Compare Two Lists Via LINQ

Question

I have two lists will say ListA and ListB. I need to loop through ListB and compare ID's to ListA. If there is a match then I need to remove that item from ListB and replace it with the matching item/object from ListA.

I've been looking at THIS article. I've also had a look at Intersect. But I'm really not sure on how to get this to work with Linq.

Here is my code:

ListB is a query generated else where and passed in

var itemsForListA = Context.Set<Item>().AsQueryable();
var ListA = from i in itemsForListA 
            where i.ReplacementItemID != null
                  && (i.ItemStatus == "DISC" || i.ItemStatus == "ACT" 
                  && i.StoreID == null)
            select i;

foreach (var i in ListB)
{
    ListB = ListA.Where(x => x.Id == ListA.Id);
}

I thought I could do something like that. Do I first have to find the id in ListB and remove it then append on the new id from ListA to B?

Answer 1

I thought you can use left join in linq as below.

var list = from lb in ListB
    join la in ListA
        on lb.Id equals la.Id into ListC
    from lc in ListC.DefaultIfEmpty()
    select lc ?? lb;

It won't remove and replace the items, but it will give the same result and you can reassign to ListB

Answer 2

Use the built in Enumerable.Except() method using the IEqualityComparer.

Class to compare

public class Item
{
  public int Id { get; set; }
}

Comparer

public class ItemIdComparer<Item>
{
  public bool Equals(Item left, Item right)
  {
    return left.Id == right.Id;
  }
  public int GetHashCode(Item item)
  {
    return item.Id.GetHashCode();
  }
}

Usage

var all = new List<Item>();
var existing = new List<Item>();

var nonExisting = all.Except(existing, new ItemIdComparer())

I can't test this exactly... but it should be very close.

Answer 3

you can do as follow:

var list = listB.Join(listA, x => x.Id, y => y.Id, (x, y) => y).ToList();

list.AddRange(listB.Where(b => listA.Any(c => c.Id != b.Id)).ToList());

1. First, I compare the listA with listB and selected objects from listA with same ID.
2. to the selected objects I add it the elements from listB which have no comon id with listA.

Answer 4

I'm not sure how your model looks like, so I created my own model:

public class Person
{
    public int Id { get; set; }
    public string FullName { get; set; }
    public byte Age { get; set; }
}

Create equality comparer:

public class PersonComaparer : IEqualityComparer<Person>
{
    public bool Equals(Person x, Person y)
    {
        if (x == null && y == null)
            return true;

        if ((x == null && y != null)
            || (x != null && y == null))
            return false;

        return x.Id == y.Id;
    }

    public int GetHashCode(Person obj)
    {
        if (obj == null)
            return 0;

        return obj.Id.GetHashCode();
    }
}

Then use them to compare lists:

List<Person> ListA = new List<Person>
{
    new Person { Id = 1, FullName = "Someone" }
};

List<Person> ListB = new List<Person>
{
    new Person { Id = 1 },
    new Person { Id = 2 }
};

PersonComaparer comparer = new PersonComaparer();
ListB = ListA
    .Intersect(ListB, comparer)
    .Union(ListB, comparer)
    .ToList();

The result will be:

Id  |   Full name
1       Someone
2       null

Answer 5

I compared my answer below to Bapaiah Malasani's join solution using 10,000 items. My solution consistently took six seconds or more. His was consistently 10ms or less. Ouch. I have to go back and look at a whole ton of code I've written.

---

Should you use an IEqualityComparer<Item>? I would, because you're probably going to have lots and lots of code like this. Write this once and use it forever:

public class ItemComparer: IEqualityComparer<Item>
{
    public bool Equals(Item i1, Item i2)
    {
        if(i1 == null && i2 == null) return true;
        if(i1 == null ^ i2 ==null) return false;
        return(i1 == i2 || i1.Id == i2.Id);
    }

    public int GetHashCode(Item item)
    {
        return item != null ? item.Id.GetHashcode() : 0;
    }
}

Then you could wrap this whole thing up in a function which makes it easier to read. With a series of Linq statements it might not be clear what your intent is. But a function with a clear name helps:

IEnumerable<T> ReplaceMatchingItems<T>(IEnumerable<T> discardFrom,
    IEnumerable<T> replaceWith,
    IEqualityComparer<T> comparer = null)
    {
        //prevent multiple enumerations
        var discardFromArray = discardFrom as T[] ?? discardFrom.ToArray();
        var replacements = replaceWith.Where(item => discardFromArray.Contains(item, comparer)).ToArray();
        var newList = discardFromArray.Except(replacements, comparer).ToList();
        newList.AddRange(replacements);
        return newList;
    }

Now, the original function you wanted looks like this:

var listWithReplacements = ReplaceMatchingItems(ListB, ListA, new ItemComparer());

It looks like more code at first, but the IEqualityComparer is going to save you a lot of time down the road. And when someone sees a function call like ReplaceMatchingItems they're more likely to understand what the code is doing rather than just looking at a bunch of Linq queries.