Distinct is not working with IEquatable<T>

Question

I have a class Bar that looks like this:

public class Bar : IEquatable<Bar>
{
    public string Stringbar1{ get; set; }
    public string Stringbar2{ get; set; }
    public string Stringbar3{ get; set; }
    public string Stringbar4{ get; set; }

    [JsonConverter(typeof(StringEnumConverter))]
    public EnumFoo1 Enumfoo1{ get; set; }

    public bool IsBar{ get; set; }
    public List<string> StringBars{ get; set; }
    [BsonSerializer(SerializerType = typeof(NullableDateTimeOffsetToUtcSerializer))]
    public DateTimeOffset? FooDate{ get; set; }
    public string Stringbar5{ get; set; }
    [JsonConverter(typeof(StringEnumConverter))]
    public EnumFoo2 EnumFoo2 { get; set; }
    public string StringBar6{ get; set; }
    public int Foo{ get; set; }
 
    public Bar()
    {
        EnumFoo1= EnumFoo1.Unknown;
        EnumFoo2= EnumFoo2.Other;
        StringBars= new List<string>();
    }

 public bool Equals(Bar other)
    {
        if (other == null)
        {
            return false;
        }

        return Stringbar1 == other.Stringbar1&& Stringbar2== other.Stringbar2 && Stringbar3== other.Stringbar3 && Stringbar4== other.Stringbar4 && EnumFoo1== other.EnumFoo1 && IsBar== other.IsBar&&  BothNullOrEquals(StringBars,other.StringBars) && StringBar5==other.StringBar5&& FooDate== other.FooDate && ContractType == other.ContractType && LotNumber == other.LotNumber && Rank == other.Rank;
    }


    public override int GetHashCode()
    {
        var stringbar1Hashcode = Stringbar1== null ? 0 : Stringbar1.GetHashCode();
        var stringbar2HashCode = Stringbar2== null ? 0 : Stringbar2.GetHashCode();
        var stringbar3CodeHashCode = Stringbar3== null ? 0 : Stringbar3.GetHashCode();
        var EnumFoo1HashCode =  EnumFoo1.GetHashCode();
        var Stringbar4HashCode = Stringbar4== null ? 0 : Stringbar4.GetHashCode();
        var isBarHashCode =  IsBar.GetHashCode();
        var strtingBarsHashCode = StringBars== null ? 0 : StringBars.GetHashCode();
        var stringbar5HashCode = Stringbar5== null ? 0 : Stringbar5.GetHashCode();
        var fooDateHashCode = FooDate== null ? 0 : FooDate.GetHashCode();
        var enumFoo2HashCode= EnumFoo2.GetHashCode();
         var stringBar6HasCode = StringBar6== null ? 0 : StringBar6.GetHashCode();
        var fooHashCode= Foo.GetHashCode();
        return stringbar1Hashcode ^ stringbar2HashCode ^ stringbar3CodeHashCode ^ EnumFoo1HashCode ^ Stringbar4HashCode ^ isBarHashCode ^ strtingBarsHashCode ^ stringbar5HashCode ^ fooDateHashCode ^ enumFoo2HashCode ^ stringBar6HasCode  ^ fooHashCode ;
    }
    
    
    public static bool BothNullOrEquals<T>(IEnumerable<T> left, IEnumerable<T> right)
    {
        if (left == null && right == null)
        {
            return true;
        }
        if (left != null && right != null)
        {
            return left.SequenceEqual(right);
        }
        return false;
    }
}

Equals is working as expected but it seems that I am missing something when it comes to GetHashCode cause extension methods like LINQ Distinct are not working as expected. and I know that Distinct uses the GetHashCode method to compare references so any idea what am I doing wrong ?

Answer 1

The problem I can see is in the

var strtingBarsHashCode = StringBars== null ? 0 : StringBars.GetHashCode();

Please, note that for List<string> StringBars HashCode is reference based: hash codes are guaranteed to be equal if StringBars share the same reference only. However, you compare these collections by more relaxing scheme:

BothNullOrEquals(StringBars, other.StringBars)

So, StringBars doesn't have to share the same reference to be equal, they should have equal items only. Note that GetHashCode must not return different codes for equal instances.

Don't make GetHashCode that elaborated; please, note, that GetHashCode should be a fast estimation of Equals; to be so 2 .. 3 selective properties are enough:

public override int GetHashCode() {
  // Let .Net check for null and combine hash codes for you
  return HashCode.Combine(Stringbar1, Stringbar2, Stringbar3);
}

Edit: If you can't use HashCode.Combine, well let it be ^, but, please, don't put many fields and properties into GetHashCode just a few the most selective ones:

public override int GetHashCode() {
  // Let .Net check for null and combine hash codes for you
  return (Stringbar1?.GetHashCode() ?? 0) ^
         (Stringbar2?.GetHashCode() ?? 0) ^
         (Stringbar3?.GetHashCode() ?? 0);
}