C# Update a List from Another List

Answer 1

You could also use the Union method with an IEqualityComparer:

var newList = ListB.Union(ListA, new PersonEqualityComparer());

class PersonEqualityComparer : IEqualityComparer<Person>
{
    public bool Equals(Person person1, Person person2)
    {
        if (person1 == null && person2 == null)
            return true;
        else if ((person1 != null && person2 == null) ||
                (person1 == null && person2 != null))
            return false;

        return person1.Id.Equals(person2.Id);
    }

    public int GetHashCode(Person item)
    {
        return item.Id.GetHashCode();
    }
}

Answer 2

As I consider, you want to update only an age. Also you don't need to use Where().First() you can use just First().

foreach (var x in ListB)
{
    var itemToChange = ListA.First(d => d.Name == x.Name).Age = x.Age;
}

If you are not sure, that item exists in ListA you should use FirstOrDefault() and if statement to check it.

foreach (var x in ListB)
{
    var itemToChange = ListA.FirstOrDefault(d => d.Name == x.Name);
    if (itemToChange != null)
         itemToChange.Age = x.Age;
}

Answer 3

to elaborate aershov's answer:

ListA.Where(d => d.Name == x.Name).First().CopyFrom(x);

then in your Person class:

public class Person
{
   // ... Name, Id, Age properties...

   public void CopyFrom(Person p)
   {
      this.Name = p.Name;
      this.Id = p.Id;
      this.Age = p.Age;
   }
}

of course check nulls and everything.

Answer 4

Where and First return IEnumerable - you can modify only node of the list, but not reassign.

option 0 - generic approach

using System.Collections.Generic;

//...

   var itemToUpdate = ListA.FirstOrDefault(d => d.Name == x.Name);
   if (itemToUpdate != null) {
       ListA[ListA.IndexOf(itemToUpdate)] = x;
   }

option 1 - implement the update method or perform field update manually

ListA.First(d => d.Name == x.Name).Update(x);

Answer 5

You could remove all elements of ListB from ListA based on Id, add ListB to ListA and then sort using Id.

var newlist = ListA.Where(s => !ListB.Any(p => p.Id == s.Id)).ToList();
newlist.AddRange(ListB);
ListA = newlist.OrderBy(o => o.Id).ToList();