public void ISet() { JsonSerializerOptions options = new JsonSerializerOptions(); options.SetupExtensions(); const string json = @"{""Collection"":[12,13]}"; ObjectWithISet obj = JsonSerializer.Deserialize <ObjectWithISet>(json, options); Assert.NotNull(obj); Assert.NotNull(obj.Collection); Assert.IsType <HashSet <int> >(obj.Collection); Assert.Equal(2, obj.Collection.Count); Assert.Equal(12, obj.Collection.ElementAt(0)); Assert.Equal(13, obj.Collection.ElementAt(1)); string actual = JsonSerializer.Serialize(obj, obj.GetType(), options); Assert.Equal(json, actual); }