How to use HashSet as a mathematical set?-CodePudding

I try to create a mathematical set using HashSet. I have the following code:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        HashSet<int> A = new HashSet<int>() { 1, 2 };
        HashSet<int> B = new HashSet<int>() { 1, 2 };

        HashSet<HashSet<int>> SET = new HashSet<HashSet<int>>() { A, B };

        // Desired: 1 (A and B are expected being equal)
        // Actual:  2
        Console.WriteLine(SET.Count);
        
        Console.ReadKey();
    }
}

It seems HashSet equality is not suitable because A and B must be considered the same, but for HashSet they are different objects. How can I redefine equality for HashSet?

CodePudding user response：

You should explain .Net how to compare your custom data (HashSet<T> in your case) with a help of IEqualityComparer<T>:

public sealed class HashSetComparer<T> : IEqualityComparer<HashSet<T>> {
  public bool Equals(HashSet<T>? left, HashSet<T>? right) {
    if (ReferenceEquals(left, right))
      return true;
    if (left == null || right == null)
      return false;

    return left.SetEquals(right);
  }

  public int GetHashCode(HashSet<T> item) {
    //TODO: improve; simplest, but not that good implementation
    return item == null ? -1 : item.Count;
  }
}

then mention the comparing rules when creating SET:

...
HashSet<HashSet<int>> SET = new HashSet<HashSet<int>>(new HashSetComparer<int>()) { 
  A, B 
};
...