Check out example codes for "select from list where not in other list c#". It will help you in understanding the concepts better.

Code Example 1

var result = peopleList2.Where(p => peopleList1.All(p2 => p2.ID != p.ID));

Code Example 2

var result = peopleList2.Where(p => !peopleList1.Any(p2 => p2.ID == p.ID));

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