Hello after I migrate my system, the framework 2.0 to 4.5.1 , I found some problems, I am solving... , However I have a problem I am unable to solve , I sought for a long time and did not get a solution.....

I Use Visual Studio 2013

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>)
     System.Web.UI.ControlCollection.Add(Control child) +9718890
       AjaxControlToolkit.ToolkitResourceManager.RegisterCssReferences(Control control) +861
       AjaxControlToolkit.ExtenderControlBase.OnLoad(EventArgs e) +99
       System.Web.UI.Control.LoadRecursive() +59
       System.Web.UI.Control.LoadRecursive() +131
       System.Web.UI.Control.LoadRecursive() +131
       System.Web.UI.Control.LoadRecursive() +131
       System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +780

Some of the que solutions did not work .: The Controls collection cannot be modified because the control contains code blocks

Thanks all!