获取数据列表。

//获取数据列表,Model是类

IList<Model> list = dao.getmx(Model, pageInfo);
//DataTable数据
DataTable dt = ......;

//GroupBy

//单条件,并返回序列中满足指定条件的第一个元素(相当于list按照user_type去重,可以是多条).

list = list.GroupBy(a => a.user_type).Select(it => it.First()).ToList();

//多条件,使用了匿名函数.

var quary = list.Where(a => a.p_num == "1" || a.c_num == "1")

            .GroupBy(a => new { student_id = a.student_id, user_type = a.user_type })

            .Select(group => new

            {

                student_id = group.Key.student_id,

                user_type = group.Key.user_type,

                Count = group.Count()

            });

//group by

//单条件

var quary = from a in list.Where(a => "1,8".Contains(a.user_type))

            group a by new { a.school } into m

            select new

            {

                school = m.Key.school

            };

//多条件

var quary = from a in list.Where(a => "1,8".Contains(a.user_type))

            group a by new { a.provice, a.school } into m

            select new

            {

                provice = m.Key.provice,

                school = m.Key.school

            };

//quary取值方法

foreach (var item in quary.ToList())

{

    //取值item

}

//将provice=吉林的筛选出来。

list = list.Where(x => x.provice == "吉林").ToList();

//将包含1,5条件的数据筛选出来，相当于sql里的in用法：select * from 表 where user_type in (1,5)

list= list.Where(a => "1,5".Contains(a.user_type)).ToList();

//此处等同于上面

list= list.Where(a => a.user_type == "1" || a.user_type == "5").ToList();

//另一种形式，from开头

IList<Model> query = (from item in list

                      where ("," + projectmodel.ids + ",").Contains("," + item.id + ",")

                      select item).ToList<Model>();

//方式一

//分成key-value的数组

string[] id = list.Select(a => a.id.ToString()).ToArray();

//dt是datatable类型的，执行LINQ语句,这里的.AsEnumerable()是延迟发生,不会立即执行，实际上什么都没有发生

string[] id = dt.AsEnumerable().Select(a => a.Field<int>("id").ToString()).ToArray();

//将数组变成1,2,3,4,5的字符串

string ids = string.Join(",", id);

//方式二

//效果等同于foreach循环

foreach (var i in list)

{

    ids += i.id + ",";

}

ids = ids.TrimEnd(',');

上述代码使用LINQ 针对数据集中的数据进行筛选和整理，同样能够以一种面向对象的思想进行数据集中数据的筛选。
在使用LINQ 进行数据集操作时，LINQ 不能直接从数据集对象中查询，因为数据集对象不支持LINQ 查询，所以需要使用AsEnumerable 方法返回一个泛型的对象以支持LINQ的查询操作。

.AsEnumerable()与相对应的.AsQueryable()的区别：

1) AsEnumerable()是 LINQ TO OBJECT，AsQueryable()是 LINQ TO SQL

2) AsEnumerable将一个序列向上转换为一个IEnumerable, 强制将Enumerable类下面的查询操作符绑定到后续的子查询当中。AsQueryable将一个序列向下转换为一个IQueryable, 它生成了一个本地查询的IQueryable包装。

3) AsEnumerable()延迟执行，不会立即执行。当你调用.AsEnumerable()的时候，实际上什么都没有发生。当真正使用对象的时候（例如调用：First, Single, ToList....的时候）才执行。

4) .ToList()立即执行

string[] id = list.Where(a => !string.IsNullOrEmpty(a.user_type)).Select(a => a.id).ToArray();

//ids="1,2,3,4,5,6,7";

string ids = string.Join(",", id);

//当on为单条件时;   dt,dt1,dt2都为DataTable类型

//左联比内联，需要多写into，需要多写from gc1 in corr.DefaultIfEmpty()，需要多写corr=gc1。

var results = from student in dt.AsEnumerable()

              join user in dt1.AsEnumerable() on student.Field<int>("student_id") equals user.Field<int>("id")//内联

              join corr in dt2.AsEnumerable() on student.Field<int>("id") equals corr.Field<int?>("studentproject_id") into corr//左联

              from gc1 in corr.DefaultIfEmpty()

              select new

              {

                  student,

                  user,

                  corr = gc1

              };

//当on为多条件时,借助于匿名类型：其实和解决按多条件分组的思路是一样的。

var results = from student in dt.AsEnumerable()

              join zrs in result_zrs on new { districtcounty = student.Field<string>("districtcounty"), school = student.Field<string>("school") }

              equals new { districtcounty = zrs.districtcounty, school = zrs.school } into zrs

              from gc1 in zrs.DefaultIfEmpty()

              select new

              {

                  student,

                  corr = gc1

              };

//取值方式

foreach (var i in results.ToList())

{

    name = i.user.Field<string>("name");

}

//1个排序

list.OrderBy(a => a.student_id);

//2个排序

list.OrderBy(a => a.student_id).ThenBy(a => a.name);

//多个排序

list.OrderBy(a => a.student_id).ThenBy(a => a.name).ThenBy(a => a.sex);

//方式一
Random rd = new Random();

List<string> liststr = new List<string>();

liststr.Add("aaa");

liststr.Add("bbb");

liststr.Add("ccc");

liststr.Add("111");

liststr.Add("222");

liststr.Add("333");

//随机一个

var s = liststr.OrderBy(_ => Guid.NewGuid()).First();

//随机两个

var ss = liststr.OrderBy(_ => Guid.NewGuid()).Take(2);

//乱序

var sss = liststr.OrderBy(o => rd.Next(0, liststr.Count())).ToList();

//方式二
Random rd = new Random();

list.OrderBy(_=>rd.Next(1,99)).First();

//随机排序通用方法
//就是从原List中每次随机取一项，添加到新的List中，并在原List中删除。这样重复，直到原List为空为止。
//不过要注意，如果要保护原List不受变化，就必须先Copy一份List，再在Copy上进行操作
public static List<T> GetRandomList<T>(List<T> inputList)

{

    //Copy to a array

    T[] copyArray = new T[inputList.Count];

    inputList.CopyTo(copyArray);

    //Add range

    List<T> copyList = new List<T>();

    copyList.AddRange(copyArray);

    //Set outputList and random

    List<T> outputList = new List<T>();

    Random rd = new Random(DateTime.Now.Millisecond);

    while (copyList.Count > 0)

    {

        //Select an index and item

        int rdIndex = rd.Next(0, copyList.Count - 1);

        T remove = copyList[rdIndex];

        //remove it from copyList and add it to output

        copyList.Remove(remove);

        outputList.Add(remove);

    }

    return outputList;

}

//Skip是起始数据,表示从第n+1条数据开始.（此处pageIndex应从0开始）

//pageIndex：页数,pageSize：一页多少条

list = list.Skip(pageIndex * pageSize).Take(pageSize).ToList();
//取前1-10条
list = list.Skip(0).Take(10).ToList();
//取第11-20条
list = list.Skip(10).Take(10).ToList();
//只取10条数据
list = list.Take(10).ToList();

//字符串数组

string[] idlist =  new string[ ]{"aaa","bbb","aaa"};

//去除重复aaa

idlist = idlist.Distinct().ToArray();