Не содержит определения и не может быть найден метод расширения, принимающий первый аргумент типа(отсутствует ли директива using или ссылка? )

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I have been working developing as asp.net application. Have there hundreds of pages.
I have never been through such error.
i am getting the same error in all the pages of any folders of my application.
when ever i try to add any button with underlying code for the button. it says ::::




does not contain a definition for and no extension method accepting a first argument of type could be found(are you missing a using directive or an reference?)

Что я уже пробовал:

have written the underlying  code the button event properly.
I am an asp.net developer since 5 years.

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