To display a child form inside a top-level form?该怎么解决

To display a child form inside a top-level form?
我有两个form:frmMain,frmAddProduct,我在frmMain中想显示frmAddProduct,frmAddProduct上有添加产品的控件了，怎么解决啊？
  private void frmMain_Load(object sender, EventArgs e)
  {
  //this.Hide();
  //frmAddProduct frm = new frmAddProduct();
  //frm.ShowDialog();
  Common common = new Common();
  common.ShowChildForm(frmAddProduct.CreateObject(), this, true);
  }
common.cs:
  public void ShowChildForm(System.Windows.Forms.Form childForm, System.Windows.Forms.Form parentForm, bool isModalWin)
  {
  if (childForm != null)
  {
  childForm.MdiParent = parentForm;
  if (isModalWin)
  childForm.ShowDialog(); //error  
  //Form that is not a top-level form cannot be displayed as a modal dialog box. Remove the form from any parent form before calling showDialog.
  else
  childForm.Show();
  }
  }


[解决办法]

C# code

 childForm.TopLevel = false;            childForm.MdiParent = parentForm;
[解决办法]
探讨

按照代码改依然出现同样的错误