Filtering lists using LINQ

Question

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.

I would just use the FindAll method on the List class. i.e.:

List<Person> filteredResults = 
    people.FindAll(p => return !exclusions.Contains(p));

Not sure if the syntax will exactly match your objects, but I think you can see where I'm going with this.

Many thanks for this guys.

I mangaged to get this down to one line:

  var results = from p in People 
                where !(from e in exclusions 
                        select e.CompositeKey).Contains(p.CompositeKey) 
                select p;

Thanks again everyone.

var thisList = new List<string>{ "a", "b", "c" };
var otherList = new List<string> {"a", "b"};

var theOnesThatDontMatch = thisList
        .Where(item=> otherList.All(otherItem=> item != otherItem))
        .ToList();

var theOnesThatDoMatch = thisList
        .Where(item=> otherList.Any(otherItem=> item == otherItem))
        .ToList();

Console.WriteLine("don't match: {0}", string.Join(",", theOnesThatDontMatch));
Console.WriteLine("do match: {0}", string.Join(",", theOnesThatDoMatch));

//Output:
//don't match: c
//do match: a,b

Adapt the list types and lambdas accordingly, and you can filter out anything.

https://dotnetfiddle.net/6bMCvN

You can use the "Except" extension method (see http://msdn.microsoft.com/en-us/library/bb337804.aspx)

In your code

var difference = people.Except(exclusions);

I couldn't figure out how to do this in pure MS LINQ, so I wrote my own extension method to do it:

public static bool In<T>(this T objToCheck, params T[] values)
{
    if (values == null || values.Length == 0) 
    {
        return false; //early out
    }
    else
    {
        foreach (T t in values)
        {
            if (t.Equals(objToCheck))
                return true;   //RETURN found!
        }

        return false; //nothing found
    }
}