you set a custom value (like text) before adding it to the hierarchy then viewstate won't be able to detet that you changed it when it's rehydrated. For this example, it shouldn't matter. However, for textboxes etc it will. We ran into this dynamically creating a Table and adding lots of textboxes and dropdowns to it. Nothing worked right until we did the property assignment AFTER associating the new controls with the container.

M4N Over a year ago

I agree regarding ViewState - but as you said, for this example it's not required.

ChrisLively Over a year ago

@M4N: rats, caught in my own logic loop.. ;)

Andrew Peacock · Accepted Answer · 2011-07-04 15:02:22Z

0

Try adding a placeholder control onto your form. Then instead of calling Form1.Controls.Add(....) use PlaceHolder1.Controls.Add(.....)

answered Jul 4, 2011 at 15:02

Andrew Peacock

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1 Comment

kobe Over a year ago

@menacheb, the placeholder1 concept should work..we used it many times to load user controls at runtime and adding them to place holders

Community · Accepted Answer · 2017-05-23 12:04:24Z

0

When you create dynamic controls on a Web Form, the controls must be created and added to the controls collection either in the OnInit or in the Page_Load events.

Please refer this question in order to solve your issue.

edited May 23, 2017 at 12:04

CommunityBot

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answered Jul 4, 2011 at 15:12

Kirill

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Comments

Antonio Dlp · Accepted Answer · 2011-07-04 15:14:01Z

0

You can try what AndyPeacock says but looking at your code I think what you need to research into is actually Repeaters and other databound controls like DataListView.

Here's a quick example of how you could implement this with a Repeater:

In your ASPX file:

<asp:Repeater ID="RepCars" runat="server">
    <ItemTemplate>
        <%# Eval("name") %>
        <img src='<% Eval("imageURL") %>' alt="" />
    </ItemTemplate>
</asp:Repeater>

And on your .CS part, probably in Page_Load:

   RepCarImages.DataSource = cars;
   RepCarImages.DataBind();

Have a look at http://www.w3schools.com/ASPNET/aspnet_repeater.asp

answered Jul 4, 2011 at 15:14

Antonio Dlp

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M4N · Accepted Answer · 2011-07-04 15:51:17Z

0

Not an answer, but the following stripped-down sample works for me:

<%@ Page Language="C#" AutoEventWireup="true" %>
<html>
<head runat="server"></head>
<body>
    <form id="form1" runat="server">

        <script runat="server">
        protected void Button1_Click(object sender, EventArgs e)
        {
            var lit = new Literal();
            Controls.Add(lit);
            Controls.Add(new Literal() { Text = "aaaa" });
            lit.Text = "some text -- ";
        }
        </script>

    <asp:Button ID="Button1" runat="server" Text="Button"
      onclick="Button1_Click" />
    </form>
</body>
</html>

So it seems you have not posted enough information to answer your question?

answered Jul 4, 2011 at 15:51

M4N

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