LINQ on Datatable Find where all rows are empty ans remove asp.net

Just move the NOT (!) out one level. You want the items where "all rows are null" is not true, rather than where "all of the rows are not null" is true.

DataTable filteredRows = dt.Rows.Cast<DataRow>()
    .Where(row => !row.ItemArray.All(field => field is System.DBNull))
    .CopyToDataTable();