Open MDI Parent after standard form

Question

basically I need to show MDI Parent after standard login form authenticate user credential. From the MDI Parent form I have a menu to recall the same login form and lock the application and the cycle loop. Also I need to maintain the same MDI Parent instance every time user re-login.

Any help will be appreciated. TIA.

I tried below but MDIParent was not showing.

 // Login Form
 void loginbutton_Click(object sender, EventArgs e){
      Form _mdiParent = new MDIParent2();

     _mdiParent.WindowState = FormWindowState.Normal;
     _mdiParent.Show();
     _mdiParent.Focus();
 }