How would you do a "not in" query with LINQ?

I don't know if this will help you but..

NorthwindDataContext dc = new NorthwindDataContext();    
dc.Log = Console.Out;

var query =    
    from c in dc.Customers    
    where !(from o in dc.Orders    
            select o.CustomerID)    
           .Contains(c.CustomerID)    
    select c;

foreach (var c in query) Console.WriteLine( c );

from The NOT IN clause in LINQ to SQL by Marco Russo

You want the Except operator.

var answer = list1.Except(list2);

Better explanation here: https://docs.microsoft.com/archive/blogs/charlie/linq-farm-more-on-set-operators

NOTE: This technique works best for primitive types only, since you have to implement an IEqualityComparer to use the Except method with complex types.