var filteredTables = tables.Where(t => t.id == m_TableID);
var filteredRows = filteredTables.SelectMany(t => t.rows.Where(r => r.id == m_RowID));
var filteredColumns = filteredRows.SelectMany(r => new { Row = r, Columns = r.columns.Where(c => c.id == m_ColumnID) });
var content = filteredColumns.SelectMany(c => c.Columns.SelectMany(c2 => new { Header = c.Row.Header, Column = c2.Content } ));