/// <summary>
/// 打开MDI窗体,同一个窗体只能运行一个实例
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="parent"></param>
public static void OpenForm<T>(Form parent) where T : Form,new()
{
    //已打开则激活
    foreach (Form f in parent.MdiChildren)
    {
        if (f is T)
        {
            f.Activate();
            return;
        }
    }

    //未打开则创建
    T frm = new T();
    frm.MdiParent = parent;
    frm.Show();
}