C#学习(十一)——Array和Collection

一、集合

集合重要且常用
孤立的数据是没有意义的,集合可以作为大量数据的处理,可进行数据的搜索、迭代、添加、删除。
C#中,所有集合都必须实现ICollection接口(数组Array除外)

集合 说明
Array 数组,固定长度、固定类型
ArrayList 列表,可变长度、任意类型
List 列表,可变长度、固定类型
Dictionary 字典,键值对结构
Queue 队列,先进先出(FIFO)集合
Stack 栈,后进先出(LIFO)集合
IEnumerable 可迭代集合

C#集合的特点

  • [ 可以储存无限个元素(除了数组) ]
  • [ 任何一个集合都支持搜索、排序、复制、添加 、删除等操作 ]

二、数组

特点
1.固定长度
2.有明确顺序

使用数组是十分安全的,不会返回任何不存在的数据
例如:

class Program
{
    static void Main(string[] args)
    {
        string[] daysOfWeek =
        {
            "Monday",
            "Tuesday",
            "Wednesday",
            "Thuresday",
            "Friday",
            "Saturday",
            "Sunday"
        };
        foreach (string day in daysOfWeek)
        {
            Console.WriteLine(day);
        }
        //零索引 0-indexed
        Console.WriteLine(daysOfWeek[0]);//周一
        Console.WriteLine(daysOfWeek[1]);//周二
        //固定长度
        string[] monthsOfYear = new string[12];
        monthsOfYear[0] = "January";
        monthsOfYear[1] = "February";
        monthsOfYear[2] = "March";
        monthsOfYear[3] = "April";
        monthsOfYear[4] = "May";
        monthsOfYear[5] = "June";
        monthsOfYear[6] = "July";
        monthsOfYear[7] = "August";
        monthsOfYear[8] = "September";
        monthsOfYear[9] = "October";
        monthsOfYear[10] = "November";
        monthsOfYear[11] = "December";

        Console.Read();
    }
}

三、列表与数组列表

1.List
在底层实现中,list依然使用数组承载数据,不过在数组装满数据以后,list会立刻创建新的数组来代替旧的数组,并且把所有数据复制装载到新的数组中,因此列表又可以成为动态数组。列表的容量不仅可以动态调整,也可以手动调整,对系统的动态调优取得很大帮助。
数组的访问速度略高于列表,但是列表对于空间的利用优于数组。

//List 列表
List<string> daysOfWeek2 = new List<string>();
daysOfWeek2.Add("Monday");
daysOfWeek2.Add("Tuesday");
daysOfWeek2.Add("Wednesday");
daysOfWeek2.Add("Thuresday");
daysOfWeek2.Add("Friday");
daysOfWeek2.Add("Saturday");
daysOfWeek2.Add("Sunday");
//只需要在前面加上I就可以声明接口的列表
IList<string> daysOfWeek3 = new List<string>();

2.ArrayList

List支持泛型,ArrayList不支持泛型,仅能保存对象

ArrayList装载数据很方便,但是提取数据较为麻烦,需要进行拆箱,影响性能

//ArrayList
var array = new ArrayList();
array.Add(daysOfWeek);
array.Add("123");
array.Add(1);

3.List的基本操作
List的有参数构造器

//List的有参数构造器
var daysOfWeek4 = new List<string>(daysOfWeek);//数组
var daysOfWeek5 = new List<string>(daysOfWeek2);//列表
var daysOfWeek6 = new List<string>(7);
List<string> daysOfWeek7 = new List<string>
{
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thuresday",
    "Friday",
    "Saturday",
    "Sunday"
};

列表的插入数据,可以使用Insert或者InsertRange

//列表插入,Insert,InsertRange
daysOfWeek7.InsertRange(2, daysOfWeek);//将daysOfWeek插入到第二个位置后

如果要将列表7插入到列表6的最前面

daysOfWeek6.InsertRange(0, daysOfWeek7);

但是更推荐

daysOfWeek7.AddRange(daysOfWeek6);

