Remove items from list 1 not in list 2

Question

I am learning to write lambda expressions, and I need help on how to remove all elements from a list which are not in another list.

var list = new List

        

Answer 1

Solution for objects (maybe easier than horaces solution):

If your list contains objects, rather than scalars, it is that simple, by removing by one selected property of the objects:

    var a = allActivePatientContracts.RemoveAll(x => !allPatients.Select(y => y.Id).Contains(x.PatientId));