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<int> {1, 2, 2, 4, 5}; var list2 = new List<int> { 4, 5 };  // Remove all list items not in List2 // new List Should contain {4,5}     // The lambda expression is the Predicate. list.RemoveAll(item => item. /*solution expression here*/ );  // Display results. foreach (int i in list) {     Console.WriteLine(i); } 

Answer 1

You can do this via RemoveAll using Contains:

list.RemoveAll( item => !list2.Contains(item)); 

Alternatively, if you just want the intersection, using Enumerable.Intersect would be more efficient:

list = list.Intersect(list2).ToList(); 

The difference is, in the latter case, you will not get duplicate entries. For example, if list2 contained 2, in the first case, you'd get {2,2,4,5}, in the second, you'd get {2,4,5}.

Answer 2

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));