Using SQLDataSource OnSelecting Event

I have two SQLDataSource controls on my page that are dynamically fed an SQL SELECT statement. I was thinking that the best way to do this was to give it the select statement that it needs inside the OnSelecting event. Here is the revelent code.

<asp:SqlDataSource OnSelecting="GetData_Selecting" ID="DS1" ConnectionString="<%$ ConnectionStrings:MyConnectionString %>" ProviderName="<%$ ConnectionStrings:MyConnectionString.ProviderName %>" runat="server" SelectCommand="" />
<asp:SqlDataSource OnSelecting="GetData_Selecting" ID="DS2" ConnectionString="<%$ ConnectionStrings:MyConnectionString %>" ProviderName="<%$ ConnectionStrings:MyConnectionString.ProviderName %>" runat="server" SelectCommand="" />



protected void GetData_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
System.Diagnostics.Debug.WriteLine("Getting Data...");

switch ((sender as SqlDataSource).ID)
{
case "DS1":
if (Checkbox1.Checked)
{
e.Command.CommandText = "SELECT * FROM table1 WHERE " + BuildQuery(getMylarColumns(), SearchBox.Text);
DS1Panel.Visible = true;
}
break;

case "DS2":
if (Checkbox2.Checked)
{
e.Command.CommandText = "SELECT * FROM table2 WHERE " + BuildQuery(getFlatFileColumns(), SearchBox.Text);
DS2Panel.Visible = true;
}
break;
}


The problem with this is that the GetData_Selecting method is never executed and thus when I try to execute the query the page PostBacks and nothing happens. Putting equivalent code in the Page_Load method works fine, however I believe having the code execute on each PostBack is the reason I'm having another problem sorting the data in the DataGrids these controls are bound to.

Why is the function never being executed? Is this the ideal way to handle the inclusion of the query in the SQLDataSource?