Visual Studio .NET - why is GridView3 SelectedDataKey Value null when

I change the select command of a data source?
I have a data grid which has a data source assigned to it. On the page load event I change the data source SelectCommand to use a different table with the same fields in it. 
ie 
 Public Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        dsFileRegMain.SelectCommand = "my SELECT command " 
End Sub
The datagrid displays correctly. but when I use the OnSelectedIndexChange event I get an error on  the selecteddatakey value?  
Session("SelectedFileRegRecordNo") = GridView3.SelectedDataKey.Value.ToString()
The SelectedIndex works ok but the selected date key doesn't.  I'm not sure why?  the Data Key is set ok. and the datagrid work ok if I don't change it in the page load event. 
I've had this problem for a while now but I'm clueless at moment.
Help please.
Paddy

Hi paddy.
I had the same problem with ASP.NET 2.0. Frankly I don't know why ASP is not updating the Grid object when he enter the EVENT OnSelectedDataKey. Probably a bug. I took the same code of an other project and fall on that bug with no reason.
What I did to "bypass" the bug is calling the DataBind() method of the Grid just before using the SelectedDataKey property. I though it would update itself and it did. 
(it's just too bad to use some machine time to bypass the bug.)
Hope it help :)
Dan.
-here's my code-
    protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {
        ((GridView)sender).DataBind();
        if (((GridView)sender).SelectedDataKey != null)
        {
            String primaryKey;
            primaryKey = ((GridView)sender).SelectedDataKey.Value.ToString();
            showInformations(primaryKey);
        }
    }

It's me again. I just wanted to tell you that all my Select, Update and Delete button work in the grid after I updated FrameWork 2.0 beta2 to the release version.
good luck !
Apo.

sorry I didn't get back for a while. on leave.
This is very useful.
Thanks
paddy