Textbox changed event is not firing in Composite control ....

public RadGrid Grid = new RadGrid();
 private RadTextBox txtbox = new RadTextBox();

 protected override void CreateChildControls()
        {
txtbox.TextChanged += new EventHandler(txtbox_TextChanged);

 Controls.Add(txtbox);
 Controls.Add(Grid);
 txtbox.AutoPostBack = true;
 txtbox.Attributes.Add("onkeyup","SendTextBox():"); 

}

 protected override void OnPreRender(EventArgs e)
        {
            ClientScriptManager cs = Page.ClientScript;
          
            cs.RegisterStartupScript(this.GetType(), "SendTextBox", "setTimeout('__doPostBack(\'" + txtbox.ID + "\',\'\')', 0);", true);
            base.OnPreRender(e);
        }

  protected void txtbox_TextChanged(object sender, EventArgs e)
        {
          // code for searching in grid 
            Grid.Rebind();
        }