How to maintain state in (asp.net) custom server control?

Question

Hi, I am trying to make a custom server control which inherits from DropDownList. I give the control an XML input containing some key/value pairs and my control shows them as a DropDownList. I make the list items in the override Render method like this:

foreach (XElement child in root.Elements("Choice"))
{
    string title = child.Element("Title").Value;
    string score = child.Element("Score").Value;
    item = new ListItem();
    item.Text = title;
    item.Value = score;

    this.Items.Add(item);
}

The problem is that, when the user selects and item in the list, and the page posts back, the selected item is lost, and the list is re-initialized with the default data.

Does anyone have any idea how to keep the selected item, i.e. maintain the state?

Here is the complete source:

public class MultipleChoiceQuestionView2 : DropDownList

{ public MultipleChoiceQuestionView2() : base() { }

protected override void Render(HtmlTextWriter writer) { writer.RenderBeginTag(HtmlTextWriterTag.Table); writer.RenderBeginTag(HtmlTextWriterTag.Tr);

writer.RenderBeginTag(HtmlTextWriterTag.Td); #region Parse Contets if (!String.IsNullOrEmpty(this.Contents)) { XElement root = XElement.Parse(this.Contents);

if (root.HasAttributes) { this.NoOfChoices = Int32.Parse(root.Attribute("ItemCount").Value); }

this.Items.Clear(); this.Style.Add("width", "100px"); this.Style.Add("font-family", "Tahoma"); this.Items.Clear(); ListItem item = new ListItem(); item.Text = ""; item.Value = "0"; this.Items.Add(item);

foreach (XElement child in root.Elements("Choice")) { string title = child.Element("Title").Value; string score = child.Element("Score").Value; item = new ListItem(); item.Text = title; item.Value = score;

this.Items.Add(item);

} } #endregion base.Render(writer); writer.RenderEndTag();

if (this.Required) { RequiredFieldValidator rfv = new RequiredFieldValidator(); rfv.ControlToValidate = this.ID; rfv.InitialValue = "0"; rfv.Text = "*"; if (!String.IsNullOrEmpty(this.ValidationGroup)) { rfv.ValidationGroup = this.ValidationGroup; } writer.RenderBeginTag(HtmlTextWriterTag.Td); rfv.RenderControl(writer); writer.RenderEndTag(); }

writer.RenderEndTag(); writer.RenderEndTag(); } #region Properties public string Contents { get { return ViewState["Contents"] == null ? "" : ViewState["Contents"].ToString(); } set { ViewState["Contents"] = value; } }

private int mNoOfChoices; public int NoOfChoices { get { return mNoOfChoices; } set { mNoOfChoices = value; } }

private string mValidationGroup; public string ValidationGroup { get { return mValidationGroup; } set { mValidationGroup = value; } }

public string SelectedChoice { get { return ""; } }

private bool mRequired = false; public bool Required { get { return mRequired; } set { mRequired = value; } }

#endregion }

Thanks in advance.

Answer 1

You've got two options: ViewState or ControlState.

The difference being ViewState can be overriden by setting EnableViewState="false" in the page directive, whilst ControlState cannot.

Essentially you need to hook into the state bag when you're getting/setting the values of the dropdown.

There's a decent example here where a custom control is derived from the Button class and maintains state between page requests - should fit your scenario nicely.

Hopefully that gets you started.

Answer 2

Hi, Thanks for your help, I tried the link you specified (ControlState), but I still can't maintain the selected value. Here is what I have:

public class MultipleChoiceQuestionView2 : DropDownList
{
    public MultipleChoiceQuestionView2()
        : base()
    {
    }
    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        Page.RegisterRequiresControlState(this);
    }
    protected override object SaveControlState()
    {
        Object obj = base.SaveControlState();
        if (obj != null)
        {
            return new Pair(obj, this.SelectedValue);
        }
        else
        {
            return (this.SelectedValue);
        }
    }
    protected override void LoadControlState(object savedState)
    {
        if (savedState != null)
        {
            Pair p = savedState as Pair;
            if (p != null)
            {
                base.LoadControlState(p.First);
                try
                {
                    this.SelectedValue = (string)p.Second;
                }
                catch (Exception)
                {
                }
            }
            else
            {
                if (savedState is string)
                {
                    this.SelectedValue = (string)savedState;
                }
                else
                {
                    base.LoadControlState(savedState);
                }
            }
        }
    }

    protected override void Render(HtmlTextWriter writer)
    {
        writer.RenderBeginTag(HtmlTextWriterTag.Table);
        writer.RenderBeginTag(HtmlTextWriterTag.Tr);

        writer.RenderBeginTag(HtmlTextWriterTag.Td);
        #region Parse Contets
        if (!String.IsNullOrEmpty(this.Contents))
        {
            XElement root = XElement.Parse(this.Contents);

            if (root.HasAttributes)
            {
                this.NoOfChoices = Int32.Parse(root.Attribute("ItemCount").Value);
            }

            this.Items.Clear();
            this.Style.Add("width", "100px");
            this.Style.Add("font-family", "Tahoma");
            this.Items.Clear();
            ListItem item = new ListItem();
            item.Text = "";
            item.Value = "0";
            this.Items.Add(item);

            foreach (XElement child in root.Elements("Choice"))
            {
                string title = child.Element("Title").Value;
                string score = child.Element("Score").Value;
                item = new ListItem();
                item.Text = title;
                item.Value = score;

                this.Items.Add(item);
            }
        }
        #endregion
        base.Render(writer);
        writer.RenderEndTag();

        if (this.Required)
        {
            RequiredFieldValidator rfv = new RequiredFieldValidator();
            rfv.ControlToValidate = this.ID;
            rfv.InitialValue = "0";
            rfv.Text = "*";
            if (!String.IsNullOrEmpty(this.ValidationGroup))
            {
                rfv.ValidationGroup = this.ValidationGroup;
            }
            writer.RenderBeginTag(HtmlTextWriterTag.Td);
            rfv.RenderControl(writer);
            writer.RenderEndTag();
        }

        writer.RenderEndTag();
        writer.RenderEndTag();
    }
    #region Properties
    public string Contents
    {
        get
        {
            return ViewState["Contents"] == null ? "" : ViewState["Contents"].ToString();
        }
        set
        {
            ViewState["Contents"] = value;
        }
    }

    private int mNoOfChoices;
    public int NoOfChoices
    {
        get { return mNoOfChoices; }
        set { mNoOfChoices = value; }
    }

    private string mValidationGroup;
    public string ValidationGroup
    {
        get { return mValidationGroup; }
        set { mValidationGroup = value; }
    }

    public string SelectedChoice
    {
        get
        {
            return "";
        }
    }

    private bool mRequired = false;
    public bool Required
    {
        get { return mRequired; }
        set { mRequired = value; }
    }

    #endregion
}

Do you have any idea what is going wrong?