Uniquness of a record containing a collection

Question

I am using C# records in a HashSet to enforce uniqueness. This works well, but as soon as the record contains a Collection then it stops working.

ie. this passes:

public record Person
{
    public string FirstName { get; set; }
}

HashSet<Person> uniquePeople = new();
uniquePeople.Add(new Person { FirstName = "John" });
uniquePeople.Add(new Person { FirstName = "John" });
Assert.That(uniquePeople.Count() == 1);

But this fails:

public record Person
{
    public string FirstName { get; set; }
    public HashSet<int> FavouriteNumbers { get; set; } = new();
}

HashSet<Person> uniquePeople = new();
uniquePeople.Add(new Person { FirstName = "John" });
uniquePeople.Add(new Person { FirstName = "John" });
Assert.That(uniquePeople.Count() == 1);

How can I enforce uniqueness on nested collections using records?

Answer 1

You want custom Equals and GetHashCode since you want to compare FavouriteNumbers as sets, not as references; that's why I suggest turning record (which has its own implementation of these methods) into class:

// Note class, not record
public class Person : IEquatable<Person> {
  public string FirstName { get; set; }
  public HashSet<int> FavouriteNumbers { get; set; } = new();

  public override int GetHashCode() =>
    HashCode.Combine(FirstName, FavouriteNumbers.Count);

  public bool Equals(Person? other) => other is not null &&
    string.Equals(FirstName, other.FirstName) &&
    FavouriteNumbers.SetEquals(other.FavouriteNumbers);

  public override bool Equals(object? obj) => Equals(obj as Person);

  public override string ToString() => $"{FirstName}";
}

If you have to deal with record (you can't change it), you can implement custom comparer:

public sealed class PersonEqualityComparer : IEqualityComparer<Person> {
  public bool Equals(Person? x, Person? y) {
    if (ReferenceEquals(x, y))
      return true;
    if (x is null || y is null)
      return false;

    return string.Equals(x.FirstName, y.FirstName) &&
           x.FavouriteNumbers.SetEquals(y.FavouriteNumbers);
  }

  public int GetHashCode(Person obj) =>
    obj is null ? 0 : HashCode.Combine(obj.FirstName, obj.FavouriteNumbers.Count);
}

and then use it:

// note PersonEqualityComparer
HashSet<Person> uniquePeople = new(new PersonEqualityComparer());

uniquePeople.Add(new Person { FirstName = "John" });
uniquePeople.Add(new Person { FirstName = "John" });