>I can see SqlDataSource1_Selecting executing when the form starts. However, when I put something in the textbox, this method doesn't get called. I tried putting

>SqlDataSource1_Selecting(sender, e )

in the textbox_TextChanged method, but this results in an error about being unable to cast into System.Web.UI.WebControls.SqlDataSourceSelectingEventArgs.
>
>Assuming that I should be calling the Selecting event from the textbox, what arguements should I be providing? If the assumption is incorrect, how do I call the Selecting event when I exit the textbox?

No, you're not calling Selecting event from the textbox. It is called automatically every time the page is Loaded. If you want to re-load the information when you changed your text, you may try calling

SQLDataSource.Select() from the Textbox_TextChanged method.

You also would need to re-bind the gridview, I think.

E.g. after SQLDataSource.Select()

GridView.DataBind() (if I recall the methods correctly)