名称空间“System.Web.UI.WebControls”中不存在类型或命名空间名称“ToolkitScriptManager”

I am getting the following errors

我收到以下错误

  The type or namespace name 'ToolkitScriptManager' does not exist in the namespace 'System.Web.UI.WebControls' (are you missing an assembly reference?)  
  The type or namespace name 'ModalPopupExtender' does not exist in the namespace 'System.Web.UI.WebControls' (are you missing an assembly reference?)  
  The type or namespace name 'Accordion' does not exist in the namespace 'System.Web.UI.WebControls' (are you missing an assembly reference?)