Check out example codes for "c# an object on upper level cannot be added to an object". It will help you in understanding the concepts better.

Code Example 1

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

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