Problème de Gridview et sorting

 
//page load
if(!IsPostBack)
{


                ChargerGrid();
 
}
 
private DataTable GetData()
{
 

#region Requête CAML sur la liste des projets
            SPQuery objSPQuery = new SPQuery();
            objSPQuery.ViewFields =
@"
               <FieldRef Name='ID' />
               <FieldRef Name='NomProjet' />
               <FieldRef Name='ChefDeProjet'  />
               <FieldRef Name='DateDebut'  />
               <FieldRef Name='DateFinProjet'  />";
 
            objSPQuery.Query =
@" <OrderBy>
                  <FieldRef Name='DateDebut' Ascending='False' />
                  </OrderBy>";
 
            objSPListItemCollection = objSPList.GetItems(objSPQuery);
#endregion
 
 
#region Affecter la valeur de la requête au grid
            DataTable datatable = objSPListItemCollection.GetDataTable();
#endregion

 
return datatable;
}
 
publicvoid ChargerGrid()
{
try
{
 
 

            GVProjet.DataSource = GetData();
            GVProjet.DataBind();


}
 
catch(Exception E)
{
            LblErreur.Text = E.Message;
}
 
}
privatestring GridViewSortDirection
{
 
get{return ViewState["SortDirection"]asstring ?? "ASC"; }
 
set{ ViewState["SortDirection"] = value; }
 
}
 
 
 
privatestring GridViewSortExpression
{
 
get{return ViewState["SortExpression"]asstring ?? string.Empty; }
 
set{ ViewState["SortExpression"] = value; }
 
}
 
 
 
privatestring GetSortDirection()
{
 
switch(GridViewSortDirection)
{
 
case"ASC":
 
                GridViewSortDirection = "DESC";
 
break;
 
 
 
case"DESC":
 
                GridViewSortDirection = "ASC";
 
break;
 
}
 
 
 
return GridViewSortDirection;
 
}
 
 
 
 
protected DataView SortDataTable(DataTable dataTable, bool isPageIndexChanging)
{
 
if(dataTable != null)
{
 
            DataView dataView = new DataView(dataTable);
 
if(GridViewSortExpression != string.Empty)
{
 
if(isPageIndexChanging)
{
 
                    dataView.Sort = string.Format("{0} {1}", GridViewSortExpression, GridViewSortDirection);
 
}
 
else
{
 
                    dataView.Sort = string.Format("{0} {1}", GridViewSortExpression, GetSortDirection());
 
}
 
}
 
return dataView;
 
}
 
else
{
 
returnnew DataView();
 
}
 
}
 
 
 
protectedvoid GVProjet_Sorting(object sender, GridViewSortEventArgs e)
{
 
        GridViewSortExpression = e.SortExpression;
 
int pageIndex = GVProjet.PageIndex;
 
//DataTable table = (DataTable)Session["datatable"];
 
        GVProjet.DataSource = SortDataTable(GetData()as DataTable, false);
 
        GVProjet.DataBind();
 
        GVProjet.PageIndex = pageIndex;
 
}