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;
        }

------解决方案--------------------
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();
  }
 }