Not thread safe..

WeakCache.cs

  public class WeakCache<T> where T : class {
    private Dictionary<object, WeakReference<T>> Index
      = new Dictionary<object, WeakReference<T>>();

    public bool TryGetValue(object key, out T val) {
      if (Index.TryGetValue(key, out WeakReference<T> weak))
        if (weak.TryGetTarget(out T target)) {
          val = target;
          return true;
        }

      val = null;
      return false;
    }

    public T Get(object key, Func<T> gen) {
      if (TryGetValue(key, out T target))
        return target;

      T val = gen();
      Index[key] = new WeakReference<T>(val);
      return val;
    }

    public void Set(object key, T val) {
      Index[key] = new WeakReference<T>(val);
    }

    public T this[object key] {
      get => TryGetValue(key, out T val) ? val : throw new KeyNotFoundException();
      set => Set(key, value);
    }

    public void Clear() => Index.Clear();

    public void Sweep() {
      Index.Where(x => !x.Value.TryGetTarget(out T d))
        .Select(x => x.Key).ToList().ForEach(x => {
          Index.Remove(x);
      });
    }
  }