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Given in web.config: 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;system.web&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;pages&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;controls&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;add tagPrefix=&quot;asp&quot; namespace=&quot;System.Web.UI&quot; assembly=&quot;System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35&quot;/&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;add namespace=&quot;AjaxControlToolkit&quot; assembly=&quot;AjaxControlToolkit&quot; tagPrefix=&quot;ajaxToolkit&quot;/&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/controls&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/pages&gt; 
 
Why does the following HTML in Default.aspx's design view says:&quot;This control cannot be displayed because its TagPrefix is not registered in this Web Form&quot;: 
&nbsp; &nbsp; &nbsp; &nbsp; &lt;ajaxToolkit:CascadingDropDown ID=&quot;CascadingDropDown1&quot; runat=&quot;server&quot; TargetControlID=&quot;DropDownList1&quot; 
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Category=&quot;Make&quot; &nbsp;PromptText=&quot;Please select a make&quot; &nbsp;LoadingText=&quot;[Loading makes...]&quot; 
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ServicePath=&quot;CarsService.asmx&quot; ServiceMethod=&quot;GetDropDownContents&quot; /&gt; 
&nbsp; &nbsp; &nbsp; 
Thanks.
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Given in web.config:
	<system.web>
		<pages>
			<controls>
				<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
				<add namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" tagPrefix="ajaxToolkit"/>
			</controls>
		</pages>

Why does the following HTML in Default.aspx's design view says:"This control cannot be displayed because its TagPrefix is not registered in this Web Form":
 <ajaxToolkit:CascadingDropDown ID="CascadingDropDown1" runat="server" TargetControlID="DropDownList1"
 Category="Make" PromptText="Please select a make" LoadingText="[Loading makes...]"
 ServicePath="CarsService.asmx" ServiceMethod="GetDropDownContents" />
 
Thanks.

This control cannot be displayed because its TagPrefix is not registered in this Web Form

The successor to Active Server Pages, ASP.NET websites utilize the .NET framework to produce dynamic, data and content-driven web applications and services. ASP.NET code can be written using any .NET supported language. As of 2009, ASP.NET can also apply the Model-View-Controller (MVC) pattern to web applications