using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Demo1
{
  class Program
  {
    static void Main(string[] args)
    {
      DataTable dt = Query();
      List<Usr> usrs = new List<Usr>(dt.Rows.Count);
      //硬编码，效率比较高，但灵活性不够，如果实体改变了，都需要修改代码
      foreach (DataRow dr in dt.Rows)
      {
        Usr usr = new Usr { ID = dr.Field<Int32?>("ID"), Name = dr.Field<String>("Name") };
        usrs.Add(usr);
      }
      usrs.Clear();
    }
    /// <summary>
    /// 查询数据
    /// </summary>
    /// <returns></returns>
    private static DataTable Query()
    {
      DataTable dt = new DataTable();
      dt.Columns.Add("ID", typeof(Int32));
      dt.Columns.Add("Name", typeof(String));
      for (int i = 0; i < 1000000; i++)
      {
        dt.Rows.Add(new Object[] { i, Guid.NewGuid().ToString() });
      }
      return dt;
    }
  }
  class Usr
  {
    public Int32? ID { get; set; }
    public String Name { get; set; }
  }
}

static class EntityConvert
  {
    /// <summary>
    /// DataTable转为List<T>
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="dt"></param>
    /// <returns></returns>
    public static List<T> ToList<T>(this DataTable dt) where T : class, new()
    {
      List<T> colletion = new List<T>();
      PropertyInfo[] pInfos = typeof(T).GetProperties();
      foreach (DataRow dr in dt.Rows)
      {
        T t = new T();
        foreach (PropertyInfo pInfo in pInfos)
        {
          if (!pInfo.CanWrite) continue;
          pInfo.SetValue(t, dr[pInfo.Name]);
        }
        colletion.Add(t);
      }
      return colletion;
    }
  }

Func<DataRow, Usr> func = dr => new Usr { ID = dr.Field<Int32?>("ID"), Name = dr.Field<String>("Name") };

static void Main(string[] args)
    {
      DataTable dt = Query();
      Func<DataRow, Usr> func = dr => new Usr { ID = dr.Field<Int32?>("ID"), Name = dr.Field<String>("Name") };
      List<Usr> usrs = new List<Usr>(dt.Rows.Count);
      Stopwatch sw = Stopwatch.StartNew();
      foreach (DataRow dr in dt.Rows)
      {
        Usr usr = func(dr);
        usrs.Add(usr);
      }
      sw.Stop();
      Console.WriteLine(sw.ElapsedMilliseconds);
      usrs.Clear();
      Console.ReadKey();
    }

public static class EntityConverter
  {
    /// <summary>
    /// DataTable生成实体
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="dataTable"></param>
    /// <returns></returns>
    public static List<T> ToList<T>(this DataTable dataTable) where T : class, new()
    {
      if (dataTable == null || dataTable.Rows.Count <= 0) throw new ArgumentNullException("dataTable", "当前对象为null无法生成表达式树");
      Func<DataRow, T> func = dataTable.Rows[0].ToExpression<T>();
      List<T> collection = new List<T>(dataTable.Rows.Count);
      foreach (DataRow dr in dataTable.Rows)
      {
        collection.Add(func(dr));
      }
      return collection;
    }
    /// <summary>
    /// 生成表达式
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="dataRow"></param>
    /// <returns></returns>
    public static Func<DataRow, T> ToExpression<T>(this DataRow dataRow) where T : class, new()
    {
      if (dataRow == null) throw new ArgumentNullException("dataRow", "当前对象为null 无法转换成实体");
      ParameterExpression paramter = Expression.Parameter(typeof(DataRow), "dr");
      List<MemberBinding> binds = new List<MemberBinding>();
      for (int i = 0; i < dataRow.ItemArray.Length; i++)
      {
        String colName = dataRow.Table.Columns[i].ColumnName;
        PropertyInfo pInfo = typeof(T).GetProperty(colName);
        if (pInfo == null) continue;
        MethodInfo mInfo = typeof(DataRowExtensions).GetMethod("Field", new Type[] { typeof(DataRow), typeof(String) }).MakeGenericMethod(pInfo.PropertyType);
        MethodCallExpression call = Expression.Call(mInfo, paramter, Expression.Constant(colName, typeof(String)));
        MemberAssignment bind = Expression.Bind(pInfo, call);
        binds.Add(bind);
      }
      MemberInitExpression init = Expression.MemberInit(Expression.New(typeof(T)), binds.ToArray());
      return Expression.Lambda<Func<DataRow, T>>(init, paramter).Compile();
    }
  }