How to use SPSiteDataQuery in sharepoint

Below is the example how we can use spsitedataquery in sharepoint

using (SPSite site = new SPSite(ServerText.Text))
{
    using (SPWeb web = site.OpenWeb())
    {
        // Fetch using SPSiteDataQuery
        SPSiteDataQuery query = new SPSiteDataQuery();
        query.Lists = "<Lists ServerTemplate=\"105\" />";
        query.ViewFields = "<FieldRef Name=\"Title\" />" +  /* Title is LastName column */
                "<FieldRef Name=\"FirstName\" Nullable=\"TRUE\" Type=\"Text\"/>";
        query.Webs = "<Webs Scope=\"SiteCollection\" />";

        DataTable dataTable = web.GetSiteData(query);
        dataGridView3.DataSource = dataTable;
    }
}