泛型和单例模式解决MDI子窗体只打开一次办法

调用类：

/// <summary>
    /// 窗口基类，使用基类创建实例，采用单例模式，避免重复创建
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class CellFrom<T> where T :Form
    {
        private static T ThisFrom;

        /// <summary>
        /// 获取实例
        /// </summary>
        /// <returns></returns>
        public static T GetInstance()
        {
            if (ThisFrom == null || ThisFrom.IsDisposed)
            {
                //ThisFrom = new T();
                ThisFrom = System.Activator.CreateInstance<T>();
                ThisFrom.FormClosed += new System.Windows.Forms.FormClosedEventHandler(ThisFromClosed);
            }
            return ThisFrom;
        }

        /// <summary>
        /// 关闭时释放资源
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private static void ThisFromClosed(object sender, FormClosedEventArgs e)
        {
            ThisFrom.Dispose();
        }

    }

 调用方法：

private void JoinTheProgramItem_Click(object sender, EventArgs e)
        {
            JoinTheProgramForm joinTheProgramFrom = CellFrom<JoinTheProgramForm>.GetInstance();
            joinTheProgramFrom.MdiParent = this;
            joinTheProgramFrom.WindowState = FormWindowState.Maximized;
            joinTheProgramFrom.Show();
        }