c#中List<T>find使用示例（转）

最新推荐文章于 2023-10-22 13:58:21 发布

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本文展示了如何利用C#的泛型集合（如List<T>）进行数据筛选、排序、查找等操作，包括寻找偶数整数、特定类型元素、计算整数总和、对订单进行排序及转换订单编号。通过实际代码示例，阐述了如何利用这些功能提升代码的效率和可读性。

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转自：http://blogs.msdn.com/b/kcwalina/archive/2004/06/22/162533.aspx

Generics and Anonymous Methods/Delegates

make up a powerful pair that can be used to create elegant query APIs. Here are some that we just added to the .NET Framework’s Collection libraries. I used List<T> to illustrate the APIs but most of those were also added to System.Array.

API Design

Delegates

namespace System {

public delegate void Action<T>(T obj);

public delegate bool Predicate<T>(T obj);

public delegate U Converter<T,U>(T from);

public delegate int Comparison<T>(T x, T y);

}

List<T>

public class List<T> : … {

public int FindIndex(Predicate<T> match);

public int FindIndex(int index, Predicate<T> match);

public int FindIndex(int index, int count, Predicate<T> match);

public int FindLastIndex(Predicate<T> match);

public int FindLastIndex(int index, Predicate<T> match);

public int FindLastIndex(int index, int count, Predicate<T> match);

public List<T> FindAll(Predicate<T> match);

public T Find(Predicate<T> match);

public T FindLast(Predicate match);

public bool Exists(Predicate<T> match);

public bool TrueForAll(Predicate<T> match);

public int RemoveAll(Predicate<T> match);

public void ForEach(Action<T> action);

public void Sort(Comparison<T> comparison);

public List<U> ConvertAll<U>(Converter<T,U> converter);

}

Finding Even Integers in List<T>

List<int> integers = new List<int>();

For(int i=1; i<=10; i++) integers.Add(i);

List<int> even = integers.FindAll(delegate(int i){

return i%2==0;

});

Finding Complex Type in List<T>

public class Order {

public Order(int number, string item) { … }

public int Number { get { return number; } }

public string Item { get { return item; } }

…

}

List<Order> orders = new List<Order>();

int orderNumber = 10;

Order order = orders.Find(delegate(Order o){

return o.Number==orderNumber;

});

Computing Sum of Integers in List<T>

List<int> integers = new List<int>();

for(int i=1; i<=10; i++) integers.Add(i);

int sum;

integers.ForEach(delegate(int i){ sum+=i; });

Sort Orders in List<T>

List<Order> orders = new List<Order>();

orders.Add(new Order(10,”Milk”));

orders.Add(new Order(5,”Cheese”));

orders.Sort(delegate(Order x, Order y){

return Comparer<int>.Default.Compare(x.Number,y.Number);

});

Convert Orders to Order Numbers

List<Order> orders = new List<Order>();

orders.Add(new Order(10,”Milk”));

orders.Add(new Order(5,”Cheese”));

List<int> numbers = orders.ConvertAll(delegate(Order x){

return o.Number;

});

转自：http://www.dotblogs.com.tw/puma/archive/2009/05/28/asp.net-generic-list-sort-find-findall-exsit.aspx#12190

最近寫案子常常用到List<T>,這個東西還真好用

因為它有下列東西：

List<T>.Sort() → 排序T

List<T>.Find() → 找出一個T

List<T>.FindAll() →找出多個T

List<T>.Exist() →判斷T是否存在

小弟就寫個範例介紹這些東西吧..

GenericList.aspx

view source print ?

`01`	`<%@ Page Language="C#" AutoEventWireup="true" CodeFile="GenericList.aspx.cs" Inherits="GenericList" %>`

02

`03`	`<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">`

04

`05`	`<html` `xmlns="http://www.w3.org/1999/xhtml">`

`06`	`<head` `runat="server">`

`07`	`<title>GenericList</title>`

08 </head>

09 <body>

`10`	`<form` `id="form1"` `runat="server">`

11 <div>

12 原始資料：

`13`	`<asp:GridView` `ID="GridView1"` `runat="server">`

`14`	`</asp:GridView>`

15 </div>

16 </form>

17 </body>

18 </html>

GenericList.aspx.cs

view source print ?

