Joining 2 tables data in Linq with Contains

Question

I have a property 'CountryId' in res1list and I want to use it in join condition if c.CountryId exists in res1 but intellisense doesnt have Contains() method onc.CountryId, all I have inc.ToString() ,c.CompareTo()` and other inbuilt functions. How do I check if c.CountryId is among res1's CountryId?

 List<ClassA> res1 = new List<ClassA>();
  
  var result = await (from enumeration in db.Enumerations e
        join cust c in e.Id equals c.Id
        where c.CountryId .... 
        select c);