linq常用扩展方法
定义一个员工信息类
class Employee
{
public long Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
public bool Gender { get; set; }
public int Salary { get; set; }
public override string ToString()
{
return $"id={Id},name={Name},age={Age},gender={Gender},salary={Salary}";
}
}api方法调用：聚合函数使用
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
int maxSalary = list.Max(e => e.Salary);//薪资最大值
Console.WriteLine(maxSalary);
double minSalary = list.Min(e => e.Salary);//薪资最小值
Console.WriteLine(minSalary);
double avgSalary = list.Average(e => e.Salary);//薪资平均值
Console.WriteLine(avgSalary);
double listCount = list.Count();//数量
Console.WriteLine(listCount);
Console.Read();
}执行结果：
linq中所有的扩展方法几乎都是针对IEnumerable接口的，而几乎所有能返回集合的都返回IEnumerable，所以可以把几乎所有方法“链式使用”。例如：
list.Where(e => e.Age > 25).Min(e => e.Salary);//求年龄大于25岁员工的薪资最小值api方法调用：分组
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
//根据年龄分组
IEnumerable<IGrouping<int, Employee>> sources = list.GroupBy(e => e.Age);
foreach (IGrouping<int, Employee> group in sources)
{
Console.WriteLine(group.Key);
foreach (var item in group)
{
Console.WriteLine(item);
}
Console.WriteLine("*********************");
Console.WriteLine();
}
Console.Read();
}执行结果：
GroupBy()方法：参数是分组条件表达式，返回值为IGrouping<TKey, TSource>类型的泛型IEnumerable，也就是每一组以一个IGrouping对象的形式返回。IGrouping是一个继承自IEnumerable的接口，IGrouping中key属性表示这一组的分组数据的值。
例子：根据年龄分组，获取每组人数、最高工资、最低工资、平均工资
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
//根据年龄分组
IEnumerable<IGrouping<int, Employee>> sources = list.GroupBy(e => e.Age);
foreach (IGrouping<int, Employee> group in sources)
{
Console.WriteLine("年龄:" + group.Key);
Console.WriteLine("人数:" + group.Count());
Console.WriteLine("最高工资:" + group.Max(e => e.Salary));
Console.WriteLine("最低工资:" + group.Min(e => e.Salary));
Console.WriteLine("平均工资:" + group.Average(e => e.Salary));
foreach (var item in group)
{
Console.WriteLine(item);
}
Console.WriteLine("*********************");
Console.WriteLine();
}
Console.Read();
}执行结果：
api方法调用：投影
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
//投影
IEnumerable<string> ages = list.Select(e => e.Name);
foreach (var item in ages)
{
Console.WriteLine(item);
}
Console.Read();
}执行结果：
投影指把集合中每一项转换为另外一种类型。它能够选择数据源中的元素，并指定元素的表现形式。
投影(select)和匿名类型(var)结合重新生成新的对象
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
//select和var结合
var newList = list.Select(e => new
{
xingming = e.Name,
nianling = e.Age,
xinzi = e.Salary
});
foreach (var item in newList)
{
Console.WriteLine($"{item.xingming} {item.nianling} {item.xinzi}");
}
Console.Read();
}执行结果：
通过select生成一个新的集合，集合中的对象时匿名类。
api方法调用：集合转换
有一些地方需要数组类型或者List类型的变量，可以用ToArray()方法和ToList()方法分别把IEnumerable<T>转换成数组类型和List<T>类型。
示例：
List<Employee> newList = list.Where(e => e.Age > 30).ToList();
Employee[] newArr = list.Where(e => e.Age > 30).ToArray();综合应用一：求相同年龄的人数，及相同年龄中的最高工资、最低工资。
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
//求相同年龄的人数，及相同年龄中的最高工资、最低工资。
var testList = list.GroupBy(e => e.Age).Select(g => new
{
nianling = g.Key,
renshu = g.Count(),
maxgz = g.Max(e => e.Salary),
mingz = g.Min(e => e.Salary)
});
foreach (var item in testList)
{
Console.WriteLine($"年龄:{item.nianling} 人数:{item.renshu} 最高工资:{item.maxgz} 最低工资:{item.mingz}");
}
Console.Read();
}执行结果：
综合应用二：获取ID>2的数据,然后安装age分组,并且把分组按照age排序,然后取前3条,最后再投影取得年龄、人数、平均工资。
static void Main(string[] args)
{
List<Employee> list = new List<Employee>();
list.Add(new Employee { Id = 1, Name = "zhangsan", Age = 15, Gender = false, Salary = 5000 });
list.Add(new Employee { Id = 2, Name = "lisi", Age = 50, Gender = true, Salary = 8000 });
list.Add(new Employee { Id = 3, Name = "wangwu", Age = 30, Gender = false, Salary = 1000 });
list.Add(new Employee { Id = 4, Name = "zhaoliu", Age = 20, Gender = true, Salary = 3000 });
list.Add(new Employee { Id = 5, Name = "mazi", Age = 25, Gender = false, Salary = 2000 });
list.Add(new Employee { Id = 6, Name = "jack", Age = 36, Gender = false, Salary = 6000 });
list.Add(new Employee { Id = 7, Name = "rose", Age = 20, Gender = false, Salary = 7600 });
//获取ID>2的数据,然后安装age分组,并且把分组按照age排序,然后取前3条,最后再投影取得年龄、人数、平均工资
var testList1 = list.Where(e => e.Id > 2).GroupBy(e => e.Age).OrderBy(e => e.Key).Take(3).Select(g => new
{
nianling = g.Key,
renshu = g.Count(),
pingjungz = g.Average(e => e.Salary)
});
foreach (var item in testList1)
{
Console.WriteLine($"年龄:{item.nianling} 人数:{item.renshu} 平均工资:{item.pingjungz}");
}
Console.Read();
}执行结果：
