最好将您的信息存储在单个父类或中央静态类中

public partial class frmEmployeeManager: Form
    {
        List<Employee> lstEmployees = List<Employee>();
        public frmEmployeeManager()
        {
            InitializeComponent();
        }
        public void updatePay(float Pay, int ID)
        {
            //Where ID = ID change the Pay
            //(Omitted the foreach loop here for brevity)
        }
        private void btnDisplayData_Click(object sender, EventArgs e)
        {
             frmUpdatePay dlgUpdatePay = new frmUpdatePay(this);
             dlgUpdatePay.ShowDialog();
        }

     }

public partial class frmUpdatePay : Form
{
    private frmEmployeeManager ParentEmployeeManager;

    public frmUpdatePay(frmEmployeeManager EmployeeManager)
    {
        InitializeComponent();
        ParentEmployeeManager = EmployeeManager;
    }
            AddPersonParent.updatePay(fltPayInput, intID);
}