How to make a form close mandatory before opening another in window based C# project

if(Application.OpenForms.Count > 0)
{
   bool flag = false;
   foreach(Form frm in Application.OpenForms)
   {
      if(frm.Name == "frmMenu")
      {
          //Ignoring the MDI form or the form with menu
      }
      else
      {
          flag = true;
          break;
      }
   }

   if(flag)
   {
       MessageBox.Show("Kindly close the current Form");
   }
   else
   {
      Form2 frm = new Form2();
      frm.Show();
      //Open the new form
   }
}
else
{
   Form2 frm = new Form2();
      frm.Show();
      //Open the new form
}