C# Group items on multiple values and keep null values as partial matches

Question

I have a list of items that need to be grouped on 3 fields. However, I need to include items in a group if some of these fields are null, but the rest match. It's kind of hard to explain, but here is a scenario.

So for example, we have a list of the following:

Items:
Amount  Type    Code   
50      Test    AA
50      Test2   BB
null    Test    AA
45      Test3   CC
50      Test    null
50      Test    AB

So we would get 4 groups

Group 1: 50/null + Test + AA/null
Group 2: 50 + Test2 + BB
Group 3: 45 + Test3 + CC
Group 4: 50 + Test + AB

We include the null rows in Group 1 because though we had null values, the rest was a match. I'm not sure how I can accomplish this via linq or C# and could use some help!

Your requirement is little bit unclear. Can you please explain more that why Null and AA is grouped together in "Group 1" but Null is not grouped with last Group 4 with "AB". There must be some criteria to group Null with first group but not with last? — Mittal Patel, Jun 08 '23 at 14:37
Also can you please give your expected output as list/table form? — Mittal Patel, Jun 08 '23 at 14:44
C# is a language of types - show your classes / types for your sample data. — NetMage, Jun 09 '23 at 18:13

score 0 · Answer 1 · answered Jun 13 '23 at 22:49

It took me a few re-reads to see what you meant. But, I think you mean to group by each of the list object's properties, while ignoring null values in the property compare.

So here's a solution by using a custom equality comparer (called ItemComparer in the code sample below) that meets your special requirement. You can then plug this comparer into the linq GroupBy function and voila:

void Main()
{
    var items = GetItems();

    foreach (var grp in items.GroupBy(i => i, new ItemComparer()))
    {
        foreach (var g in grp)
        {
            Console.WriteLine($"Grouping {grp.Key.Code}: {g.Amount?.ToString() ?? "null"} + {g.Type ?? "null"} + {g.Code?? "null"}");
        }   
        Console.WriteLine("--------------------------------------------------");
    }   
}

public class ItemComparer : IEqualityComparer<Item>
{
    // Here we consider a null on either side of the compare to be true as per your 
    // requirements. Normally a null compare returns: false.
    public bool Equals(Item a, Item b)
    {
        return 
            (a.Amount == null || b.Amount == null || a.Amount == b.Amount) &&
            (a.Type == null || b.Type == null || a.Type == b.Type) &&
            (a.Code == null || b.Code == null || a.Code == b.Code);
    }
    
    // The linq operators will try a hash code comparison first, if they are the same, 
    // then it will use the Equals function. So always returning 0 will force it to use 
    // the Equals method.
    public int GetHashCode(Item item)
    {
        return 0;       
    }
}

public List<Item> GetItems()
{
    return new List<Item>
    {
        new Item { Amount = 50, Type = "Test", Code = "AA" },
        new Item { Amount = 50, Type = "Test2", Code = "BB" },
        new Item { Amount = null, Type = "Test", Code = "AA" },
        new Item { Amount = 45, Type = "Test3", Code = "CC" },
        new Item { Amount = 50, Type = "Test", Code = null },
        new Item { Amount = 50, Type = "Test", Code = "AB" }
    };
}

public class Item 
{
    public int? Amount { get; set; }
    public string Type { get; set; }
    public string Code { get; set; }
}

This returns the following:

Grouping AA: 50 + Test + AA
Grouping AA: null + Test + AA
Grouping AA: 50 + Test + null
--------------------------------------------------
Grouping BB: 50 + Test2 + BB
--------------------------------------------------
Grouping CC: 45 + Test3 + CC
--------------------------------------------------
Grouping AB: 50 + Test + AB
--------------------------------------------------

C# Group items on multiple values and keep null values as partial matches

1 Answers1