因为使用Insert操作,会自动将原来的列表分成两个列表,在进行插入操作,影响性能。
删除数据

//删除数据,RemoveAt, RemoveRange
daysOfWeek7.RemoveAt(0);
daysOfWeek7.RemoveRange(2, 6);
daysOfWeek7.Remove("Monday"); //只删除遍历到的第一个数据
daysOfWeek7.RemoveAll(i => i == "Monday");//删除遍历到的所有的Monday
daysOfWeek7.RemoveAll(i => i.Contains("day"));//删除所有包含day的数据

四、迭代器Enumerator与循环遍历ForEach

读取列表

//读取列表
var a = daysOfWeek6.Count;//读取数据个数
var b = daysOfWeek6.Capacity;//读取列表容量

索引器,方括号就是索引器,准确查找位置

//索引器,方括号就是索引器,准确查找位置
var c = daysOfWeek6[3];

迭代器,将集合按照一定规律全部访问一遍

var enumerator = daysOfWeek6.GetEnumerator();
var d = enumerator.Current;//迭代器当前所指的元素(此时为空)
enumerator.MoveNext();//此时指向第一个元素,遍历完成返回TRUE
while (enumerator.MoveNext())
{
    Console.WriteLine(enumerator.Current);
    //使用while循环,会在最后输出一个null,是由于Current悬空
}
//为了解决上面问题,所有可以使用foreach
foreach ( var day in daysOfWeek6)
{
    Console.WriteLine(day);
}

Foreach遍历时不允许容量发生变化,所有元素均为只读数据,不允许修改

但是我们可以举个例子
Customers.cs

public class Customer
    {
        public Customer(int id, string name, string address)
        {
            Id = id;
            Name = name;
            Address = address;
        }
        public int Id { get; set; }
        public string Name { get; set; }
        public string Address { get; set; }
    }

Program.cs

List<Customer> customers = new List<Customer>();
    customers.Add(new Customer(1, "小赵", "广州"));
    customers.Add(new Customer(2, "小钱", "北京"));
    customers.Add(new Customer(3, "小王", "上海"));
    customers.Add(new Customer(4, "小孙", "深圳"));

    foreach(var customer in customers)
    {
        customer.Name = "123";
        Console.WriteLine(customer.Name);
    }

此时,使用foreach却可以修改数据,为什么?
原因很简单,这里的customer是引用类型的数据,对于foreach,引用类型的数据本身地址是不改变的,因此这里的数据就可以完成修改。

IEumerableIEnumerator

**由于不带泛型需要涉及到装箱拆箱,因此以下只讨论带泛型的版本 *

创建Bank类,使用迭代器遍历列表
Bank.cs

public class Bank : IEnumerable<Customer>
{
    public List<Customer> Customers { get; set; } = new List<Customer>();

    public Bank() 
    {
        Customers.Add(new Customer(1, "小赵", "广州"));
        Customers.Add(new Customer(2, "小钱", "北京"));
        Customers.Add(new Customer(3, "小王", "上海"));
        Customers.Add(new Customer(4, "小孙", "深圳"));
    }

    public IEnumerator<Customer> GetEnumerator()
    {
        return Customers.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

Program.cs

var bank = new Bank();

foreach (var item in bank)
{
    Console.WriteLine(item.Name);
}

下面,我们尝试自己创建一个迭代器,实现对于列表的遍历
创建MyEnumerator.cs

public class MyEnumerator<T> : IEnumerator<T>
{
    T[] _data;
    int _position = -1;//开始时,要让current悬空

    public MyEnumerator(T[] data)
    {
        _data = data;
    }

    public T Current { get => _data[_position]; }

    object IEnumerator.Current { get => Current; }

    public void Dispose()
    {
    }

    public bool MoveNext()
    {
        _position++;
        return _position < _data.Length;
    }

    public void Reset()
    {
        _position = -1;
    }
}

MyList.cs

public class MyList<T> : IEnumerable<T>
{
    private T[] _data;
    int cuttentIndex;

    public MyList(int length)
    {
        this._data = new T[length];
        cuttentIndex = 0;
    }

    public void Add(T item)
    {
        _data[cuttentIndex] = item;
        cuttentIndex++;

    }

    public IEnumerator<T> GetEnumerator()
    {
        return new MyEnumerator<T>(_data);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

