Linq 聚合函数

var WReserve = (from m in _db.W_RESERVE select m).ToList().LastOrDefault();

必须ToList() 之后才能 last first 函数，否则报错。这样会不会从数据库 取全部数据 然后再 last()；未测试。

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Text;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Collections.Generic;
using System.Collections;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        #region 用Count()得出查询结果的个数
        Response.Write("<hr>用Count()得出查询结果的个数<br>");
        var langCount = (from s in GetStudents()
                           select s.Language)
                           .Distinct()
                           .Count();
        Response.Write(string.Format("<div class='result'>{0}</div>", langCount));
        #endregion

        #region 用Count()的Lambda表达式满足条件的元素个数
        Response.Write("<hr>用Count()得出查询结果的个数<br>");
        var ageCoun = (from s in GetStudents()
                          select s.Age)
                          .Count(a=>a>22);
        Response.Write(string.Format("<div class='result'>{0}</div>", ageCoun));
        #endregion

        #region 用Sum()将查询结果相加
        Response.Write("<hr>用Sum()将查询结果相加<br>");
        var ageSum = (from s in GetStudents()
                      select s.Age)
                       .Sum();
        //var ageSum = GetStudents().Sum(s=>s.Age);
        Response.Write(string.Format("<div class='result'>{0}</div>", ageSum));
        #endregion

        #region 用Min()得出查询结果的最小值
        Response.Write("<hr>用Min()得出查询结果的最小值<br>");
        var ageMin = (from s in GetStudents()
                      select s.Age)
                       .Min();
        //var ageMin = GetStudents().Min(s => s.Age);
        Response.Write(string.Format("<div class='result'>{0}</div>", ageMin));
        #endregion

        #region 用Max()得出查询结果的最大值
        Response.Write("<hr>用Max()得出查询结果的最大值<br>");
        var ageMax = (from s in GetStudents()
                      select s.Age)
                       .Max();
        //var ageMax = GetStudents().Max(s => s.Age);
        Response.Write(string.Format("<div class='result'>{0}</div>", ageMax));
        #endregion

        #region 用Average()得出查询结果的平均值
        Response.Write("<hr>用Average()得出查询结果的平均值<br>");
        var ageAverage = (from s in GetStudents()
                      select s.Age)
                      .Average();
        //var ageAverage = GetStudents().Average(s => s.Age);
        Response.Write(string.Format("<div class='result'>{0}</div>", ageAverage));
        #endregion
    }

    #region 构造一个学生集合体
    private List<Student> GetStudents()
    {
        List<Student> students = new List<Student> {
             new Student{ Name="YOUNG", Age=25, Language="Chinese"},
             new Student{ Name="JESSIE", Age=21, Language="Scotland"},
             new Student{ Name="KELLY", Age=18, Language="English"},
             new Student{ Name="JUNE", Age=20, Language="English"},
             new Student{ Name="ADRIAN", Age=22, Language="Italy"},
             new Student{ Name="BRUCE", Age=17, Language="Scotland"},
             new Student{ Name="BRANT", Age=30, Language="Germany"},
             new Student{ Name="BEN", Age=25, Language="Chinese"}
        };
        return students;
    }
    #endregion
}

#region 学生类
class Student
{
    public int Age { get; set; }
    public string Name { get; set; }
    public string Language { get; set; }
}
#endregion