2
這些枚舉之一是否比另一個快？或者差不多？ （在C＃示例）哪個.NET集合更快：枚舉foreach Dictionary <>。Values或List <>？
情況1：
Dictionary<string, object> valuesDict;
// valuesDict loaded with thousands of objects
foreach (object value in valuesDict.Values) { /* process */ }
情況2：
List<object> valuesList;
// valuesList loaded with thousands of objects
foreach (object value in valuesList) { /* process */ }
UPDATE：
背景：
字典將是鍵控搜索有益別處（而不是迭代一個列表），但如果迭代，益處會減少通過字典比通過列表慢得多。
更新： 聽取了很多人的意見，我做了我自己的測試。
首先，這些是結果。以下是該計劃。
迭代整個集合 快譯通：78 Keyd：131 名單：76
鍵控搜索集合 快譯通：178 Keyd：194 列表：142800
using System;
using System.Linq;
namespace IterateCollections
{
public class Data
{
public string Id;
public string Text;
}
public class KeyedData : System.Collections.ObjectModel.KeyedCollection<string, Data>
{
protected override string GetKeyForItem(Data item)
{
return item.Id;
}
}
class Program
{
static void Main(string[] args)
{
var dict = new System.Collections.Generic.Dictionary<string, Data>();
var list = new System.Collections.Generic.List<Data>();
var keyd = new KeyedData();
for (int i = 0; i < 10000; i++)
{
string s = i.ToString();
var d = new Data { Id = s, Text = s };
dict.Add(d.Id, d);
list.Add(d);
keyd.Add(d);
}
var sw = new System.Diagnostics.Stopwatch();
sw.Start();
for (int r = 0; r < 1000; r++)
{
foreach (Data d in dict.Values)
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
var dictTime = sw.ElapsedMilliseconds;
sw.Reset();
sw.Start();
for (int r = 0; r < 1000; r++)
{
foreach (Data d in keyd)
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
var keydTime = sw.ElapsedMilliseconds;
sw.Reset();
sw.Start();
for (int r = 0; r < 1000; r++)
{
foreach (Data d in list)
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
var listTime = sw.ElapsedMilliseconds;
Console.WriteLine("Iterate whole collection");
Console.WriteLine("Dict: " + dictTime);
Console.WriteLine("Keyd: " + keydTime);
Console.WriteLine("List: " + listTime);
sw.Reset();
sw.Start();
for (int r = 0; r < 1000; r++)
{
for (int i = 0; i < 10000; i += 10)
{
string s = i.ToString();
Data d = dict[s];
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
dictTime = sw.ElapsedMilliseconds;
sw.Reset();
sw.Start();
for (int r = 0; r < 1000; r++)
{
for (int i = 0; i < 10000; i += 10)
{
string s = i.ToString();
Data d = keyd[s];
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
keydTime = sw.ElapsedMilliseconds;
sw.Reset();
sw.Start();
for (int r = 0; r < 10; r++)
{
for (int i = 0; i < 10000; i += 10)
{
string s = i.ToString();
Data d = list.FirstOrDefault(item => item.Id == s);
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
listTime = sw.ElapsedMilliseconds * 100;
Console.WriteLine("Keyed search collection");
Console.WriteLine("Dict: " + dictTime);
Console.WriteLine("Keyd: " + keydTime);
Console.WriteLine("List: " + listTime);
}
}
}
更新：
按照@Blam建議的字典與KeyedCollection的比較。
最快的方法是遍歷一個KeyedCollection項目數組。
但是，請注意，迭代字典值比在KeyedCollection上迭代更快，而不轉換爲數組。
請注意，迭代字典值比字典集合要快得多，快得多。
Iterate 1,000 times over collection of 10,000 items
Dictionary Pair: 519 ms
Dictionary Values: 95 ms
Dict Val ToArray: 92 ms
KeyedCollection: 141 ms
KeyedC. ToArray: 17 ms
計時時間來自Windows控制檯應用程序（發佈版本）。這裏是源代碼：
using System;
using System.Collections.Generic;
using System.Linq;
namespace IterateCollections
{
public class GUIDkeyCollection : System.Collections.ObjectModel.KeyedCollection<Guid, GUIDkey>
{
// This parameterless constructor calls the base class constructor
// that specifies a dictionary threshold of 0, so that the internal
// dictionary is created as soon as an item is added to the
// collection.
