private void LeaveFromField(object sender, EventArgs e)
        {

            var item = (TabPage)sender;
            DialogResult result = MessageBox.Show(
            null,
            "Внимание",
            MessageBoxButtons.YesNo,
            MessageBoxIcon.Information);
            if (result == DialogResult.No)
            {
                //
            }

        }

private void SelectTab(object sender, TabControlCancelEventArgs e)
        {
            DialogResult result = MessageBox.Show
            (
                "Выйти?",
                "Внимание",
                MessageBoxButtons.YesNo,
                MessageBoxIcon.Error
            );
            if (result == DialogResult.No)
            {
                e.Cancel = true;
            }
            if (result == DialogResult.Yes)
            {
                Clear();
            }
        }