001 using System;

002 using System.Collections.Generic;

003 using System.Web;

004 using System.Web.UI;

005 using System.Web.UI.WebControls;

006

007 public partial class GenericList : System.Web.UI.Page

008 {

009

010 protected void Page_Load(object sender, EventArgs e)

011 {

012 List<Person> lstPerson = new List<Person>();

013 lstPerson.Add(new Person(1, "puma", 10));

014 lstPerson.Add(new Person(2, "F6 Team", 20));

015 lstPerson.Add(new Person(3, "ASP.NET", 30));

016 lstPerson.Add(new Person(4, "Dotblogs", 40));

017

018 //原始資料顯示在GridView上

019 this.GridView1.DataSource = lstPerson;

020 this.GridView1.DataBind();

021

022

023

024 //List<T>.Find()

025 //找出Name='puma'的Person

026 Response.Write("找出Name='puma'的Person→ ");

027 Response.Write(lstPerson.Find(delegate(Person p) { return p.Name == "puma"; }).ToString() + "<p>");

028

029

030

031 //List<T>.FindAll()

032 //找出Age>10的數目

033 Response.Write("找出Age>10的數目→ ");

034 Response.Write(lstPerson.FindAll(delegate(Person p) { return p.Age > 10; }).Count.ToString() + "<p>");

035

036

037

038 //List<T>.Exists()

039 //檢查Name='F6'是否存在

040 Response.Write("檢查Name='F6'是否存在→ ");

041 Response.Write(lstPerson.Exists(delegate(Person p) { return p.Name == "F6"; }).ToString() + "<p>");

042

043

044

045 //List<T>.Sort()

046 //依Name升冪排序

047 Response.Write("<p>依Name升冪排序↑<br/>");

048 lstPerson.Sort(delegate(Person p1, Person p2) { return Comparer<string>.Default.Compare(p1.Name, p2.Name); });

049 foreach (Person p in lstPerson)

050 {

051 Response.Write(p.ToString() + "<br/>");

052 }

053

054

055

056 //List<T>.Sort()

057 //依Name降冪排序

058 Response.Write("<p>依Name降冪排序↓<br/>");

059 lstPerson.Sort(delegate(Person p1, Person p2) { return Comparer<string>.Default.Compare(p2.Name, p1.Name); });

060 foreach (Person p in lstPerson)

061 {

062 Response.Write(p.ToString() + "<br/>");

063 }

064 }

065 }

066

067 public class Person

068 {

069 private int _ID;

070 private string _Name;

071 private int _Age;

072

073 public Person(int ID, string Name, int Age)

074 {

075 _ID = ID;

076 _Name = Name;

077 _Age = Age;

078 }

079

080 public int ID

081 {

082 set { _ID = value; }

083 get { return _ID; }

084 }

085

086 public string Name

087 {

088 set { _Name = value; }

089 get { return _Name; }

090 }

091

092 public int Age

093 {

094 set { _Age = value; }

095 get { return _Age; }

096 }

097

098 public override string ToString()

099 {

100 return string.Format("ID：{0}，Name：{1}，Age：{2}", _ID, _Name, _Age);

101 }

102 }

執行結果：

转自微软官方：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

namespace Find
{
    class Program
    {
        private static string IDtoFind = "bk109";

        private static List<Book> Books = new List<Book>();
        public static void Main(string[] args)
        {
            FillList();

            // Find a book by its ID.
            Book result = Books.Find(
            delegate(Book bk)
            {
                return bk.ID == IDtoFind;
            }
            );
            if (result != null)
            {
                DisplayResult(result, "Find by ID: " + IDtoFind);
            }
            else
            {
                Console.WriteLine("\nNot found: {0}", IDtoFind);
            }

