A rough HashableWeakRef<T> implementation. Missing many details and cases.

HashableWeakRefOfT.cs

using System;

class HashableWeakRef<T> : IEquatable<HashableWeakRef<T>> where T : class
{
    private WeakReference weakRef;

    public T Target { get { return (T) weakRef.Target; } }

    public HashableWeakRef(T target)
    {
        weakRef = new WeakReference(target);
    }

    public override int GetHashCode()
    {
        var tmpTarget = weakRef.Target;
        if (tmpTarget == null) {
            throw new InvalidOperationException("Can't hash a null weak ref");
        }
        return tmpTarget.GetHashCode();
    }

    public override bool Equals(object other)
    {
        if (other is HashableWeakRef<T>) {
            return this.Equals((HashableWeakRef<T>) other);
        }
        return false;
    }

    public bool Equals(HashableWeakRef<T> other)
    {
        var tmpOtherTarget = other.weakRef.Target;
        var tmpThisTarget = this.weakRef.Target;
        if (tmpOtherTarget == null || tmpThisTarget == null) {
            throw new ArgumentNullException("Can't compare null weakrefs");
        }
        return tmpThisTarget.Equals(tmpOtherTarget);
    }

    public static bool operator ==(HashableWeakRef<T> a, HashableWeakRef<T> b)
    {
        var tmpTargetA = (T) a.weakRef.Target;
        var tmpTargetB = (T) b.weakRef.Target;
        if (tmpTargetA == null || tmpTargetB == null) {
            throw new ArgumentNullException("Can't test equality for null weakrefs");
        }
        return tmpTargetA == tmpTargetB;
    }

    public static bool operator !=(HashableWeakRef<T> a, HashableWeakRef<T> b)
    {
        return !(a == b);
    }
}