Question

I have two lists (ObservableCollections) of following class:

public class Data
{
    public string Key { get; set; }
    public string Value { get; set; }
}

One representing old objects (listA) and second representing updated ones (listB). I want to merge new data from listB to listA without breaking any references. More precisely I want to do the following:

• Remove from listA all objects that do not exist in listB (objects are compared by Key property)
• Add to listA all objects from listB that do not exist in listA
• Update Value property for all objects in listA that are present in both lists

Can you propose some efficient way to do that? My solution is big and seems very ineffective.

Update:
Current solution is:

public void MergeInstanceList(List<Instance> instances)
{
    var dicOld = Instances.ToDictionary(a => a.Ip);
    var dicNew = instances.ToDictionary(a => a.Ip);
    var forUpdate = instances.Intersect(Instances, new Comparer()).ToList();
    Instances.Where(a => !dicNew.Keys.Contains(a.Ip)).ToList().ForEach(a => Instances.Remove(a));
    instances.Where(a => !dicOld.Keys.Contains(a.Ip)).ToList().ForEach(a => Instances.Add(a));
    forUpdate.ForEach(a => dicOld[a.Ip].Name = a.Name);
}
public class Comparer : IEqualityComparer<Instance>
{

    public bool Equals(Instance x, Instance y)
    {
        return x.Ip == y.Ip;
    }

    public int GetHashCode(Instance obj)
    {
        return obj.Ip.GetHashCode();
    }
}

Answer 1

var listA = ...;
var listB = ...;

var itemsToRemove = new HashSet<Data>(listA.Except(listB));
var itemsToAdd = listB.Except(listA);
var itemsToUpdate = listA.Join(listB, a => listA.Key, b => listB.Key, 
            (a, b) => new
            {
                First = a,
                Second = b
            });

listA.AddRange(itemsToAdd);
listA.RemoveAll(item => itemsToRemove.Contains);
foreach(var pair in itemsToUpdate)
{
  //set properties in First to be that of second
}

As is mentioned in another answer, you will need to create a custom comparer and pass it into the two Except methods for them to work properly, or you will need to override the Equals and GetHashCode methods to be based off of just Key.