//
public GUIDkeyCollection() : base() { }
// This is the only method that absolutely must be overridden,
// because without it the KeyedCollection cannot extract the
// keys from the items.
//
protected override Guid GetKeyForItem(GUIDkey item)
{
// In this example, the key is the part number.
return item.Key;
}
public GUIDkey[] ToArray()
{
return Items.ToArray();
}
//[Obsolete("Iterate using .ToArray()", true)]
//public new IEnumerator GetEnumerator()
//{
// throw new NotImplementedException("Iterate using .ToArray()");
//}
}
public class GUIDkey : Object
{
private Guid key;
public Guid Key
{
get
{
return key;
}
}
public override bool Equals(Object obj)
{
//Check for null and compare run-time types.
if (obj == null || !(obj is GUIDkey)) return false;
GUIDkey item = (GUIDkey)obj;
return (Key == item.Key);
}
public override int GetHashCode() { return Key.GetHashCode(); }
public GUIDkey(Guid guid)
{
key = guid;
}
}
class Program
{
static void Main(string[] args)
{
const int itemCount = 10000;
const int repetitions = 1000;
const string resultFormat = "{0,18}: {1,5:D} ms";
Console.WriteLine("Iterate {0:N0} times over collection of {1:N0} items", repetitions, itemCount);
var dict = new Dictionary<Guid, GUIDkey>();
var keyd = new GUIDkeyCollection();
for (int i = 0; i < itemCount; i++)
{
var d = new GUIDkey(Guid.NewGuid());
dict.Add(d.Key, d);
keyd.Add(d);
}
var sw = new System.Diagnostics.Stopwatch();
long time;
sw.Reset();
sw.Start();
for (int r = 0; r < repetitions; r++)
{
foreach (KeyValuePair<Guid, GUIDkey> w in dict)
{
if (null == w.Value) throw new ApplicationException();
}
}
sw.Stop();
time = sw.ElapsedMilliseconds;
Console.WriteLine(resultFormat, "Dictionary Pair", time);
sw.Reset();
sw.Start();
for (int r = 0; r < repetitions; r++)
{
foreach (GUIDkey d in dict.Values)
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
time = sw.ElapsedMilliseconds;
Console.WriteLine(resultFormat, "Dictionary Values", time);
sw.Reset();
sw.Start();
for (int r = 0; r < repetitions; r++)
{
foreach (GUIDkey d in dict.Values.ToArray())
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
time = sw.ElapsedMilliseconds;
Console.WriteLine(resultFormat, "Dict Val ToArray", time);
sw.Reset();
sw.Start();
for (int r = 0; r < repetitions; r++)
{
foreach (GUIDkey d in keyd)
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
time = sw.ElapsedMilliseconds;
Console.WriteLine(resultFormat, "KeyedCollection", time);
sw.Reset();
sw.Start();
for (int r = 0; r < repetitions; r++)
{
foreach (GUIDkey d in keyd.ToArray())
{
if (null == d) throw new ApplicationException();
}
}
sw.Stop();
time = sw.ElapsedMilliseconds;
Console.WriteLine(resultFormat, "KeyedC. ToArray", time);
}
}
}
第二是更快，最有可能的，因爲字典值將是稀疏。列表值取決於實施情況，可能會更好。 – Lucas 2013-04-09 14:27:12
您在尋找應用程序的時間或者有特定原因不使用優化的Linq查詢時需要幾毫秒的時間？ – Jasper 2013-04-09 14:28:38
過早的微觀優化是邪惡的。 – JustAnotherUserYouMayKnow 2013-04-09 14:29:14