C#

public static DataTable ToPivotTable<T, TPivot>(
    this IEnumerable<T> source,
    IReadOnlyList<Expression<Func<T, object>>> groupColumns,
    Func<T, string> pivotColumn, Func<T, TPivot> pivotValue)
{
    var result = new DataTable();
    var groupColumnMembers = groupColumns.Select(c => ((MemberExpression)c.Body).Member).Cast<PropertyInfo>().ToList();
    foreach (var groupColumn in groupColumnMembers)
    {
        result.Columns.Add(new DataColumn(groupColumn.Name, groupColumn.PropertyType));
    }
    
    var p = Expression.Parameter(typeof(T), "p");
    var tupleType = Type.GetType($"System.Tuple`{groupColumns.Count}");
    var groupColumnTypes = groupColumnMembers.Select(x => x.PropertyType).ToArray();
    var tupleConstructor = tupleType.MakeGenericType(groupColumnTypes).GetConstructor(groupColumnTypes);
        
    var args = groupColumnMembers.Select(c => Expression.Property(p, c));
    var body = Expression.New(tupleConstructor, args);
    var groupByExpression = Expression.Lambda<Func<T, System.Runtime.CompilerServices.ITuple>>(body, p);
    
    foreach (var group in source.GroupBy(groupByExpression.Compile()))
    {
        foreach (T element in group)
        {
            string pivotColumnName = pivotColumn(element);
            if (!result.Columns.Contains(pivotColumnName))
            {
                result.Columns.Add(new DataColumn(pivotColumnName, typeof(TPivot)));
            }
        }
        
        var row = result.NewRow();
        for (int index = 0; index < group.Key.Length; index++)
        {
            row[index] = group.Key[index];
        }
        
        foreach (T element in group)
        {
            string pivotColumnName = pivotColumn(element);
            TPivot pivotColumnValue = pivotValue(element);
            row[pivotColumnName] = pivotColumnValue;
        }
        
        result.Rows.Add(row);
    }
    
    return result;
}

DataTable pivotedData = _data.ToPivotTable(
    new List<Expression<Func<Data, object>>>
    {
        d => d.Section, 
        d => d.Lineitem, 
        d => d.BrokerCode, 
        d => d.BrokerName 
    }, 
    d => d.Period, 
    d => d.PeriodValue);

Section         | Lineitem           | BrokerCode | BrokerName             | 2012 FYA | 2013 FYA | 1Q 2014A | 2Q 2014A
----------------------------------------------------------------------------------------------------------------------
Consensus Model | Net Revenue        | ZB         | B Securities           |          | null     | null     | null 
Consensus Model | Net Revenue        | ZB         | B. Riley Securities    | null     |          | 204.45   | 205.00 
Consensus Model | Net Revenue        | TU         | Cantor Fitzgerald & Co | 101.33   |          | 204.45   | 201.00 
Consensus Model | Cost of Goods Sold | ZB         | B. Riley Securities    |          |          | 204.45   | 201.00 
Consensus Model | Cost of Goods Sold | TU         | Cantor Fitzgerald & Co | 101.33   | 222.30   | 784.45   | 555.00 

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,
        });
    }
}