Step 1: Create a class extending the DropDownList control and add private RequiredFieldValidator variable.
public class DropDownReqValidator : DropDownList
{
#region Validator
/// <summary>
/// Validator
/// </summary>
private RequiredFieldValidator req;
#endregionStep 2: Add properties you will want to set on the validator.
Notice that the Validation Group property overrides the textbox property and also sets the corresponding property on the validator if the validator is not null. The Required property will be used to enable and disable the validator. The rest of the properties will be used to set properties on the validator control.#region Properties
/// <summary>
/// ValidationGroup Property
/// </summary>
public virtual string ValidationGroup
{
get
{
string o = base.ValidationGroup;
return o;
}
set
{
String oldValue = ValidationGroup;
if (value != oldValue)
{
if (req != null)
{
req.ValidationGroup = value;
}
base.ValidationGroup = value;
}
}
}
/// <summary>
/// InvalidMessage Property
/// </summary>
[Browsable(true)]
[Description("InvalidMessage"), Category("Custom"), DefaultValue("")]
public virtual String InvalidMessage
{
get
{
object o = ViewState["InvalidMessage"];
if (o != null)
return (String)o;
return string.Empty; // Default value
}
set
{
String oldValue = InvalidMessage;
if (value != oldValue)
{
if (req != null)
{
req.ErrorMessage = value;
}
ViewState["InvalidMessage"] = value;
}
}
}
/// <summary>
/// Required Property
/// </summary>
[Browsable(true)]
[Description("Required"), Category("Custom"), DefaultValue(false)]
public virtual bool Required
{
get
{
object o = ViewState["Required"];
if (o != null)
return (bool)o;
return false; // Default value
}
set
{
bool oldValue = Required;
if (value != oldValue)
{
ViewState["Required"] = value;
}
}
}
/// <summary>
/// ClientScript Property
/// </summary>
[Browsable(true)]
[Description("ClientScript"), Category("Custom"), DefaultValue(true)]
public virtual bool ClientScript
{
get
{
object o = ViewState["ClientScript"];
if (o != null)
return (bool)o;
return true; // Default value
}
set
{
bool oldValue = ClientScript;
if (value != oldValue)
{
if (req != null)
{
req.EnableClientScript = value;
}
ViewState["ClientScript"] = value;
}
}
}
#endregionStep 3: On Init
We will instantiate the RequiredFieldValidator in the OnInit event, we also set the validator properties. You can see the full life cycle of the page and controls and the order of the events here. Since we don't want our code to execute at design time we check if we're in design mode at the top of our method.#region On Init
/// <summary>
/// Init
/// </summary>
/// <param name="e"></param>
protected override void OnInit(EventArgs e)
{
//if we want this control to render at design time we need this code.
if (this.DesignMode)
{
base.OnInit(e);
return;
}
//instantiate and setup the validator
req = new RequiredFieldValidator();
req.Enabled = false;
req.ControlToValidate = this.ID;
req.ErrorMessage = this.InvalidMessage;
req.Text = "Required";
req.EnableClientScript = ClientScript;
req.ValidationGroup = this.ValidationGroup;
req.Display = ValidatorDisplay.Dynamic;
Controls.Add(req);
}
#endregionStep 4: Render
If the field is required we will render the textbox and the validator controls.#region Render
/// <summary>
/// Render
/// </summary>
/// <param name="writer">Writer</param>
public override void RenderControl(HtmlTextWriter writer)
{
//if we want this control to render at design time we need this code.
if (this.DesignMode)
{
base.RenderControl(writer);
return;
}
//if required property is set to true enable the validator
if (Required == true)
{
base.Render(writer);
writer.Write("<span style=\"color:red;\">*</span>");
req.Enabled = true;
req.RenderControl(writer);
}
else
{
base.RenderControl(writer);
}
}
#endregionStep 5: Override the CreateCustomControl method.
In order to add the validator control to the drop down list we need to override the Create Custom Control method.#region Create Control Collection
/// <summary>
/// Override of the Create Control Collection Method
/// based on http://www.codeproject.com/Articles/20055/Does-Not-Allow-Child-Controls-Error
/// </summary>
/// <returns></returns>
protected override System.Web.UI.ControlCollection CreateControlCollection()
{
return new ControlCollection(this);
}
#endregionUsage Example:
Markup<%@ Page Title="" Language="C#" MasterPageFile="~/Site1.Master" AutoEventWireup="true" CodeBehind="DropDownReq.aspx.cs" Inherits="CustomControls.DropDownReq" %>
<%@ Register assembly="CustomControls" namespace="CustomControls.Controls" tagprefix="cc1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<cc1:DropDownReqValidator ID="DropDownReqValidator1" runat="server" Required="true">
<asp:ListItem Text="" Value=""></asp:ListItem>
<asp:ListItem Text="Option 1" Value="1"></asp:ListItem>
<asp:ListItem Text="Option 2" Value="2"></asp:ListItem>
</cc1:DropDownReqValidator>
<br />
<asp:checkbox id="DropDownReqCheckBox" runat="server" checked="true" text="Required"
autopostback="true" oncheckedchanged="DropDownReqCheckBox_CheckedChanged" />
<asp:button id="Submit" runat="server" causesvalidation="true" text="Submit" />
</asp:Content>
Code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace CustomControls
{
public partial class DropDownReq : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void DropDownReqCheckBox_CheckedChanged(object sender, EventArgs e)
{
DropDownReqValidator1.Required = DropDownReqCheckBox.Checked;
}
}
}Examples:
Required Field:
Required Field after validation:
