使用LINQ从List <T>中删除元素

Question

假设我有LINQ查询,例如:

var authors = from x in authorsList
              where x.firstname == "Bob"
              select x;

鉴于它authorsList是类型List<Author>,我如何删除查询返回的Author元素？authorsListauthors

或者换句话说,如何删除所有名字等于Bob的名字authorsList？

注意:这是用于问题目的的简化示例.

Answer 1

好吧,首先要排除它们会更容易:

authorsList = authorsList.Where(x => x.FirstName != "Bob").ToList();

但是,这只会改变值,authorsList而不是从前一个集合中删除作者.或者,您可以使用RemoveAll:

authorsList.RemoveAll(x => x.FirstName == "Bob");

如果你真的需要基于另一个集合来做,我会使用HashSet,RemoveAll和Contains:

var setToRemove = new HashSet<Author>(authors);
authorsList.RemoveAll(x => setToRemove.Contains(x));

Answer 2

最好使用List <T> .RemoveAll来完成此任务.

authorsList.RemoveAll((x) => x.firstname == "Bob");

Answer 3

如果你真的需要删除项目,那么Except()呢？
您可以根据新列表删除,也可以通过嵌套Linq来即时删除.

var authorsList = new List<Author>()
{
    new Author{ Firstname = "Bob", Lastname = "Smith" },
    new Author{ Firstname = "Fred", Lastname = "Jones" },
    new Author{ Firstname = "Brian", Lastname = "Brains" },
    new Author{ Firstname = "Billy", Lastname = "TheKid" }
};

var authors = authorsList.Where(a => a.Firstname == "Bob");
authorsList = authorsList.Except(authors).ToList();
authorsList = authorsList.Except(authorsList.Where(a=>a.Firstname=="Billy")).ToList();

Answer 4

您无法使用标准LINQ运算符执行此操作,因为LINQ提供查询,而不是更新支持.

但是您可以生成一个新列表并替换旧列表.

var authorsList = GetAuthorList();

authorsList = authorsList.Where(a => a.FirstName != "Bob").ToList();

或者您可以authors在第二次传递中删除所有项目.

var authorsList = GetAuthorList();

var authors = authorsList.Where(a => a.FirstName == "Bob").ToList();

foreach (var author in authors)
{
    authorList.Remove(author);
}

Answer 5

简单方案:

static void Main()
{
    List<string> myList = new List<string> { "Jason", "Bob", "Frank", "Bob" };
    myList.RemoveAll(x => x == "Bob");

    foreach (string s in myList)
    {
        //
    }
}

Answer 6

我在游荡,如果RemoveAll和Except以及使用HashSet的优点之间有任何区别,那么我已经做了快速的性能检查:)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;

namespace ListRemoveTest
{
    class Program
    {
        private static Random random = new Random( (int)DateTime.Now.Ticks );

        static void Main( string[] args )
        {
            Console.WriteLine( "Be patient, generating data..." );

            List<string> list = new List<string>();
            List<string> toRemove = new List<string>();
            for( int x=0; x < 1000000; x++ )
            {
                string randString = RandomString( random.Next( 100 ) );
                list.Add( randString );
                if( random.Next( 1000 ) == 0 )
                    toRemove.Insert( 0, randString );
            }

            List<string> l1 = new List<string>( list );
            List<string> l2 = new List<string>( list );
            List<string> l3 = new List<string>( list );
            List<string> l4 = new List<string>( list );

            Console.WriteLine( "Be patient, testing..." );

            Stopwatch sw1 = Stopwatch.StartNew();
            l1.RemoveAll( toRemove.Contains );
            sw1.Stop();

            Stopwatch sw2 = Stopwatch.StartNew();
            l2.RemoveAll( new HashSet<string>( toRemove ).Contains );
            sw2.Stop();

            Stopwatch sw3 = Stopwatch.StartNew();
            l3 = l3.Except( toRemove ).ToList();
            sw3.Stop();

            Stopwatch sw4 = Stopwatch.StartNew();
            l4 = l4.Except( new HashSet<string>( toRemove ) ).ToList();
            sw3.Stop();


            Console.WriteLine( "L1.Len = {0}, Time taken: {1}ms", l1.Count, sw1.Elapsed.TotalMilliseconds );
            Console.WriteLine( "L2.Len = {0}, Time taken: {1}ms", l1.Count, sw2.Elapsed.TotalMilliseconds );
            Console.WriteLine( "L3.Len = {0}, Time taken: {1}ms", l1.Count, sw3.Elapsed.TotalMilliseconds );
            Console.WriteLine( "L4.Len = {0}, Time taken: {1}ms", l1.Count, sw3.Elapsed.TotalMilliseconds );

            Console.ReadKey();
        }


        private static string RandomString( int size )
        {
            StringBuilder builder = new StringBuilder();
            char ch;
            for( int i = 0; i < size; i++ )
            {
                ch = Convert.ToChar( Convert.ToInt32( Math.Floor( 26 * random.NextDouble() + 65 ) ) );
                builder.Append( ch );
            }

            return builder.ToString();
        }
    }
}

结果如下:

Be patient, generating data...
Be patient, testing...
L1.Len = 985263, Time taken: 13411.8648ms
L2.Len = 985263, Time taken: 76.4042ms
L3.Len = 985263, Time taken: 340.6933ms
L4.Len = 985263, Time taken: 340.6933ms

我们可以看到,在这种情况下最好的选择是使用RemoveAll(HashSet)

Answer 7

这是一个非常古老的问题,但我发现了一个非常简单的方法:

authorsList = authorsList.Except(authors).ToList();

请注意,由于返回变量authorsList是a List<T>,因此IEnumerable<T>返回的Except()必须转换为a List<T>.

Answer 8

LINQ 起源于函数式编程，强调对象的不变性，因此它不提供内置方法来就地更新原始列表。

关于不变性的注意事项（取自另一个SO答案）：

这是维基百科中不变性的定义。

在面向对象和函数式编程中，不可变对象是指其状态在创建后无法修改的对象。

Answer 9

您可以通过两种方式删除

var output = from x in authorsList
             where x.firstname != "Bob"
             select x;

要么

var authors = from x in authorsList
              where x.firstname == "Bob"
              select x;

var output = from x in authorsList
             where !authors.Contains(x) 
             select x;

我有同样的问题,如果你想根据你的条件得到简单的输出,那么第一个解决方案就更好了.

Answer 10

说authorsToRemove是一个IEnumerable<T>包含您要移除的元素authorsList.

然后,这是完成OP提出的删除任务的另一种非常简单的方法:

authorsList.RemoveAll(authorsToRemove.Contains);

Answer 11

下面是从列表中删除元素的示例。

 List<int> items = new List<int>() { 2, 2, 3, 4, 2, 7, 3,3,3};

 var result = items.Remove(2);//Remove the first ocurence of matched elements and returns boolean value
 var result1 = items.RemoveAll(lst => lst == 3);// Remove all the matched elements and returns count of removed element
 items.RemoveAt(3);//Removes the elements at the specified index

Answer 12

我想你可以做这样的事情

    authorsList = (from a in authorsList
                  where !authors.Contains(a)
                  select a).ToList();

虽然我认为已经给出的解决方案以更易读的方式解决了问题.