Simple Custom Web Control

Question

Hello I am designing a custom .net web control that inherits from the aspx dropdownlist. The idea is to have a dropdownlist that will display year values up until the current year. I want to be able to set a "StartYear" property that the control can start from or else use a default date. I am able to create this control but it is always using the defult date. It seems I am unable to use the property setting in the aspx code in the code behind. My front end code is ....

<customControls:YearDropDownList StartYear="2000" ID="ddlYear" runat="server"/>

and the code behind is

using System;
using System.ComponentModel;
using System.Globalization;
using System.Web.UI;
using System.Web.UI.WebControls;

    namespace customControls {
    [ToolboxData("<{0}:ServerControl1 runat=server></{0}:ServerControl1>")]
    [DefaultProperty("StartYear")]
    public class YearDropDownList : DropDownList
    {
        public YearDropDownList() {
            for (int i = Int32.Parse(StartYear); i <= DateTime.Now.Year; i++)
            {
                this.Items.Add(new ListItem(i.ToString(), i.ToString()));
            }
        }
        public string StartYear {
            get{
                String s = (String)ViewState["StartYear"];
                return ((s == null) ? "2009":s);
            }
            set{
                ViewState["StartYear"] = value;
            }
        }
    }
    }

Answer 1

you need to regenerate the list when the property is set like so;

[ToolboxData("<{0}:YearDropDownList runat=\"server\" StartYear=\"[StartYear]\"></{0}:YearDropDownList>")]
[DefaultProperty("StartYear")]
public class YearDropDownList : DropDownList
{
    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        RegenerateList();
    }
    public string StartYear
    {
        get
        {
            String s = (String)this.ViewState["StartYear"];
            return s ?? "2009";
        }
        set
        {
            ViewState["StartYear"] = value;
            RegenerateList();
        }
    }
    public void RegenerateList()
    {
        Items.Clear();
        for (int i = Int32.Parse(this.StartYear); i <= DateTime.Now.Year; i++)
        {
            this.Items.Add(new ListItem(i.ToString(), i.ToString()));
        }
    }
}

I have tested and verified the code above and it most definitely works. An interesting thing that I did notice was that I was able to reproduce your issue for a while in that the property setter was not being hit. I resolved this by right-clicking the solution and clicking Clean. After the solution was cleaned, I right-clicked on the solution again but this time selected Rebuild. That seemed to solve the property setter issue.

Answer 2

To my way of thinking, it doesn't make sense to regenerate the list in the constructor. At the time the control is instantiated, ViewState probably won't be populated yet (though I am not sure of this) and therefore you'll always get your default value being used.

Here's what I'd do:

[ToolboxData("<{0}:ServerControl1 runat=server></{0}:ServerControl1>")]
[DefaultProperty("StartYear")]
public class YearDropDownList : DropDownList
{
    public string StartYear
    {
        get
        {
            String s = (String)ViewState["StartYear"];
            return ((s == null) ? "2009" : s);
        }
        set
        {
            ViewState["StartYear"] = value;
            RegenerateList();
        }
    }

    // Defer regenerating the list until as late as possible    
    protected void OnPreRender(EventArgs e)
    {
        RegenerateList();
        base.OnPreRender(e);
    }

    public void RegenerateList()
    {
        // Remove any existing items.
        this.Items.Clear();
        for (int i = Int32.Parse(StartYear); i <= DateTime.Now.Year; i++)
        {
            this.Items.Add(new ListItem(i.ToString(), i.ToString()));
        }
    }
}