"Control can not be created because visual studio can not find the controls 's type in control assembly"

This error message was suddenly popping up in visual studio when I tried to drag my custom web control on a web page then tried to compile the code and found two main error messages in error list. Some thing like
"only one section per config file is allowed" since I had just added ajax configuration to web.config file like

<httpModules >

<add type="URLRewrite" name="URLRewrite" />

<httpHandlers>

<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>

>

<httpModules>

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

httpmodule tag has been declared twice in web.config.

Solution:

merge two httpModules tags in one tag.

<httpModules>

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

<add type="URLRewrite" name="URLRewrite" />

These two sections of httpmodules in web.config were causing this error.