Top-level control cannot be added to a control

Question

How to Load other Form inside panel in a primary form.

i was trying something like

private void frmInitialEntryBooks_Load(object sender, EventArgs e) {     frmSujbect objForm = new frmSujbect();     pnl.Controls.Add(objForm);     objForm.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;     objForm.Dock = DockStyle.Fill;     objForm.Show(); } 

but it throw exception Top-level control cannot be added to a control at line pnl.Controls.Add(objForm);