how to create merge item method in c#

Question

//Items.cs
public class Items
{
    public int ItemNumber { get; set; }
    public DateTime ItemDate { get; set; }
    public List<ItemsLine> ItemsLines { get; set; } = new List<ItemsLine>();

    /// MergeItem appends the items from the sourceItem to the current item
    /// </summary>
    /// <param name="sourceItem">Item to merge from</param>
    public void MergeItem( Items sourceItem)
    {
        //IEnumerable<ItemsLine> items = new List<ItemsLine>();
        //IEnumerable<Items> items = ItemsLines.Union<item>
        
        throw new NotImplementedException();
    }
}

//ItemLine.cs
public class ItemsLine
{
    public int ItemLineId { get; set; }
    public int Quantity { get; set; }
    public double Cost { get; set; }

    // public double Total { get { return this.Cost * this.Quantity; } }

    public ItemsLine(int itemLineId, int quantity, double cost)
    {
        this.ItemLineId = itemLineId;
        this.Quantity = quantity;
        this.Cost = cost;
    }

    public ItemsLine() { }
}

//Program.cs
private static void MergeItem()
{
    var item1 = new Items();

    item1.ItemsLines.Add(new ItemsLine()
    {
        ItemLineId = 1,
        Cost = 10.33,
        Quantity = 4,
    });

    var item2 = new Items();

    item2.ItemsLines.Add(new ItemsLine()
    {
        ItemLineId = 2,
        Cost = 5.22,
        Quantity = 1,
    });

    item2.ItemsLines.Add(new ItemsLine()
    {
        ItemLineId = 3,
        Cost = 6.27,
        Quantity = 3,
    });

    item1.MergeItem(item2);
    Console.WriteLine("Grand total after merge = {0}", item1.ItemsLines.Sum(i => i.Cost * i.Quantity));
}

The rules for merging items are:

• The ItemsLine instances in the .ItemsLines of the Item passed into Merge, are added to the this.ItemsLines, or used to update existing items
• ItemsLine has an ItemLineID that is unique in the list
• If the incoming ItemsLine.ItmeLineID already exists in the list the Quantity of the incoming item should be added to the existing item and existing Cost should be adjusted to the average of the existing and the incoming costs
• If the incoming ItemsLine.ItemLineID doesn't exist in the list the ItemsLine should be added

I tried using below code but that didn't work

• IEnumerable<ItemsLine> items = new List<ItemsLine>();

• IEnumerable<Items> items = ItemsLines.Union<item>()

To merge do i need to use Concat?, so you can combine two collections that implement IEnumerable or the Linq join query to achieve mergeritem?

Answer 1

You need an implementation of MergeItem that merges the local item with the incoming:

public void MergeItem(Item incoming)
{
    if(this.ItemNumber != incoming.ItemNumber)
      throw new BlahException($"Item with number {this.ItemNumber} cannot be merged with different item having number {incoming.ItemNumber}");

    this.ItemDate = DateTime.UtcNow;

    foreach(var il in incoming.ItemLines){

      var existing = this.ItemLines.SingleOrDefault(t => t.ItemLineId == il.ItemLineId);

      if(existing == null) //not known, add 
        this.ItemLines.Add(il);
      else{ //merge
        existing.Quantity += il.Quantity;
        existing.Cost = (existing.Cost + il.Cost) / 2.0;
      }
    }
}

Please don't name your classes in the plural form, it's bad practice because it makes it hard to name your variables in the plural if they are a collection of your things. A List<Items> should be called Itemss (multiple Items), which is a bit awkward. If you create classes that are a collection of something that don't really have an accepted name that people understand to include multiples of something (like a Library has List<Book> Books, an Order has List<Product> Products etc) then name your class with a Collection suffix or similar that implies it is a group of things, but be careful to make it so that the class's purpose is really nothing more than "to be a collection of the things", like Microsoft did with DataTable (has a DataColumnCollection called Columns, and a DataRowCollection called Rows - these things are literally just collections of those things)

Answer 2

You ask how to merge two lists using a "Linq join query". Conceptually, your proposed algorithm corresponds to a full outer join on your two ItemsLine lists, matching on the ItemLineId, preserving unmatched items from both lists as-is and merging matched items using the logic described in your question:

(Diagram from this answer by Pratik.)

Unfortunately, there is no full outer join built into LINQ currently. Fortunately there are several working imlementations from LINQ - Full Outer Join. I copied Ext.FullOuterJoin<TLeft, TRight, TKey, TResult>() from this answer by NetMage, and was able to merge your Items lists as follows:

public class Items
{
    public int ItemNumber { get; set; }
    public DateTime ItemDate { get; set; }
    public List<ItemsLine> ItemsLines { get; set; } = new List<ItemsLine>();

    /// MergeItem appends the items from the sourceItem to the current item
    /// </summary>
    /// <param name="sourceItem">Item to merge from</param>
    public void MergeItem( Items sourceItem)
    {
        var mergedItems = ItemsLines.FullOuterJoin(sourceItem.ItemsLines, i => i.ItemLineId, i => i.ItemLineId, (i1, i2) =>
                                                   {
                                                       if (i1 == default)
                                                           return i2;
                                                       else if (i2 == default)
                                                           return i1;
                                                       //It's not clear from your question whether the average cost should be weighted by the quantities.
                                                       //i1.Cost = (i1.Cost + i2.Cost) / 2.0,
                                                       i1.Cost = (i1.Quantity * i1.Cost + i2.Quantity * i2.Cost) / (i1.Quantity + i2.Quantity);
                                                       i1.Quantity = i1.Quantity + i2.Quantity;
                                                       return i1;
                                                   })
            .ToList();
        ItemsLines.Clear();
        ItemsLines.AddRange(mergedItems);
    }
}

Working demo fiddle here.