LINQ: Use .Except() on collections of different types by making them convertible/comparable?

Question

Given two lists of different types, is it possible to make those types convertible between or comparable to each other (eg with a TypeConverter or similar) so that a LINQ query can compare them? I've seen other similar questions on SO but nothing that points to making the types convertible between each other to solve the problem.

Collection Types:

public class Data
{
    public int ID { get; set; }
}

public class ViewModel
{
    private Data _data;

    public ViewModel(Data data)
    {
        _data = data;
    }
}

Desired usage:

    public void DoMerge(ObservableCollection<ViewModel> destination, IEnumerable<Data> data)
    {
        // 1. Find items in data that don't already exist in destination
        var newData = destination.Except(data);

        // ...
    }

It would seem logical that since I know how to compare an instance of ViewModel to an instance of Data I should be able to provide some comparison logic that LINQ would then use for queries like .Except(). Is this possible?

Answer 1

I assume that providing a projection from Data to ViewModel is problematic, so I'm offering another solution in addition to Jason's.

Except uses a hash set (if I recall correctly), so you can get similar performance by creating your own hashset. I'm also assuming that you are identifying Data objects as equal when their IDs are equal.

var oldIDs = new HashSet<int>(data.Select(d => d.ID));
var newData = destination.Where(vm => !oldIDs.Contains(vm.Data.ID));

You might have another use for a collection of "oldData" elsewhere in the method, in which case, you would want to do this instead. Either implement IEquatable<Data> on your data class, or create a custom IEqualityComparer<Data> for the hash set:

var oldData = new HashSet<Data>(data);
//or: var oldData = new HashSet<Data>(data, new DataEqualityComparer());
var newData = destination.Where(vm => !oldData.Contains(vm.Data));