窗体单例问题

    public class frmCreate<T> where T : Form, new()
    {
        private static T instance;
        public static T createFrom(Form mdi)
        {
            if (instance == null || instance.IsDisposed)
            {
                instance = new T();
                instance.StartPosition = FormStartPosition.CenterScreen;
                instance.MdiParent = mdi;
                instance.Show();
            }
            instance.WindowState = FormWindowState.Normal;
            return instance;
        }
    }

    public class frmCreateByName
    {
        public static Form createForm(Form mdi, string frmName)
        {
            string className = frmName.Substring(frmName.IndexOf(".") + 1);
            //保证单例问题
            foreach (Form frm in Application.OpenForms)
            {
                if (frm.Name.Equals(className))
                    return frm;
            }
            Assembly assembly = Assembly.GetExecutingAssembly();
            Form instance = assembly.CreateInstance("珠宝加工管理系统." + frmName) as Form;
            instance.StartPosition = FormStartPosition.CenterScreen;
            instance.MdiParent = mdi;
            instance.WindowState = FormWindowState.Normal;
            instance.Show();
            return instance;
        }
    }