Published: 06/03/2008

A short time ago I was confronted with a serious problem. What I needed to do was dynamically choose a UserControl as well as fire methods from that UserControl. The problem lies in the fact that a UserControl does not implement my custom methods that I needed for my controls. Each control was similar and would have the same methods but it would have different display characteristics.

That was when I had a small epiphany. Why can’t I just make an abstract base class? Well the answer is you can! Sometimes I am prone to forget how .NET allows me to customize pre-defined classes. What we can do is create an abstract base class that inherits the UserControl class, then have our UserControls inherit from our base class.

First we will create our abstract base class.

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using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
/// <summary>
/// Base class to be inherited by a UserControl that displays the date
/// </summary>
public abstract class DateTimeDisplayControl : UserControl
{
/// <summary>
/// Updates the date time inside the user control.
/// </summary>
public abstract void UpdateDateTime();
}

So in the snippet above, we have created a class called DateTimeDisplayControl. This inherits from UserControl and will have to override the abstract method UpdateDateTime().

Now we can create a couple of UserControls that inherit from our DisplayDateTimeControl class. The first control will be called "ControlOne".

Here is the *.ascx code:

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<%@ Control Language="C#" AutoEventWireup="true" CodeFile="ControlOne.ascx.cs" Inherits="UserControls_ControlOne" %>
<p>User control one</p>
<asp:UpdatePanel ID="udp1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Label ID="lblDateTime" runat="server" />
</ContentTemplate>
</asp:UpdatePanel>

Here is the code behind:

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using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
public partial class UserControls_ControlOne : DateTimeDisplayControl
{
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
UpdateDateTime();
}
public override void UpdateDateTime()
{
lblDateTime.Text = DateTime.Now.ToString();
udp1.Update();
}
}

As you can see, ControlOne contains an UpdatePanel with a Label inside of it. The Label will display the date and time. We will call our second control "ControlTwo" and it will look exactly like ControlOne, only it will say "User control two" inside of it.

Now we will create the actual *.aspx page to display the controls.

Here is the *.aspx code

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<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!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&quot;>
<head runat="server">
<title>Strongly Typed Dynamic User Controls</title>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="sm1" runat="server" />
<asp:UpdatePanel ID="udp" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Panel ID="pnlContent" runat="server" />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnUseControlOne" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="btnUseControlTwo" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="btnUpdateDateTime" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
<p>
<asp:Button ID="btnUseControlOne" runat="server" Text="Use Control One"
onclick="btnUseControlOne_Click" />&nbsp;
<asp:Button ID="btnUseControlTwo" runat="server" Text="Use Control Two"
onclick="btnUseControlTwo_Click" />&nbsp;
<asp:Button ID="btnUpdateDateTime" runat="server" Text="Update Content"
onclick="btnUpdateDateTime_Click" />
</p>
</form>
</body>
</html>

Here is the code behind:

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using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
public partial class _Default : System.Web.UI.Page
{
private string _controlVirtualPath = string.Empty;
private DateTimeDisplayControl _DateTimeDisplayControl = null;
private string ControlVirtualPath
{
get
{
if (string.IsNullOrEmpty(_controlVirtualPath))
_controlVirtualPath = ViewState["ControlVirtualPath"].ToString();
ViewState["ControlVirtualPath"] = _controlVirtualPath;
if (string.IsNullOrEmpty(_controlVirtualPath))
throw new ApplicationException("The control virtual path was not found");
return _controlVirtualPath;
}
}
private DateTimeDisplayControl LoadedAjaxControl
{
get
{
if (_DateTimeDisplayControl == null)
{
_DateTimeDisplayControl = (DateTimeDisplayControl)Page.LoadControl(ControlVirtualPath);
}
return _DateTimeDisplayControl;
}
}
private void LoadAndDisplayUserControl()
{
pnlContent.Controls.Add(LoadedAjaxControl);
}
private void LoadAndDisplayUserControl(string controlVirtualPath)
{
_controlVirtualPath = controlVirtualPath;
pnlContent.Controls.Add(LoadedAjaxControl);
}
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
LoadAndDisplayUserControl("~/UserControls/ControlOne.ascx");
}
protected void btnUseControlOne_Click(object sender, EventArgs e)
{
LoadAndDisplayUserControl("~/UserControls/ControlOne.ascx");
}
protected void btnUseControlTwo_Click(object sender, EventArgs e)
{
LoadAndDisplayUserControl("~/UserControls/ControlTwo.ascx");
}
protected void btnUpdateDateTime_Click(object sender, EventArgs e)
{
LoadAndDisplayUserControl();
LoadedAjaxControl.UpdateDateTime();
}
}

As you may see, the *.aspx page will load up ControlOne by default. There are 3 buttons on the page that will allow you to swap out ControlOne and ControlTwo as well as call the UpdateDateTime() method of the controls.

That is all there is to it! One important note is that I am using a private property to get the loaded control, this is important to note because you will need to call that UpdateDateTime() method on the instance of the control that you rendered to the page. I don’t know why I didn’t think of this long ago, but I hope you will find it as useful as I did!

Happy coding!


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