Bank.cs

public class Bank : IEnumerable<Customer>
{
    public MyList<Customer> Customers { get; set; } = new MyList<Customer>(4);

    public Bank() 
    {
        Customers.Add(new Customer(1, "小赵", "广州"));
        Customers.Add(new Customer(2, "小钱", "北京"));
        Customers.Add(new Customer(3, "小王", "上海"));
        Customers.Add(new Customer(4, "小孙", "深圳"));
    }

    public IEnumerator<Customer> GetEnumerator()
    {
        return Customers.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

五、迭代与yield return

示例一个场景,有一百万个用户数据,我们需要调取前1000个客户数据

static void Main(string[] args)
{
	var customers = GetCustomers(1000000);
	foreach (var customer in customers)
	{
	    if(customer.Id < 1000)
	    {
	        Console.WriteLine($"客户Id{customer.Id}, 客户姓名:{customer.Name}");
	    }
	    else
	    {
	        break;
	    }
	}
}
static IEnumerable<Customer> GetCustomers(int count)
{
    var customers = new List<Customer>();
    for (int i = 0; i < count; i++)
    {
        customers.Add(new Customer(i, $"Crackpot{i}", "天津"));
    }
    return customers;
}

此时,创建了1000000个数据,但是实际上只需要前1000个数据,对于其余的数据,内存空间完全浪费了,因为根本不会用到
因此,使用yield关键词,就可以实现一个懒加载的效果;yield语句的执行次数与if语句执行次数相同,可以极大地提升内存利用率。yield语句并不返回数据,而是返回数据的迭代。

static void Main(string[] args)
{
	var customers = GetCustomersYield(1000000);;
	foreach (var customer in customers)
	{
	    if(customer.Id < 1000)
	    {
	        Console.WriteLine($"客户Id{customer.Id}, 客户姓名:{customer.Name}");
	    }
	    else
	    {
	        break;
	    }
	}
}
static IEnumerable<Customer> GetCustomersYield(int count)
{
    var customers = new List<Customer>();
    for (int i = 0; i < count; i++)
    {
        yield return new Customer(i, $"Crackpot{i}", "天津");
         

举例2

static void Main(string[] args)
{
	foreach (var i in Createnumerable())
	{
	    Console.WriteLine(i);
	}
}
static IEnumerable<int> Createnumerable()
{
    yield return 3;
    yield return 2;
    yield return 1;
}

输出结果为3 2 1 ,可以看到yield可以实现非必要,不创建的原则

六、Benchmark性能基准测试

首先安装NuGet包
Benchmark
我们使用Benchmark来查看两种方法使用不使用yield的差距
创建BenchmarkTester.cs

[MemoryDiagnoser]
public class BenchmarkTester
{
    [Benchmark]
    public void ProcessCustomer()
    {
        var customers = GetCustomers(1000000);
        foreach (var customer in customers)
        {
            if (customer.Id < 1000)
            {
                Console.WriteLine($"客户id:{customer.Id}, 客户姓名:{customer.Name}");
            }
            else
            {
                break;
            }
        }
    }

    [Benchmark]
    public void ProcessCustomerYield()
    {
        var customers = GetCustomersYield(1000000);
        foreach (var customer in customers)
        {
            if (customer.Id < 1000)
            {
                Console.WriteLine($"客户id:{customer.Id}, 客户姓名:{customer.Name}");
            }
            else
            {
                break;
            }
        }
    }
    static IEnumerable<Customer> GetCustomersYield(int count)
    {
        var customers = new List<Customer>();
        for (int i = 0; i < count; i++)
        {
            yield return new Customer(i, $"Crackpot{i}", "天津");
        }
    }

    static IEnumerable<Customer> GetCustomers(int count)
    {
        var customers = new List<Customer>();
        for (int i = 0; i < count; i++)
        {
            customers.Add(new Customer(i, $"Crackpot{i}", "天津"));
        }
        return customers;
    }
}

