Retaining State for Dynamically Created Controls in ASP.NET applications

Introduction

When dynamically adding controls to an ASP.NET page in runtime the object references are lost at postback because they have no handle in the codebehind. The values entered by the user is not availible when accessing these objects after postback, and they seem empty. This article describes how to use ViewState to recreate and reinsert the postback values into the objects to access their values.

Background

An ASP.NET application with pages that use dynamic generation of controls that need to retain state across postbacks. The solution described in this paper enables you to create a totally dynamic number of controls at runtime and retreive their values after postback.

Solution

Dynamically added controls have no object reference variable in the codebehind class. They appear only in the control collection of the containing control, i.e. the Page.Controls collection. When the page is posted back to the server as a result of user interaction a new instance of the codebehind class is instantiated, and all the variables of the class is set with values from the ViewState.

This means that the objects we are accessing from the codebehind class, that "feels" like the same objects as we worked on before postback, actually are new ones that got their predecessors values via ViewState and ASP.NET state management.

So, the controls that were dynamically created are no longer there and consequently the values returned from these controls have no place to go. They are lost in the viewstate.

In order to catch these values the dynamically generated controls needs to be re-generated at Page_Load. The important thing is to assign the same ID to each control. The ViewState uses the ID property of the Control objects to reinstate the values.

Add the following elements to your System.UI.Page class:

public class DynamicallyAddingControls : System.Web.UI.Page
{
    // a Property that manages a counter stored in ViewState
    protected int NumberOfControls
    {
        get{return (int)ViewState["NumControls"];}
        set{ViewState["NumControls"] = value;}
    }

    private void Page_Load(object sender, System.EventArgs e)
    {
        if(!Page.IsPostBack)
            //Initiate the counter of dynamically added controls
            this.NumberOfControls = 0;                
        else
            //Controls must be repeatedly be created on postback
            this.createControls();
    }

    // This routine creates the controls and assigns a generic ID
    private void createControls()
    {
        int count = this.NumberOfControls;

        for(int i = 0; i < count; i++)
        {
            TextBox tx = new TextBox();
            tx.ID = "ControlID_" + i.ToString();
            //Add the Controls to the container of your choice
            Page.Controls.Add(tx);
        }
    }

    // example of dynamic addition of controls
    // note the use of the ViewState variable
    private void addSomeControl()
    {
        TextBox tx = new TextBox();
        tx.ID = "ControlID_" + NumberOfControls.ToString();

        Page.Controls.Add(tx);
        this.NumberOfControls++;
    }

}

Note that the createControls method has to simulate the way that you dynamically built your page before the postback. The important thing here is obviously to assign identical ID values to the correct type of controls to that we can access them at postback.

Points of Interest

This solution works because the ASP.NET ViewState supports dynamically added controls, but does not do the re-generation for us. As long as the ID properties match ASP.NET does the rest of the job for us.

Thanks to Eric Sütter for providing valuable input on this article!

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