How do I implement IEqualityComparer on an immutable generic Pair struct?

Question

Currently I have this (edited after reading advice):

struct Pair<T, K> : IEqualityComparer<Pair<T, K>>
{
    readonly private T _first;
    readonly private K _second;

    public Pair(T first, K second)
    {
        _first = first;
        _second = second;

    }

    public T First { get { return _first; } }
    public K Second { get { return _second; } }

    #region IEqualityComparer<Pair<T,K>> Members

    public bool Equals(Pair<T, K> x, Pair<T, K> y)
    {
        return x.GetHashCode(x) == y.GetHashCode(y);
    }

    public int GetHashCode(Pair<T, K> obj)
    {
        int hashCode = obj.First == null ? 0 : obj._first.GetHashCode();

        hashCode ^= obj.Second == null ? 0 : obj._second.GetHashCode();

        return hashCode;
    }

    #endregion

    public override int GetHashCode()
    {
        return this.GetHashCode(this);
    }

    public override bool Equals(object obj)
    {
        return (obj != null) && 
    (obj is Pair<T, K>) && 
    this.Equals(this, (Pair<T, K>) obj);
    }
}

The problem is that First and Second may not be reference types (VS actually warns me about this), but the code still compiles. Should I cast them (First and Second) to objects before I compare them, or is there a better way to do this?

Edit: Note that I want this struct to support value and reference types (in other words, constraining by class is not a valid solution)

Edit 2: As to what I'm trying to achieve, I want this to work in a Dictionary. Secondly, SRP isn't important to me right now because that isn't really the essence of this problem - it can always be refactored later. Thirdly, comparing to default(T) will not work in lieu of comparing to null - try it.

Answer 1

Your IEqualityComparer implementation should be a different class (and definately not a struct as you want to reuse the reference).

Also, your hashcode should never be cached, as the default GetHashcode implementation for a struct (which you do not override) will take that member into account.

Answer 2

It looks like you need IEquatable instead:

internal struct Pair<T, K> : IEquatable<Pair<T, K>>
{
  private readonly T _first;
  private readonly K _second;

  public Pair(T first, K second)
  {
    _first = first;
    _second = second;
  }

  public T First
  {
    get { return _first; }
  }

  public K Second
  {
    get { return _second; }
  }

  public bool Equals(Pair<T, K> obj)
  {
    return Equals(obj._first, _first) && Equals(obj._second, _second);
  }

  public override bool Equals(object obj)
  {
    return obj is Pair<T, K> && Equals((Pair<T, K>) obj);
  }

  public override int GetHashCode()
  {
    unchecked
    {
      return (_first != null ? _first.GetHashCode() * 397 : 0) ^ (_second != null ? _second.GetHashCode() : 0);
    }
  }
}