Map to new model based on equal objects C#

Question

I have object:

var obj = new FileList
{
   FileInfo = new List<FileDiff>()
    {
      new ()
      {
         Name = "test1",
         Included = new Diff<int> {Added = new HashSet<int> {1, 2}},
         Excluded = new Diff<int> {Added = new HashSet<int> {1, 2}},
         Existing = new Diff<int> {Added = new HashSet<int> {1, 2, 3}}
  },
    new ()
     {
        Name = "test1",
        Included = new Diff<int> {Added = new HashSet<int> {3, 4, 5}},
        Excluded = new Diff<int> {Added = new HashSet<int> {11, 222}},
        Existing = new Diff<int> {Added = new HashSet<int>()}}
    },
   new ()
    {
       Name = "equal to test1",
       Included = new Diff<int> {Added = new HashSet<int> {1, 2}},
       Excluded = new Diff<int> {Added = new HashSet<int> {1, 2}},
       Existing = new Diff<int> {Added = new HashSet<int> {1, 2, 3}}
     },
   }
};

I want to transform this data to new model where name is not string but string[] and Included, Excluded and Existing are same for these string names:

var obj = new FileListV2
 {
   FileInfo = new List<FileDiff2>()
   {
    new ()
      {
        Names = new string[] {"test1", "equal to test1"},
        Included = new Diff<int> {Added = new HashSet<int> {1, 2}},
        Excluded = new Diff<int> {Added = new HashSet<int> {1, 2}},
        Existing = new Diff<int> {Added = new HashSet<int> {1, 2, 3}}
     },
    new ()
     {
       Names = new string[] {"test1"},
       Included = new Diff<int> {Added = new HashSet<int> {3, 4, 5}},
       Excluded = new Diff<int> {Added = new HashSet<int> {11, 222}},
       Existing = new Diff<int> {Added = new HashSet<int>()}}
     }
    }
};

I tried the following:

var obj2 = obj.FileList
    .GroupBy(f => new
    {
        f.Included,
        f.Excluded,
        f.Existing,
    })
    .Select(g => new FileListV2
    {
        Names = g.Select(f => f.Name).ToArray(),
        Included = g.Key.Included,
        Excluded = g.Key.Excluded,
        Existing = g.Key.Existing,
    });

However my values are not created I want them to. How should I compare those objects?

Answer 1

From your code, obj is a FileList object but not an array (IEnumerable<T>). Thus, I doubt that your code works, but expect that you will get a compilation error for the FileList doesn't contains the GroupBy method.

Either

• you need to use obj.FileList instead, or
• assume that obj is the List<FileDiff> type.

The problem which .GroupBy not works as Included, Excluded, and Existing are (equality) compared by reference but not by value.

You have to implement the IEquatable<T> interface and override both GetHashCode and Equals methods. Therefore, when performing an equality check, both Diff<T> objects will be compared by the value.

One of the approaches to generating the hashcode for set/list, you may refer to @JonSkeet's answer on Good GetHashCode() override for List of Foo objects respecting the order.

public class Diff<T> : IEquatable<Diff<T>>
{
    public HashSet<T> Added { get; set; }
    
    public override int GetHashCode()
    {
        unchecked
        {
            int hash = 19;
            foreach (var foo in Added)
            {
                hash = hash * 31 + foo.GetHashCode();
            }
            return hash;
        }
    }
     
    public override bool Equals(object obj)
    {
        return this.Equals(obj as Diff<T>);
    }

    public bool Equals(Diff<T> other)
    {
        if (other == null)
            return false;

        return this.Added.SetEquals(other.Added);
    }
}

public class FileDiff
{
    public string Name { get; set; }
    public Diff<int> Included { get; set; }
    public Diff<int> Excluded { get; set; }
    public Diff<int> Existing { get; set; }
}

public class FileDiff2
{
    public string[] Names { get; set; }
    public Diff<int> Included { get; set; }
    public Diff<int> Excluded { get; set; }
    public Diff<int> Existing { get; set; }
}

Console program

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.Json;
                    
public class Program
{
    public static void Main()
    {
        var obj = new List<FileDiff>()
                {
                    new FileDiff
                    {
                        Name = "test1",
                        Included = new Diff<int> {Added = new HashSet<int> {1, 2}},
                        Excluded = new Diff<int> {Added = new HashSet<int> {1, 2}},
                        Existing = new Diff<int> {Added = new HashSet<int> {1, 2, 3}}
                    },
                    new FileDiff
                    {
                        Name = "test1",
                        Included = new Diff<int> {Added = new HashSet<int> {3, 4, 5}},
                        Excluded = new Diff<int> {Added = new HashSet<int> {11, 222}},
                        Existing = new Diff<int> {Added = new HashSet<int>()}
                    },
                    new FileDiff
                    {
                        Name = "equal to test1",
                        Included = new Diff<int> {Added = new HashSet<int> {1, 2}},
                        Excluded = new Diff<int> {Added = new HashSet<int> {1, 2}},
                        Existing = new Diff<int> {Added = new HashSet<int> {1, 2, 3}}
                    },
                
            };
        
        var obj2 = obj
            .GroupBy(f => new
            {
                f.Included,
                f.Excluded,
                f.Existing,
            })
            .Select(g => new FileDiff2
            {
                Names = g.Select(f => f.Name).ToArray(),
                Included = g.Key.Included,
                Excluded = g.Key.Excluded,
                Existing = g.Key.Existing,
            })
            .ToList();
    
        
        Console.WriteLine(JsonSerializer.Serialize<List<FileDiff2>>((List<FileDiff2>)obj2, new JsonSerializerOptions { WriteIndented = true  }));
    }
}

Demo @ .NET Fiddle