To get selected Item text:

ListUser.SelectedItem.Text //this would give you "Jim Smith"

To get selected Item value:

ListUser.SelectedItem.Value //this would give you "Jim Smith" because you have not set ListUser.DataValueField

If you bind ListUser like this:

ListUser.DataSource =query("Select IDuser, username from user order by username", connection);

ListUser.DataTextField = "username";

ListUser.DataValueField = "IDuser";

ListUser.DataBind();

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ListUser.SelectedItem.Text //this would give you "Jim Smith"

ListUser.SelectedItem.Value //this would give you id of "Jim Smith" because now ListUser.DataValueField is set to IDuser

Make sure you bind ListUser inside if(!IsPostBack){} on Page_Load.

Thanks