Merge and Update Two Lists in C#

Question

I have two List objects:

For example:

List 1:
ID, Value where Id is populated and value is blank and it contains s

Answer 1

This is O(m*n) but should do the job for arbitrary lists

        foreach (var record in List1)
        {
            var other = List2.FirstOrDefault(x => x.Key == record.Key);
            if(other != null) record.Value = other.Value;
        }

If the lists are guaranteed ordered, then it could be brought down to O(n) at the cost of more code. The algortihm would be

Current items start as head of each list
While items remain in both lists
  If the current item of list1 has lower key than list2  advance to next in list1
  else if the current item of list2 has lower key than list1  advance to next in list2
  else copy value from current list2 item into list1 item and advance both lists.