这些枚举中的一个比另一个或大约相同吗？ (C#中的例子)
情况1：
Dictionary<string,object> valuesDict;
// valuesDict loaded with thousands of objects
foreach (object value in valuesDict.Values) { /* process */ }
案例2：
List<object> valuesList;
// valuesList loaded with thousands of objects
foreach (object value in valuesList) { /* process */ }
更新：
背景：
字典对于其他地方的键控搜索是有益的(而不是遍历列表),但是如果遍历字典比通过列表慢得多,那么好处将会减少.
更新：
根据许多人的建议,我已经完成了自己的测试.
首先,这些是结果.以下是该计划.
迭代整个集合
Dict：78
凯德：131
清单：76
键控搜索集合
Dict：178
凯德：194
清单：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：
@Blam建议的字典与KeyedCollection的比较.
最快的方法是迭代KeyedCollection项目数组.
但请注意,迭代字典值比KeyedCollection快,而不转换为数组.
请注意,迭代字典值比字典集合快得多.
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
计时来自Windows控制台应用程序(发布版本).这是源代码：
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); } } }
解决方法
使用秒表检查相对容易：
var d = new Dictionary<string,object>();
var s = new List<object>();
for (int i =0 ; i != 10000000 ; i++) {
d.Add(""+i,i);
s.Add(i);
}
var sw = new Stopwatch();
sw.Start();
foreach(object o in d.Values) {
if (o == null) throw new ApplicationException();
}
sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);
sw.Reset();
sw.Start();
foreach (object o in s) {
if (o == null) throw new ApplicationException();
}
sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);
这将打印相当接近的数字：
Dict List ---- ---- 136 107 139 108 136 108
List总是获胜,但考虑到两个数据结构的相对复杂性,边距并不像人们预期的那么大.
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