Причины ошибки "List" не содержит определение для "ReadPoints" - C#

namespace AdditionalClasses
{
    public class WorkWithXML
    {
        public List<Point> Points { get; set; }
 
        public void WritePoints(string fileName)
        {
            XmlSerializer serializer = new XmlSerializer(typeof(List<Point>));
            using (TextWriter textWriter = new StreamWriter(fileName))
            {
                serializer.Serialize(textWriter, Points);
            }
        }
 
        public void ReadPoints(string fileName)
        {
            XmlSerializer deserializer = new XmlSerializer(typeof(List<Point>));
            using (TextReader textReader = new StreamReader(fileName))
            {
                Points = (List<Point>)deserializer.Deserialize(textReader);
            }
        }
    }
}

private void ItemFileOpen_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog dlg = new Microsoft.Win32.OpenFileDialog();
            dlg.InitialDirectory = System.AppDomain.CurrentDomain.BaseDirectory; 
            dlg.DefaultExt = ".xml";
            dlg.Filter = "Файлы XML (*.xml)|*.xml|Все файлы (*.*)|*.*";
            if (dlg.ShowDialog() == true)
            {
                try
                {
                    lastList.ReadPoints(dlg.FileName);
                }
                catch (Exception)
                {
                    MessageBox.Show("Ошибка");
                }
            }
        }