然后再Program.cs中调用Benchmark进行测试var sumery = BenchmarkRunner.Run();
需要注意,需要在CMD中执行benchmark测试
找到项目文件,然后dotnet build -c Release
会生成文件于:项目名->bin->Release->net8.0->项目名.dll
dotnet 项目.dll
出现结果
C#学习(十一)——Array和Collection_第1张图片
可以看到,使用yield的运行时间是不使用的大约1/2,而使用的内存分配仅为约1/671,足以看到yield对于系统运行性能的提升

七、数据搜索:字典

使用示例
program.cs

static void Main(string[] args)
{
	var customers = GetCustomerDictionary(1000000);
	customers.GetValueOrDefault(999999);
	var customer = customers[999999];
	Console.WriteLine($"客户id:{customer.Id}, 客户姓名:{customer.Name}");
}
static Dictionary<int, Customer> GetCustomerDictionary(int count)
{
    var customer = new Dictionary<int, Customer>();
    for (int i = 0; i < count; i++)
    {
        customer.Add(i, new Customer(i, $"Crackpot{i}", "广州"));
    }
    return customer;
}

八、哈希表

C#中哈希表与字典几乎没有泛型,最显著区别是哈希表没有泛型而字典有泛型
哈希表的值均为object类型,因此难免会使用装箱或拆箱,因此非常耗时

static void Main(string[] args)
{
	var customerHashTable = GetCustomerHashTable(1000000);
	var customer = (Customer)customerHashTable[99999];
	Console.WriteLine($"客户id:{customer.Id}, 客户姓名:{customer.Name}");
}
static Hashtable GetCustomerHashTable(int count)
{
    var customer = new Hashtable();
    for (int i = 0; i < count; i++)
    {
        customer.Add(i, new Customer(i, $"Crackpot{i}", "广州"));
    }
    return customer;
}

九、集合的交、并、差运算(HashSet)

HashSet并不常用,但是在处理一些特殊问题时,非常便捷。
示例为:查找公交线路系统
Program.cs

static void Main(string[] args)
{
	var database = new BusRouteRepository();
	Console.WriteLine("从哪里来?");
	string startingAt = Console.ReadLine();
	
	Console.WriteLine("到哪里去?");
	string goingTo = Console.ReadLine();
	
	var startingRoutes = database.FindBusTo(startingAt);
	var destination = database.FindBusTo(goingTo);
	
	HashSet<BusRoute> routes = new HashSet<BusRoute>(startingRoutes);
	routes.IntersectWith(destination);
	
	if(routes.Count > 0)
	{
	    foreach(var route in routes)
	    {
	        Console.WriteLine($"乘坐公交车:{route}");
	    }
	}
	else
	{
	    Console.WriteLine("路线找不到");
	}
	
	Console.Read();
}

BusRoute.cs

public class BusRoute
{
    public int Number {  get; }

    public string Origin => PlacesServed[0];

    public string Destination => PlacesServed[^1];

    public string[] PlacesServed {  get; }

    public BusRoute(int number, string[] placesServed)
    {
        this.Number = number;
        this.PlacesServed = placesServed;
    }
    public override string ToString() => $"{Number}: {Origin} -> {Destination}";

    public bool Serves(string destination)
    {
        return Array.Exists(PlacesServed, place => place == destination);
    }
}

BusRouteRepository.cs

public class BusRouteRepository
{
    private readonly BusRoute[] _allRoutes;

    public BusRouteRepository()
    {
        _allRoutes = new BusRoute[]
        {
            new BusRoute(101, new string[] {"火车站","大学城","动物园","体育馆"}),
            new BusRoute(42, new string[] {"火车站","电子城","花园酒店","体育馆"}),
            new BusRoute(232, new string[] {"理工大学","海洋馆","购物中心","游泳馆","体育馆"}),
            new BusRoute(51, new string[] {"美食广场","长途汽车站","游乐园","机场"}),
            new BusRoute(6, new string[] {"南井村","双河营村","长途汽车站","火车站"}),
        };
    }

    public BusRoute[] FindBusTo(string location)
    {
        return Array.FindAll(_allRoutes, route => route.Serves(location));
    }
    public BusRoute[] FindBusesBetween(string location1, string location2)
    {
        return Array.FindAll(_allRoutes, route => route.Serves(location1) && route.Serves(location2));
    }
}

示例结果
C#学习(十一)——Array和Collection_第2张图片

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