C# List集合类常用操作：三、查找

List集合查询数据

List<Employees> employees = new List<Employees>();
employees.Add(new Employees { Id = 1, Name = "Nancy.Davolio", City = "Seattle", BirthDate = Convert.ToDateTime("1948-12-08") });
employees.Add(new Employees { Id = 2, Name = "Andrew.Fuller", City = "Tacoma", BirthDate = Convert.ToDateTime("1952-02-19") });
employees.Add(new Employees { Id = 3, Name = "Janet.Leverling", City = "Kirkland", BirthDate = Convert.ToDateTime("1963-08-30") });
employees.Add(new Employees { Id = 4, Name = "Margaret.Peacock", City = "Redmond", BirthDate = Convert.ToDateTime("1937-09-19") });
employees.Add(new Employees { Id = 5, Name = "Steven.Buchanan", City = "London", BirthDate = Convert.ToDateTime("1955-03-04") });

All<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)确定序列中的所有元素是否都满足条件。

Any<TSource>(IEnumerable<TSource>) 确定序列是否包含任何元素。

//表示集合中的任一个元素满足表达式条件，即返回true。
var res1 = employees.Any(e => e.Id == 3 || e.Id == 8);
Console.WriteLine(res1);  //out:True

var res2 = employees.Any(e => e.Id == 3 && e.Name == "Steven");
Console.WriteLine(res2);  //out:False  数据源Name中没有Steven

Contains(T) 确定某元素是否在 List<T> 中。

Equals(Object) 确定指定的对象是否等于当前对象。(继承自 Object)

Exists(Predicate<T>) 确定 List<T> 是否包含与指定谓词定义的条件匹配的元素。

Find(Predicate<T>) 搜索与指定谓词所定义的条件相匹配的元素，并返回整个 List<T> 中的第一个匹配元素。

List<Employees> employees = new List<Employees>();
employees.Add(new Employees { Id = 1, Name = "Nancy.Davolio", City = "Seattle", BirthDate = Convert.ToDateTime("1948-12-08") });
employees.Add(new Employees { Id = 2, Name = "Andrew.Fuller", City = "Tacoma", BirthDate = Convert.ToDateTime("1952-02-19") });
employees.Add(new Employees { Id = 3, Name = "Janet.Leverling", City = "Kirkland", BirthDate = Convert.ToDateTime("1963-08-30") });
employees.Add(new Employees { Id = 4, Name = "Margaret.Peacock", City = "Redmond", BirthDate = Convert.ToDateTime("1937-09-19") });
employees.Add(new Employees { Id = 5, Name = "Steven.Buchanan", City = "London", BirthDate = Convert.ToDateTime("1955-03-04") });

//如果找到与指定谓词定义的条件匹配的第一个元素，则为该元素；
var emp = employees.Find(e => e.Id == 5);
Console.WriteLine(emp.Id);          //5
Console.WriteLine(emp.Name);        //Steven.Buchanan
Console.WriteLine(emp.City);        //London
Console.WriteLine(emp.BirthDate);   //1955-03-04

FindAll(Predicate<T>) 检索与指定谓词定义的条件匹配的所有元素。

List<Employees> employees = new List<Employees>();
employees.Add(new Employees { Id = 1, Name = "Nancy.Davolio", City = "Seattle", BirthDate = Convert.ToDateTime("1948-12-08") });
employees.Add(new Employees { Id = 2, Name = "Andrew.Fuller", City = "Tacoma", BirthDate = Convert.ToDateTime("1952-02-19") });
employees.Add(new Employees { Id = 3, Name = "Janet.Leverling", City = "Kirkland", BirthDate = Convert.ToDateTime("1963-08-30") });
employees.Add(new Employees { Id = 4, Name = "Margaret.Peacock", City = "Redmond", BirthDate = Convert.ToDateTime("1937-09-19") });
employees.Add(new Employees { Id = 5, Name = "Steven.Buchanan", City = "London", BirthDate = Convert.ToDateTime("1955-03-04") });

//如果找到一个 List<T>，其中所有元素均与指定谓词定义的条件匹配，则为该数组；否则为一个空
var emp = employees.FindAll(e => e.Id >=3);
//遍历访问
foreach (var item in emp)
{
    Console.WriteLine(item.Id);
    Console.WriteLine(item.Name);
    Console.WriteLine(item.City);
    Console.WriteLine(item.BirthDate);
}

FindIndex(Predicate<T>)    搜索与指定谓词所定义的条件相匹配的元素，并返回整个 List<T> 中第一个匹配元素的从零开始的索引。

List<Employees> employees = new List<Employees>();
employees.Add(new Employees { Id = 1, Name = "Nancy.Davolio", City = "Seattle", BirthDate = Convert.ToDateTime("1948-12-08") });
employees.Add(new Employees { Id = 2, Name = "Andrew.Fuller", City = "Tacoma", BirthDate = Convert.ToDateTime("1952-02-19") });
employees.Add(new Employees { Id = 3, Name = "Janet.Leverling", City = "Kirkland", BirthDate = Convert.ToDateTime("1963-08-30") });
employees.Add(new Employees { Id = 4, Name = "Margaret.Peacock", City = "Redmond", BirthDate = Convert.ToDateTime("1937-09-19") });
employees.Add(new Employees { Id = 5, Name = "Steven.Buchanan", City = "London", BirthDate = Convert.ToDateTime("1955-03-04") });

//如果找到与 match 定义的条件相匹配的第一个元素，则为该元素的从零开始的索引；否则为 -1
var emp = employees.FindIndex(e => e.Id ==3);
Console.WriteLine("查到的索引为：" + emp);  //2

FindLast(Predicate<T>)    搜索与指定谓词所定义的条件相匹配的元素，并返回整个 List<T> 中的最后一个匹配元素。
使用方式同FindIndex

IndexOf(T)    搜索指定的对象，并返回整个 List<T> 中第一个匹配项的从零开始的索引

LastIndexOf(T)    搜索指定对象并返回整个 List<T> 中最后一个匹配项的从零开始索引。