C# - How To Add A Dynamic Number Of UserControls In Page

I have an (almost) empty aspx page and I want to insert a certain number of the same user control in this page.

I tried to add them from the CodeBehind but it seems that the UserControls are completely empty.

In the main page (MainDiv is a div with runat="server"):

protected void Page_Init(object sender, EventArgs e)
{
WebUserControl1 uc = new WebUserControl1();
WebUserControl1 uc1 = new WebUserControl1();
MainDiv.Controls.Add(uc);
MainDiv.Controls.Add(uc1);
}

(it doesn't work if i put this code on Page_Init, Page_Load or Page_PreRender)

UserControl (gw is a Gridview contained in the UserControl):
protected void Page_PreRender(object sender, EventArgs e)
{
if (_data != null)
{
gw.DataSource = _data;
gw.DataBind();
}
}

when I arrive there, gw is null (this.Controls.Count is 0).