[英]Distinct list of objects based on an arbitrary key in LINQ
我有一些對象:
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" });
我想以這樣的方式處理這個列表,即id
字段是唯一的,並拋棄非唯一對象(基於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/更快的方法來實現相同的結果。
一個非常優雅和意圖揭示的選項是在IEnumerable上定義一個新的擴展方法
所以你有了:
list = list.Distinct(foo => foo.id).ToList();
而......
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();
}
}
可以構建類似的輔助類來比較對象。 (它需要做更好的空值處理)
在我的非正式測試中,使用Distinct()
方法比使用GroupBy()快4倍。 對於100萬個Foo,我的測試在大約0.89秒內有Distinct(),以便在一個非獨特的數組中創建一個獨特的數組,其中GroupBy()需要大約3.4秒。
我的Distinct()調用看起來像,
var unique = list.Distinct(FooComparer.Instance).ToArray();
和FooComparer
看起來像,
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()
版本看起來像,
var unique = (from l in list group l by l.id into g select g.First()).ToArray();
覆蓋Equals(object obj)和GetHashCode()方法:
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();
}
}
然后只需調用Distinct() :
list = list.Distinct().ToList();
創建一個IEqualityComparer<Foo>
,如果id字段相同則返回true,並將其傳遞給Distinct()運算符。
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;
}
}
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