C#

var tupleType = Type.GetType($"System.Tuple`{groupColumns.Count}");
 var groupByExpression = Expression.Lambda<Func<T, System.Runtime.CompilerServices.ITuple>>(body, p);
 
 foreach (var group in source.GroupBy(groupByExpression.Compile()))
 {
 }

public static DataTable DataToPivotTable(this IEnumerable<Data> source)
{
    var result = new DataTable();
    result.Columns.Add(new DataColumn("Section", typeof(string)));
    result.Columns.Add(new DataColumn("Lineitem", typeof(string)));
    result.Columns.Add(new DataColumn("BrokerCode", typeof(string)));
    result.Columns.Add(new DataColumn("BrokerName", typeof(string)));
    
    var sourceList = source.ToList();
    var periods = sourceList.Select(d => d.Period).Distinct(StringComparer.OrdinalIgnoreCase).ToList();
    foreach (string period in periods)
    {
        result.Columns.Add(new DataColumn(period, typeof(string)));
    }
    
    var groupedData = source.GroupBy(d => new
    {
        d.Section,
        d.Lineitem,
        d.BrokerCode,
        d.BrokerName
    });
    
    foreach (var group in groupedData)
    {
        var row = result.NewRow();
        row["Section"] = group.Key.Section;
        row["Lineitem"] = group.Key.Lineitem;
        row["BrokerCode"] = group.Key.BrokerCode;
        row["BrokerName"] = group.Key.BrokerName;
        
        foreach (var item in group)
        {
            row[item.Period] = item.PeriodValue;
        }
        
        result.Rows.Add(row);
    }
    
    return result;
}

DataTable pivotedData = _data.DataToPivotTable();

public class Header
{
    public string Section { get; set; }
    public string LineItem { get; set; }
    public string BrokerCode { get; set; }
    public string BrokerName { get; set; }
    public List<PeriodTotal> Periods { get; set; }
}

public class PeriodTotal
{
    public string Period { get; set; }
    public decimal PeriodValue { get; set; }
}

var result = new List<Header>();

using (var reader = command.ExecuteReader())
{
    while (reader.Read())
    {
        string section = reader.GetString(0);
        string lineItem = reader.GetString(1);
        string brokerCode = reader.GetString(2);
        string brokerName = reader.GetString(3);
        
        var periods = new List<PeriodTotal>();
        for (int index = 4; index < reader.FieldCount; index++)
        {
            periods.Add(new PeriodTotal { Period = reader.GetName(index), PeriodValue = reader.GetDecimal(index) });
        }
        
        result.Add(new Header
        {
            Section = section,
            LineItem = lineItem,
            BrokerCode = brokerCode,
            BrokerName = brokerName,
            Periods = periods,
        });
    }
}