IEqualityComparer使用字符串列表作为比较器

Question

I'm attempting to setup an IEqualityComparer that uses a list of string as the comparing property.

When using Except and Intersect in the 2 lines of code below, all records are seen as 'new' and none are recognized as 'old'.

List<ExclusionRecordLite> newRecords = currentRecords.Except(historicalRecords, new ExclusionRecordLiteComparer()).ToList();
List<ExclusionRecordLite> oldRecords = currentRecords.Intersect(historicalRecords, new ExclusionRecordLiteComparer()).ToList();

This is my IEqualityComparer class (Words is a List)

public class RecordComparer : IEqualityComparer<Record>
{
    public bool Equals(Record x, Record y)
    {
        if (object.ReferenceEquals(x, y))
            return true;

        if (x == null || y == null)
            return false;

        return x.Words.SequenceEqual(y.Words);
    }

    public int GetHashCode(Record obj)
    {
        return new { obj.Words }.GetHashCode();
    }
}

Answer 1

Your GetHashCode is incorrect. Use one like this:

public override int GetHashCode()
{
    if(Words == null) return 0;
    unchecked
    {
        int hash = 19;
        foreach (var word in Words)
        {
            hash = hash * 31 + (word == null ? 0 : word.GetHashCode());
        }
        return hash;
    }
}

To answer why a collection does not override GetHashCode but uses object.GetHashCode which returns a unique value: Why does C# not implement GetHashCode for Collections?

Answer 2

Assuming you're storing Words in a List then calling GetHashCode on it won't give you back a hash of the items in it, instead it'll give you back the hash value from Object.GetHashCode .

You need to implement your own hash function that enumerates over the words and generates a hash value.