Поле недоступно из-за уровня доступа

        private void vkButton_Click(object sender, EventArgs e)
        {
            VkAuthorizationForm vkAuthorizationForm = new VkAuthorizationForm();
            vkAuthorizationForm.mainForm = this;
            this.Hide();
            vkAuthorizationForm.Show();
        }

    public partial class VkAuthorizationForm : Form
    {
        private Form mainForm { get; set; }
 
        public VkAuthorizationForm()
        {
            InitializeComponent();
        }
    }

    public partial class VkAuthorizationForm : Form
    {
        private Form mainForm;
 
        public MainForm { get; set; }
 
        public VkAuthorizationForm()
        {
            InitializeComponent();
        }
    }

private InfoFriendsJson infoFriends;
 
public InfoFriendsJson InfoFriends { get; set; }

    public partial class VkAuthorizationForm : Form
    {
        public Form mainForm { get; set; }
 
        public VkAuthorizationForm()
        {
            InitializeComponent();
        }
    }

    public partial class VkAuthorizationForm : Form
    {
        private Form _mainForm;
 
        public VkAuthorizationForm(Form mainForm)
        {
            InitializeComponent();
            _mainForm = mainForm ?? throw new ArgumentNullException(nameof(mainForm));
        }
    }