            // Find last book in collection published before 2001.
            result = Books.FindLast(
            delegate(Book bk)
            {
                DateTime year2001 = new DateTime(2001, 01, 01);
                return bk.Publish_date < year2001;
            });
            if (result != null)
            {
                DisplayResult(result, "Last book in collection published before 2001:");
            }
            else
            {
                Console.WriteLine("\nNot found: {0}", IDtoFind);
            }

            // Find all computer books.
            List<Book> results = Books.FindAll(FindComputer);
            if (results != null)
            {
                DisplayResults(results, "All computer:");
            }
            else
            {
                Console.WriteLine("\nNo books found.");
            }

            // Find all books under $10.00.
            results = Books.FindAll(
            delegate(Book bk)
            {
                return bk.Price < 10.00;
            }
            );
            if (results != null)
            {
                DisplayResults(results, "Books under $10:");
            }
            else
            {
                Console.WriteLine("\nNo books found.");
            }

            // Find index values.
            Console.WriteLine();
            int ndx = Books.FindIndex(FindComputer);
            Console.WriteLine("Index of first computer book: {0}", ndx);
            ndx = Books.FindLastIndex(FindComputer);
            Console.WriteLine("Index of last computer book: {0}", ndx);

            int mid = Books.Count / 2;
            ndx = Books.FindIndex(mid, mid, FindComputer);
            Console.WriteLine("Index of first computer book in the second half of the collection: {0}", ndx);

ndx = Books.FindLastIndex(Books.Count - 1, mid, FindComputer);
Console.WriteLine("Index of last computer book in the second half of the collection: {0}", ndx);

}

        // Populates the list with sample data.
        private static void FillList()
        {

// Create XML elements from a source file.
XElement xTree = XElement.Load(@"c:\temp\books.xml");

// Create an enumerable collection of the elements.
IEnumerable<XElement> elements = xTree.Elements();

            // Evaluate each element and set set values in the book object.
            foreach (XElement el in elements)
            {
                Book book = new Book();
                book.ID = el.Attribute("id").Value;
                IEnumerable<XElement> props = el.Elements();
                foreach (XElement p in props)
                {

                    if (p.Name.ToString().ToLower() == "author")
                    {
                        book.Author = p.Value;
                    }
                    else if (p.Name.ToString().ToLower() == "title")
                    {
                        book.Title = p.Value;
                    }
                    else if (p.Name.ToString().ToLower() == "genre")
                    {
                        book.Genre = p.Value;
                    }
                    else if (p.Name.ToString().ToLower() == "price")
                    {
                        book.Price = Convert.ToDouble(p.Value);
                    }
                    else if (p.Name.ToString().ToLower() == "publish_date")
                    {
                        book.Publish_date = Convert.ToDateTime(p.Value);
                    }
                    else if (p.Name.ToString().ToLower() == "description")
                    {
                        book.Description = p.Value;
                    }
                }

Books.Add(book);

}

DisplayResults(Books, "All books:");

}

        // Explicit predicate delegate.
        private static bool FindComputer(Book bk)
        {

            if (bk.Genre == "Computer")
            {
                return true;
            }
            {
                return false;
            }

}

        private static void DisplayResult(Book result, string title)
        {
            Console.WriteLine();
            Console.WriteLine(title);
            Console.WriteLine("\n{0}\t{1}\t{2}\t{3}\t{4}\t{5}", result.ID,
                result.Author, result.Title, result.Genre, result.Price,
                result.Publish_date.ToShortDateString());
            Console.WriteLine();

}

        private static void DisplayResults(List<Book> results, string title)
        {
            Console.WriteLine();
            Console.WriteLine(title);
            foreach (Book b in results)
            {

                Console.Write("\n{0}\t{1}\t{2}\t{3}\t{4}\t{5}", b.ID,
                    b.Author, b.Title, b.Genre, b.Price,
                    b.Publish_date.ToShortDateString());
            }
            Console.WriteLine();

}

    public class Book
    {
        public string ID { get; set; }
        public string Author { get; set; }
        public string Title { get; set; }
        public string Genre { get; set; }
        public double Price { get; set; }
        public DateTime Publish_date { get; set; }
        public string Description { get; set; }
    }
}