How to properly compare 2 objects from the same class?

Question

I have the following class dto:

public class Field
{
    public bool isEmptyField { get; set; }
    public bool isPawn { get; set; }
    public bool isDame { get; set; }
    public string color { get; set; }


    public Field(bool isEmptyField, bool isPawn, bool isDame, string color)
    {
        this.isEmptyField = isEmptyField;
        this.isPawn = isPawn;
        this.isDame = isDame;
        this.color = color;
    }
}

Can someone tell me why i can't compare objects from this class when i am using Equals() or ==?

Example:

Condition

if (TypeOfGame.gameBoard[indexFrom].Equals(Constant.PAWN_RED))

Result with Equals()

Result with ==

Can someone explain this please? I am newbie with C# i really dont know what is going on here...

`a == b` when classes are involved defaults to reference comparison, which will only evaluate to `true` if `a` and `b` are referring to the same object. It does not matter if you have two objects with the same values internally, reference comparison will deem them different. To make `a.Equals(b)` return `true` for two distinct objects that have the same values, override `Equals` and `GetHashCode`, as well as probably implement `IEquatable` on the class. — Lasse V. Karlsen, Jul 11 '20 at 22:14
Does this answer your question? [C# difference between == and Equals()](https://stackoverflow.com/questions/814878/c-sharp-difference-between-and-equals) — , Jul 11 '20 at 22:14
This is ‘reference equality’ - by default, `Equals` on a class will test if they are the same object, not that they have the same values. To use Equals like this, you should implement `IEquatable` within the class. — stuartd, Jul 11 '20 at 22:14
Sooo i need to write my custom method for something like that? :( Is there no library extension that will do it for me for simple objects like the one above? — Brarord, Jul 11 '20 at 22:17
If you use ReSharper, it can do it automatically: https://www.jetbrains.com/help/resharper/Code_Generation__Equality_Members.html It seems Visual Studio itself can do this too: https://learn.microsoft.com/en-us/visualstudio/ide/reference/generate-equals-gethashcode-methods?view=vs-2019 — , Jul 11 '20 at 22:20

score 3 · Accepted Answer · 2020-07-11T22:41:31.143

You need to implement Equals method of Object that does nothing else comparing references.

https://referencesource.microsoft.com/#mscorlib/system/object.cs,f2a579c50b414717,references

public class Field : IEquatable<Field>
{
  public bool isEmptyField { get; set; }
  public bool isPawn { get; set; }
  public bool isDame { get; set; }
  public string color { get; set; }

  public Field(bool isEmptyField, bool isPawn, bool isDame, string color)
  {
    this.isEmptyField = isEmptyField;
    this.isPawn = isPawn;
    this.isDame = isDame;
    this.color = color;
  }

  public override bool Equals(object obj)
  {
    return Equals(this, obj as Field);
  }

  public bool Equals(Field obj)
  {
    return Equals(this, obj);
  }

  static public bool Equals(Field x, Field y)
  {
    return ( ReferenceEquals(x, null) && ReferenceEquals(y, null) )
        || (    !ReferenceEquals(x, null)
             && !ReferenceEquals(y, null)
             && ( x.isEmptyField == y.isEmptyField )
             && ( x.isPawn == y.isPawn )
             && ( x.isDame == y.isDame )
             && ( x.color == y.color ) );
  }
}

IEquatable<Field> is not needed but can be usefull and it doesn't cost much.

Test

var field1 = new Field(true, true, true, "test");
var field2 = new Field(true, true, true, "test");
var field3 = new Field(false, true, true, "test");
var field4 = new Field(true, true, true, "test2");

Console.WriteLine(field1.Equals(field2));
Console.WriteLine(field1.Equals(field3));
Console.WriteLine(field1.Equals(field4));
Console.WriteLine(field1.Equals(null));
Console.WriteLine(Field.Equals(null, null));

Output

True
False
False
False
True

How to properly compare 2 objects from the same class?

1 Answers1