DemoModel.cs

    public class DemoModel : UIPickerViewModel
    {
        public Dictionary<string, string[]> options = new Dictionary<string, string[]>()
        {
            { "America", new string[] { "Mexico", "USA" } },
            { "Europe", new string[] { "Germany", "France", "Italy"} },
            { "Asia", new string[] { "Korea", "Japan"} },
        };

        public override nint GetComponentCount(UIPickerView pickerView) => 2; // This determines how many columns are rendered
        public override nfloat GetComponentWidth(UIPickerView picker, nint component) => component == 0 ? 120f : 160f;
        public override nfloat GetRowHeight(UIPickerView picker, nint component) => 40f;

        /// <summary>
        /// Determines the number of rows to render in a component
        /// </summary>
        public override nint GetRowsInComponent(UIPickerView pickerView, nint component)
        {
            if (component == 0)
                return options.Keys.Count;
            else
            {
                var driver = pickerView.SelectedRowInComponent(0);
                return options.Values.ElementAt((int)driver).Length;
            }
        }

        /// <summary>
        /// Gets the display value for a row in a component
        /// </summary>
        public override string GetTitle(UIPickerView pickerView, nint row, nint component)
        {
            if (component == 0)
                return options.Keys.ElementAt((int)row);
            else
            {
                var driver = pickerView.SelectedRowInComponent(0);
                return options.Values.ElementAt((int)driver).ElementAt((int)row);
            }
        }

        [Export("pickerView:didSelectRow:inComponent:")]
        public override void Selected(UIPickerView pickerView, nint row, nint component)
        {
            // Update the display for column 2 if the value of column 1 has changed
            if (component == 0)
            {
                pickerView.ReloadComponent(1);
                pickerView.Select(0, 1, false);
            }
        }
    }