子窗体不重复打开和再次打开有关问题

子窗体不重复打开和再次打开问题
问题描述：
我在主窗体F_Main中打开子窗体F_Child,为了不重复打开F_Child，我设置了如下代码：

public static F_Child Frm_Child=null;//首先在F_Main中申明一个全局变量，记录子窗体是否打开；


当主窗体打开子窗体时，作如下判读：

if(Frm_Child=null)
{ 
  Frm_Child=new F_Child（）;
  Frm_Child.mdiparent=this;
  Frm_Child.show;
   
}
else
{
  Frm_Child.Activate();
}

这个代码能够实现子窗体不重复打开。问题是子窗体关闭过后，再打开子窗体时就显示不出来了？？？？请高手指点怎么样实现即可不不重复打开，又可以关闭后再打开？？？

[解决办法]

C# code

 public partial class ShowImagesForm : Form    {private static ShowImagesForm sForm;        private ShowImagesForm()        {            InitializeComponent();        }        public static ShowImagesForm GetForm()        {            if (sForm==null)            {                sForm = new ShowImagesForm();            }            return sForm;        } private void ShowImagesForm_FormClosed(object sender, FormClosedEventArgs e)        {            sForm = null;        }
[解决办法]
C# code
        private void ShowChildFrom(Form   chfr, string fromname)        {             bool isfind = false;            foreach (Form fm in Application.OpenForms)            {                if (fm.Name == fromname)                {                    fm.Activate();                    fm.WindowState = FormWindowState.Normal;                    return;                }             }            if (!isfind)            {                Form fm = chfr;                fm.Show();            }          } private void ShowChildFrom(Form  chfr, string fromname)   {    bool isfind = false;   foreach (Form childrenForm in this.MdiChildren)      {     if (childrenForm.Name == fromname)   {      childrenForm.Visible = true;      childrenForm.Activate();      return;      }    }    if (!isfind) {  Form fm = chfr;   fm.Show();  } }