How to find the differences between two objects using IEquatable?

Question

I am comparing two same objects by implementing an IEquatable interface on the object. If they are not equal, then update the DB; otherwise, leave it as it is. Here the context is i need to update the table with the data coming from an excel sheet and compare the data and update only when there is a data change.

Below is the code for the same

var constructionMaterialTypes = package.GetObjectsFromSheet<ConstructionMaterialTypeExcelDto>(ConstructionDataSheetNames.CONSTRUCTION_MATERIAL_TYPE,
                                                                                              ConstructionDataTableNames.ConstructionMaterialType);
var materialTypes = new List<ConstructionMaterialType>();
foreach (var materialType in constructionMaterialTypes)
{
    var materialTypeId = GetGuidFromMD5Hash(materialType.name);
    List<string> materialTypeNotes = new();

    if (!string.IsNullOrEmpty(materialType.notes))
    {
        materialTypeNotes.Add(materialType.notes);
    }

    var existingMaterialType = ctx.ConstructionMaterialTypes.SingleOrDefault(cm => cm.Id == materialTypeId);

    var constructionMaterialType = new ConstructionMaterialType
    {
        Id = materialTypeId,
        Name = materialType.name,
        NotesHTML = materialTypeNotes
    };

    if (existingMaterialType != default)
    {
        if (existingMaterialType != constructionMaterialType) // Object comparison happening here 
        {
            existingMaterialType.Name = materialType.name;
            existingMaterialType.NotesHTML = materialTypeNotes;
        }
    }
    else
    {                       
        materialTypes.Add(constructionMaterialType);                       
    }
}

and then below is the actual class where I am implementing Iequatable interface

public sealed class ConstructionMaterialType : IIdentity<Guid>, IEquatable<ConstructionMaterialType>
{
    [Key]
    public Guid Id { get; set; }
    public string Name { get; set; }
    public List<string> NotesHTML { get; set; }
    public bool Equals(ConstructionMaterialType other)
    {
        if (other is null)
            return false;

        return this.Id == other.Id
            && this.Name == other.Name
            && this.NotesHTML == other.NotesHTML;
    }
    public override bool Equals(object obj) => Equals(obj as ConstructionMaterialType);
    public override int GetHashCode()
    {
        int hash = 19;
        hash = hash * 31 + (Id == default ? 0 : Id.GetHashCode());
        hash = hash * 31 + (Name == null ? 0 : Name.GetHashCode(StringComparison.OrdinalIgnoreCase));
        hash = hash * 31 + (NotesHTML == null ? 0 : NotesHTML.GetHashCode());
        return hash;
    }
}

this condition existingMaterialType != constructionMaterialType is always true even if both objects are holding the same values, and I have attached the images as well for reference purposes

I am not sure where I am doing wrong in the above code. Could anyone please point me in the right direction? Many thanks in advance

1. You did not override the `!=` operator, but you can use `!existingMaterialType.Equals(constructionMaterialType)` instead. 2. Instead of `this.NotesHTML == other.NotesHTML` you might want to use `this.NotesHTML.SequenceEquals(other.NotesHTML) `; in this case, also `GetHashCode()` needs to be adapted — Klaus Gütter, Jun 22 '21 at 19:19
@KlausGütter. I have used `!existingMaterialType.Equals(constructionMaterialType)` but it is still failing and yeah did not use `!=` operator — Glory Raj, Jun 22 '21 at 19:20
Did you try to debug it? I bet the problem is with `this.NotesHTML == other.NotesHTML` — Klaus Gütter, Jun 22 '21 at 19:22
The issue does seam to be the `this.NotesHTML == other.NotesHTML`, because when you equate a `List` to another `List` it just checks to see if the references are equal, and not checking if the items inside the lists are the same. You will have to use `(System.Linq) Enumerable.SequenceEqual` or write your own `List` equality method — Space, Jun 22 '21 at 19:26

Klaus Gütter · Accepted Answer · 2021-06-22T19:43:49.833

You did not override the != operator, but you can use !existingMaterialType.Equals(constructionMaterialType) instead.
this.NotesHTML == other.NotesHTML will do a reference comparison of the two list, so even if both contain excactly the same strings, it will return false is the two lists are different instances. You might want to use this.NotesHTML.SequenceEqual(other.NotesHTML) instead (might need sone adaptation if NotesHTML can be null).

Note: GetHashCode must deliver the same result for all objects that compare equal. So if you change anything in the Equals method, you probably also have to change GetHashCode. As it is not necessary that objects that compare non-equal have different hash codes, it is an option to just not take into account some properties. Here: just omit the line with NotesHTML.

I did tried `this.NotesHTML.SequenceEqual(other.NotesHTML)` the same but it is still failing somewhat and what do you mean by gethashcode needs to be adapted — Glory Raj, Jun 22 '21 at 19:32

How to find the differences between two objects using IEquatable?

1 Answers1