Convert a GridView.DataSource to DataTable or Dataview

Please find the solution:-
Page_Load()
{
    gvAssignedGroups.DataSource = dtAssignedGroups;
        gvAssignedGroups.DataBind();
        Session["dsAssignedGroup1"] = dtAssignedGroups;
}   


protected void gsAU1GroupSort(object sender, GridViewSortEventArgs e)
{
    GridView gvAvailableGroups = (GridView)gsAU1Groups.Controls[0].FindControl("gvAvailableGroups");
        GridView gvAssignedGroups = (GridView)gsAU1Groups.Controls[0].FindControl("gvAssignedGroups");


        DataTable dtAssignedGroupSort = (System.Data.DataTable)Session["dsAssignedGroup1"];
        dtAssignedGroupSort.DefaultView.Sort = e.SortExpression;

        if ((string)ViewState["sortField"] == e.SortExpression.ToString())
        {
            if ((string)ViewState["sortDirection"] == "ASC")
            {
                ViewState["sortDirection"] = "DESC";
            }
            else
            {
                ViewState["sortDirection"] = "ASC";
            }
        }
        else
        {
            ViewState["sortField"] = e.SortExpression.ToString();
            ViewState["sortDirection"] = "ASC";
        }

        gvAssignedGroups.DataSource = dtAssignedGroupSort;
        gvAssignedGroups.DataBind();



}

you can not convert datagrid datasource to datatable directly this way.
here is an article,
which explains you step by step

http://www.codeproject.com/KB/aspnet/GridView_To_DataTable.aspx