Mark method as not enumerable

I have a code that is checking something if it's not null. It works like Code Contracts (They doesn't work in my project so I use my own implementation).

So for example my test method is:

        public static void IsNotNull<T>(this T obj, string argName = null) where T : class
        {
            if (obj != null)
                return;
            if (argName == null)
                throw new ArgumentNullException();
            throw new ArgumentNullException(argName);
        }

then I call it somewhere:

            source.IsNotNull("source");
            foreach (T value in source) // working with source

and resharper warns "Possible NullReferenceException". I don't want to turn off this check but I also don't want to get spammed by this message. I tried to apply Code Annotations, but they doesn't work. I want to say "Hey, R#, this code doesn't enumerate collection so don't worry about this call".