nunit UITest xamarin.forms: how to scroll a picker?

I am trying to create UITest for Xamarin.Forms project. I have tryed using some inf available on Xamarin forum but had no luck with it.

Country list for the Picker:

List<country> countryList = new List<country>({id="GB", name="United Kingdom"}, {id="US", name="United States"}.... etc);

Code for Picker:

        var countryPicker  = new Picker();
        countryPicker.SetBinding(Picker.SelectedIndexProperty, "myIndex");
        countryPicker.BindingContext = getCountry;
        countryPicker.BackgroundColor = Color.White;
        countryPicker.AutomationId = "countryPicker";
        countryPicker.SelectedIndexChanged += ((s, e) =>
            {
                    foreach (var element in countryList)
                    {
                        if (element.name.Equals(countryPicker.Items[countryPicker.SelectedIndex]))
                        {
                            getCountry.myCountry = element.id;
                        }
                    }
            });

        //adding all the countries to the list
        foreach (var item in countryList)
        {
            countryPicker.Items.Add(item.name);

        }

nunit code:

app.Query(c=>c.Marked("countryPicker").Invoke("selectValue", "United States"));

Answer: Query for Marked("countryPicker").Invoke("selectValue", "United States") gave 1 results. But nothing is happening - selection is not done! I've tryed to select value to "US" - same, no result.

Am I missing something?