how to regard groupby in C# linq as a two dimensional list?

Question

List<History> data = new List<History>(){   
        new History() {Symbol="a", Close = 1.0m, Date = new DateTime(2016, 2, 1) },
        new History() {Symbol="a", Close = 1.2m, Date = new DateTime(2016, 2, 2) },
        new History() {Symbol="a", Close = 1.3m, Date = new DateTime(2016, 2, 3) },
        new History() {Symbol="b", Close = 1.2m, Date = new DateTime(2016, 2, 1) },
        new History() {Symbol="b", Close = 1.2m, Date = new DateTime(2016, 2, 2) },
        new History() {Symbol="b", Close = 1.3m, Date = new DateTime(2016, 2, 3) },
};

var StockGroupList = data
               .GroupBy(o => o.Symbol)
               .OrderBy(o => o.Key)
               .ToList();

What's the simplest way to call for the value of "Close" of "a" at "2016, 2, 2", is it possible to use something like StockGroupList[0][1].close?

Furthermore, how to order the second dimension? E.g, for each "Symbol", ordered by the sequence of date?

Answer 1

Let write code backward. To make StockGroupList[0][1] work, StockGroupList should be type of List<List<History>>.

var StockGroupList = data
    ... // Should be type IEnumerable<List<History>>
    .ToList();

=>

var StockGroupList = data 
    .Select( o => o.ToList()) // Should be ordered by Date
    .ToList();

=>

var StockGroupList = data 
    ... // Should be IEnumerable<IEnumerable<History>>
    .Select( o => o.Orderby(c => c.Date).ToList())
    .ToList();

Because IGrouping<out TKey, out TElement> : IEnumerable<TElement>, final code is

var StockGroupList = data
    .GroupBy(o => o.Symbol)
    .OrderBy(o => o.Key)
    .Select(o => o.OrderBy(x => x.Date).ToList())
    .ToList();

You can replace ToList with ToArray to get the same effect.