R – Why is ViewState lost when a control’s property is modified during its parent’s Control.Render method

asp.netasp.net-2.0viewstatewebforms

I have code like the following in a UserControl:

Protected Overrides Sub Render(ByVal writer As System.Web.UI.HtmlTextWriter)
    If someCondition Then
        childControl.Enabled = false
    End If

    MyBase.Render(writer)
End Sub

Whenever someCondition is true and childControl.Enabled is set to false during the Render event, the ViewState for childControl is destroyed (i.e. if it is a TextBox, the text that the user has entered is lost).

Note that only the ViewState is lost… The control still renders with the correct property values the first time around. Only on PostBack, when properties are restored from ViewState are the values actually lost.

The timeline is as follows:

Page_Load (initial)
Properties are set via code.
SaveViewState
Render
Properties are modified.
Postback occurs.
LoadViewState
Page_Load
Values of unaltered controls are still available, but controls which have had properties set during the Render method are blank.
SaveViewState

My understanding was that the ViewState became fixed during the call to Control.SaveViewState, which occurs prior to the call to Control.Render… But is there some nuance that I'm missing?

Best Answer

Do controls that are disabled post empty values? Or post nothing at all? To be honest, I forget. But if they post empty values, then your viewstate is being overridden just like if you typed something new into the textbox and then posted the page.

Related Solutions

.net – How to take more control in ASP.NET

This solution will give you programmatic access to the controls in their entirety including all attributes on the controls. Also, only the text box values will appear in the URL upon submission so your GET request URL will be more "meaningful"

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="JonSkeetForm.aspx.cs" Inherits="JonSkeetForm" %>
<!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>Jon Skeet's Form Page</title>
</head>
<body>
    <form action="JonSkeetForm.aspx" method="get">
    <div>
        <input type="text" ID="text1" runat="server" />
        <input type="text" ID="text2" runat="server" />
        <button type="submit">Submit</button>
        <asp:Repeater ID="Repeater1" runat="server">
            <ItemTemplate>
                <div>Some text</div>
            </ItemTemplate>
        </asp:Repeater>
    </div>
    </form>
</body>
</html>

Then in your code-behind you can do everything you need on PageLoad

public partial class JonSkeetForm : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        text1.Value = Request.QueryString[text1.ClientID];
        text2.Value = Request.QueryString[text2.ClientID];
    }
}

If you don't want a form that has runat="server", then you should use HTML controls. It's easier to work with for your purposes. Just use regular HTML tags and put runat="server" and give them an ID. Then you can access them programmatically and code without a ViewState.

The only downside is that you won't have access to many of the "helpful" ASP.NET server controls like GridViews. I included a Repeater in my example because I'm assuming that you want to have the fields on the same page as the results and (to my knowledge) a Repeater is the only DataBound control that will run without a runat="server" attribute in the Form tag.

Asp – Why is the viewstate growing between postbacks when using dynamic data

I don't know the direct answer to your question, but perhaps one of these tools may help you diagnose further:

(1) Fritz Onion's ViewState Decoder tool might be able to help you figure out what's getting stuffed in there. You can find ViewSate Decoder listed at the Pluralsight Tools page. Direct download link: ViewStateDecoder22.zip.

ViewState Decoder screen shot http://alt.pluralsight.com/images/decoderscreenshot.gif

(2) Another tool that may be worth looking at is ASP.NET ViewState Helper:

"[...] ASP.NET ViewState Helper is designed to help all web developers, but has specific features to help ASP.NET developers debug ViewState issues. Analyze your web pages in real-time while you browse them using Internet Explorer 6.0 or higher. [...]"

Related Topic

R – ViewState error: Concurrent users modifying dynamic controls