尝试以下操作：

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;

namespace ConsoleApplication2
{
    class Program
    { 
        static void Main(string[] args)
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("Date", typeof(DateTime));
            dt.Columns.Add("TeamNo", typeof(string));
            dt.Columns.Add("Size", typeof(string));
            dt.Columns.Add("Qty", typeof(int));

            dt.Rows.Add(new object[] { DateTime.Parse("8/4/2023"), "Team 01", "S", 10 });
            dt.Rows.Add(new object[] { DateTime.Parse("8/4/2023"), "Team 02", "M", 20 });
            dt.Rows.Add(new object[] { DateTime.Parse("8/4/2023"), "Team 03", "L", 30 });
            dt.Rows.Add(new object[] { DateTime.Parse("8/5/2023"), "Team 01", "M", 40 });
            dt.Rows.Add(new object[] { DateTime.Parse("8/5/2023"), "Team 02", "M", 50 });

            DateTime[] dates = dt.AsEnumerable().Select(x => x.Field<DateTime>("Date")).Distinct().OrderBy(x => x).ToArray();

            DataTable pivot = new DataTable();
            pivot.Columns.Add("TeamNo", typeof(string));
            pivot.Columns.Add("Size", typeof(string));
            foreach(DateTime date in dates)
            {
                pivot.Columns.Add(date.ToString("M/d/yyyy"));
            }

            var groups = dt.AsEnumerable().GroupBy(x => new { team = x.Field<string>("TeamNo"), size = x.Field<string>("Size")});

            foreach(var group in groups)
            {
                DataRow newRow = pivot.Rows.Add();
                newRow["TeamNo"] = group.Key.team;
                newRow["Size"] = group.Key.size;
                var rowDate = group.GroupBy(x => x.Field<DateTime>("Date"));
                foreach(var date in rowDate)
                {
                    newRow[date.Key.ToString("M/d/yyyy")] = date.Sum(x => x.Field<int>("Qty"));
                }
            }
        }

    }

 
}

字符串