Shared Method and Thread Safe

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I have a shared/static method that access an object from the AppDomain. If the object is not cached, the method creates an object reference and caches the reference into the Appdomain. Multiple threads will use this method. I would like to make the method thread-safe by sychronizing specific code sections in the method. Is this necessary and, if so, what is the best way to sync critical code sections in a static method?