/// 
判断窗体是否已存在实例        ///         /// 
        /// 窗体类型        /// 
             public static Form ExistedForm(Type type)        {            return Application.OpenForms.Cast
     
      ().FirstOrDefault(form => form.GetType() == type);        }
     

使用方法：

Form isExist = ExistedForm(typeof(Form1));            if (isExist == null)            {                Form1 Frm = new Form1();                Frm.Show();            }            else            {                isExist.WindowState = FormWindowState.Normal;                isExist.Activate();            }