Filtering lists using LINQ

Question

I've got a list of People that are returned from an external app and I'm creating an exclusion list in my local app to give me the option of manually removing people from the list.

I have a composite key which I have created that is common to both and I want to find an efficient way of removing people from my List using my List

e.g

class Person
{
    prop string compositeKey { get; set; }
}

class Exclusions
{
    prop string compositeKey { get; set; }
}

List<Person> people = GetFromDB;

List<Exclusions> exclusions = GetFromOtherDB;

List<Person> filteredResults = People - exclustions using the composite key as a comparer

I thought LINQ was the ideal way of doing this but after trying joins, extension methods, using yields, etc. I'm still having trouble.

If this were SQL I would use a not in (?,?,?) query.

Answer 1

Have a look at the Except method, which you use like this:

var resultingList = 
    listOfOriginalItems.Except(listOfItemsToLeaveOut, equalityComparer)

You'll want to use the overload I've linked to, which lets you specify a custom IEqualityComparer. That way you can specify how items match based on your composite key. (If you've already overridden Equals, though, you shouldn't need the IEqualityComparer.)

Edit: Since it appears you're using two different types of classes, here's another way that might be simpler. Assuming a List<Person> called persons and a List<Exclusion> called exclusions:

var exclusionKeys = 
        exclusions.Select(x => x.compositeKey);
var resultingPersons = 
        persons.Where(x => !exclusionKeys.Contains(x.compositeKey));

In other words: Select from exclusions just the keys, then pick from persons all the Person objects that don't have any of those keys.