Distinct 不适用于 IEquatable<t></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);
}