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

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;
        }开发者_如何学C

        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.

---

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.