Question

我有一些对象：

class Foo {
    public Guid id;
    public string description;
}

var list = new List<Foo>();
list.Add(new Foo() { id = Guid.Empty, description = "empty" });
list.Add(new Foo() { id = Guid.Empty, description = "empty" });
list.Add(new Foo() { id = Guid.NewGuid(), description = "notempty" });
list.Add(new Foo() { id = Guid.NewGuid(), description = "notempty2" });

我希望以id字段唯一的方式处理此列表，并丢弃非唯一对象（基于id）。

我能想到的最好的是：

list = list.GroupBy(i => i.id).Select(g=>g.First()).ToList();

是否有更好/更好/更快的方法来实现相同的结果。

Answer 1

一个非常优雅和意图揭示的选项是在IEnumerable

上定义一个新的扩展方法

所以你有：

list = list.Distinct(foo => foo.id).ToList();

而且......

    public static IEnumerable<T> Distinct<T,TKey>(this IEnumerable<T> list, Func<T,TKey> lookup) where TKey : struct {
        return list.Distinct(new StructEqualityComparer<T, TKey>(lookup));
    }


    class StructEqualityComparer<T,TKey> : IEqualityComparer<T> where TKey : struct {

        Func<T, TKey> lookup;

        public StructEqualityComparer(Func<T, TKey> lookup) {
            this.lookup = lookup;
        }

        public bool Equals(T x, T y) {
            return lookup(x).Equals(lookup(y));
        }

        public int GetHashCode(T obj) {
            return lookup(obj).GetHashCode();
        }
    }

可以构建类似的辅助类来比较对象。（它需要做更好的空值处理）

Answer 2

在我的非正式测试中，使用Distinct()方法比使用GroupBy（）快4倍。对于100万个Foo，我的测试在大约0.89秒内有Distinct（）来从一个非独特的数组中创建一个独特的数组，其中GroupBy（）大约需要3.4秒。

My Distinct（）调用看起来像，

var unique = list.Distinct(FooComparer.Instance).ToArray();

和FooComparer看起来像，

class FooComparer : IEqualityComparer<Foo> {
    public static readonly FooComparer Instance = new FooComparer();

    public bool Equals(Foo x, Foo y) {
        return x.id.Equals(y.id);
    }

    public int GetHashCode(Foo obj) {
        return obj.id.GetHashCode();
    }
}

和我的GroupBy()版似的，

var unique = (from l in list group l by l.id into g select g.First()).ToArray();

Answer 3

创建一个IEqualityComparer<Foo>，如果id字段相同则返回true，并将其传递给Distinct（）运算符。

Answer 4

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var list = new List<Foo>();
            list.Add(new Foo() { id = Guid.Empty, description = "empty" });
            list.Add(new Foo() { id = Guid.Empty, description = "empty" });
            list.Add(new Foo() { id = Guid.NewGuid(), description = "notempty" });
            list.Add(new Foo() { id = Guid.NewGuid(), description = "notempty2" });

            var unique = from l in list
                         group l by new { l.id, l.description } into g
                         select g.Key;
            foreach (var f in unique)
                Console.WriteLine("ID={0} Description={1}", f.id,f.description);
            Console.ReadKey(); 
        }
    }

    class Foo
    {
        public Guid id;
        public string description;
    }
}

Answer 5

覆盖等于（对象obj）和 GetHashCode（）方法：

imprimir(regras); // should print all values inside "regras" but only print the last value added * "number of productions found so far".
vetor[numLinha] = regras; // should assign a list into its position in the pre-defined vector of lists at position "numLinha".
i = 0; // resets string position counter to 0 so we can reuse it for next production.
numLinha++; // increment line/rule counter.
fseek(entrada, 2, SEEK_CUR);
regras = cria_lista(); // Next lista

然后只需拨打 Distinct（）：

class Foo
{
    public readonly Guid id;
    public string description;

    public override bool Equals(object obj)
    {
        return ((Foo)obj).id == id;
    }
    public override int GetHashCode()
    {
        return id.GetHashCode();
    }
}

基于LINQ中任意键的不同对象列表

5 个答案: