Diffs

Diff Generator

public static class DiffGenerator
{
    public Diff<T> Generate<T>(IEnumerable<T> left, IEnumerable<T> right)
    {       
        var added = right.Except(left).ToList();
        var removed = left.Except(right).ToList();

        var leftList = left.ToList();       
        var common = right.Where(item => leftList.Contains(item)).ToList();

        return new Diff(added, removed, common);
    }
}

public sealed class Diff<T>
{
    public IEnumerable<T> Added { get; private set; }
    public IEnumerable<T> Removed { get; private set; }
    public IEnumerable<T> Common { get; private set; }

    internal Diff(IEnumerable<T> added, IEnumerable<T> removed, IEnumerable<T> common)
    {
        Added = added;
        Removed = removed;
        Common = common;
    }
}

public class Something
{   
    public void Update(string updatedName, IEnumerable<Guid> updatedCountries)
    {
        var countriesDiff = DiffGenerator.Generate(this.countries, updatedCountries);

        var regionNameChanged = updatedName != this.name;

        this.Apply(new RegionUpdated(updatedName, regionNameChanged, countriesDiff));
    }
}