访问动态创建的控件（c＃）

Question

In my code behind (c#) I dynamically created some RadioButtonLists with more RadioButtons in each of them. I put all controls to a specific Panel. What I need to know is how to access those controls later as they are not created in .aspx file (with drag and drop from toolbox)?

I tried this:

    foreach (Control child in panel.Controls)
    {
        Response.Write("test1");
        if (child.GetType().ToString().Equals("System.Web.UI.WebControls.RadioButtonList"))
        {
            RadioButtonList r = (RadioButtonList)child;
            Response.Write("test2");
        }   
    }

"test1" and "test2" dont show up in my page. That means something is wrong with this logic. Any suggestions what could I do?

Answer 1

You must recreate your controls after each postback.

ASP.NET is stateless, that is, when you postback a page to the server, your dynamically created controls won't be part of the page anymore.

Last week I had to overcome this situation once more.

What did I do? I saved the data that I used to create the controls inside Session object. On PageLoad method I passed that same data to recreate the dynamic controls.

What I suggest is: Write a method to create the dynamic controls.

On PageLoad method check to see if it's a postback...

if(Page.IsPostBack)
{
   // Recreate your controls here.
}

A really important thing: assign unique IDs to your dynamically created controls so that ASP.NET can recreate the controls binding their existing event handlers, restoring their ViewState, etc.

myControl.ID = "myId";

I had a hard time to learn how this thing works. Once you learn you have power in your hands. Dynamically created controls open up a new world of possibilities.

As Frank mentioned: you can use the "is" keyword this way to facilitate your life...

if(child is RadioButtonList)

Note: it's worth to mention the ASP.NET Page Life Cycle Overview page on MSDN for further reference.

Answer 2

When are you doing this in your code? Be sure you do this at the right time in the ASP life cycle or your controls don't exist yet: http://msdn.microsoft.com/en-us/library/ms178472.aspx

Answer 3

I don't think creating controls in the PageLoad is the right away of doing, first the asp.net life cycle goes from Initialization;Load ViewState Data;Load PostData; Object Load etc.

if you create controls at the Page_Load you'll lose the ViewState, events etc.

The right away is doing at PageInit, or if is a control (OnInit).

The next difficult is that at PageInit, you don't have the ViewState Available, if you need to reconstruct the number of objects you need to store some context/info in a hidden field ant then retrieve that information at PageInit, Create the objects and voila!

Example:

imagine that you need to create 1..N TextBoxes, you create html hidden field (not with runat=server) eg NumberOfTextBoxes.

When you are executing PageInit Code: you retrieve the value eg numberOfTextBoxes = Request.Form["NumberOfTextBoxes"], then you create the TextBoxes.

Remember the most important thing is to match the number and the order of existent Controls stored the ViewState.