ASP.NET User Controls Overview

At times, you might need functionality in a control that is not provided by the built-in ASP.NET Web server controls. In those cases, you can create your own controls. You have two options. You can create:

• User controls. User controls are containers into which you can put markup and Web server controls. You can then treat the user control as a unit and define properties and methods for it.

• Custom controls. A custom control is a class that you write that derives from Control or WebControl.

User controls are substantially easier to create than custom controls, because you can reuse existing controls. They make it

particularly easy to create controls with complex user interface elements.

 User Control Structure

An ASP.NET Web user control is similar to a complete ASP.NET Web page (.aspx file), with both a user interface page and code. You create the user control in much the same way you create an ASP.NET page and then add the markup and child controls that you need. A user control can include code to manipulate its contents like a page can, including performing tasks such as data binding.

A user controls differs from an ASP.NET Web page in these ways:

• The file name extension for the user control is .ascx.
• Instead of an @ Page directive, the user control contains an @ Control directive that defines configuration and other properties.
• User controls cannot run as stand-alone files. Instead, you must add them to ASP.NET pages, as you would any control.
• The user control does not have html, body, or form elements in it. These elements must be in the hosting page.

You can use the same HTML elements (except the html, body, or form elements) and Web controls on a user control that you do on an ASP.NET Web page. For example, if you are creating a user control to use as a toolbar, you can put a series of Button Web server controls onto the control and create event handlers for the buttons.

<% @ Control Language="C#" ClassName="UserControl1" %>
<script runat="server">
    protected int currentColorIndex;
    protected String[] colors = {"Red", "Blue", "Green", "Yellow"};
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            currentColorIndex =
                Int16.Parse(ViewState["currentColorIndex"].ToString());
        }
        else
        {
            currentColorIndex = 0;
            DisplayColor();
        }
    }
   
    protected void DisplayColor()
    {
        textColor.Text = colors[currentColorIndex];
        ViewState["currentColorIndex"] = currentColorIndex.ToString();
    }
   
    protected void buttonUp_Click(object sender, EventArgs e)
    {
        if(currentColorIndex == 0)
        {
            currentColorIndex = colors.Length - 1;
        }
        else
        {
            currentColorIndex -= 1;
        }
        DisplayColor();
    }
    protected void buttonDown_Click(object sender, EventArgs e)
    {
        if(currentColorIndex == (colors.Length - 1))
        {
            currentColorIndex = 0;
        }   
        else
        {
            currentColorIndex += 1;
        }
        DisplayColor();
    }
</script>
<asp:TextBox ID="textColor" runat="server"
    ReadOnly="True" />
<asp:Button Font-Bold="True" ID="buttonUp" runat="server"
    Text="^" OnClick="buttonUp_Click" />
<asp:Button Font-Bold="True" ID="buttonDown" runat="server"
    Text="v" OnClick="buttonDown_Click" />