DataTable dtNew = new DataTable();

            dtNew.Columns.Add("KEY");

            dtNew.Columns.Add("ITEM");

            dtNew.Columns.Add("DateWeek");

            dtNew.Columns.Add("qty");



            foreach (DataRow item in dt.Rows)

            {

                foreach (DataColumn dc in dt.Columns)

                {

                    

                    if (dc.ColumnName != "KEY" && dc.ColumnName != "ITEM")

                    {

                        DataRow dr = dtNew.NewRow();

                        dr["KEY"] = item["KEY"];

                        dr["ITEM"] = item["ITEM"];

                        dr["DateWeek"] = dc.ColumnName;

                        dr["qty"] = item[dc.ColumnName];

                        dtNew.Rows.Add(dr);

                    }

                }

            }

还有比这种方法更简单的吗

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;



namespace T392551790

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

        }



        private void Form1_Load(object sender, EventArgs e)

        {

            DataTable dt1 = new DataTable();

            dt1.Columns.Add("key");

            dt1.Columns.Add("item");

            dt1.Columns.Add("2017/2/1");

            dt1.Columns.Add("2017/3/1");

            dt1.Columns.Add("2017/4/1");

            string data = @"123 1q 123 45 34

234 123e 45 55 66

333 eee 444 555 5656";

            foreach (var line in data.Split(new string[] { "\r\n" }, StringSplitOptions.None))

            {

                var row = dt1.NewRow();

                var rowdt = line.Split(' ');

                row[0] = rowdt[0];

                row[1] = rowdt[1];

                row[2] = rowdt[2];

                row[3] = rowdt[3];

                row[4] = rowdt[4];

                dt1.Rows.Add(row);

            }

            dataGridView1.DataSource = dt1;

            dataGridView2.DataSource = ConvTable(dt1, new string[] { "2017/2/1", "2017/3/1", "2017/4/1" }, "data", "qty");

        }



        private DataTable ConvTable(DataTable dt, string[] aggfrom, string aggto, string val)

        {

            DataTable dt1 = new DataTable();

            foreach (DataColumn col in dt.Columns)

            {

                if (!aggfrom.Contains(col.ColumnName))

                    dt1.Columns.Add(col.ColumnName);

            }

            dt1.Columns.Add(aggto);

            dt1.Columns.Add(val);

            var query = dt.Rows.Cast<DataRow>()

                .SelectMany(x => aggfrom.Select(y =>

                {

                    var row = dt1.NewRow();

                    foreach (DataColumn col in dt1.Columns)

                    {

                        if (col.ColumnName == aggto)

                        {

                            row[col.ColumnName] = y;

                        }

                        else if (col.ColumnName == val)

                        {

                            row[col.ColumnName] = x[y];

                        }

                        else

                        {

                            row[col.ColumnName] = x[col.ColumnName];

                        }

                    }

                    return row;

                }));

            foreach (var r in query)

                dt1.Rows.Add(r);

            return dt1;

        }

    }

}