C# 什么更有效：Dictionary TryGetValue或ContainsKey+；项目？

从MSDN的条目：

此方法结合了ContainsKey方法和 项属性

如果找不到键，则value参数将获得适当的 值类型TValue的默认值；例如，0（零）表示 整数类型，布尔类型为false，引用类型为null

如果代码经常尝试访问，请使用TryGetValue方法 字典中没有的键。使用这种方法更有效 比捕获项引发的KeyNotFoundException更有效 财产

此方法接近O（1）操作

从描述来看，它是否比调用ContainsKey然后执行查找更高效或更方便还不清楚。

TryGetValue

的实现是只调用ContainsKey，然后调用Item，还是实际上比执行单个查找更有效

换句话说，什么更有效（即哪一个执行的查找更少）：

字典dict；
//...//
国际竞争力；
if（dict.ContainsKey（ikey））
{
ival=dict[ikey]；
}
其他的
{
ival=默认值（int）；
}

或

字典dict；
//...//
国际竞争力；
dict.TryGetValue（ikey，out-ival）；

注意：我不是在寻找基准

为什么不测试一下呢

但是我很确定

TryGetValue

更快，因为它只进行一次查找。当然，这并不能保证，即不同的实现可能具有不同的性能特征

---

我实现字典的方法是创建一个内部的

Find

函数来查找项目的槽，然后在其上构建其余的。

TryGetValue

会更快

ContainsKey

使用与

TryGetValue

相同的检查，该检查在内部表示实际输入位置。

项

属性实际上与

TryGetValue

具有几乎相同的代码功能，只是它将抛出异常而不是返回false

---

使用

ContainsKey

后跟

项

基本上复制了查找功能，这是本例中的大部分计算

快速基准测试表明

TryGetValue

有一点优势：

    static void Main() {
        var d = new Dictionary<string, string> {{"a", "b"}};
        var start = DateTime.Now;
        for (int i = 0; i != 10000000; i++) {
            string x;
            if (!d.TryGetValue("a", out x)) throw new ApplicationException("Oops");
            if (d.TryGetValue("b", out x)) throw new ApplicationException("Oops");
        }
        Console.WriteLine(DateTime.Now-start);
        start = DateTime.Now;
        for (int i = 0; i != 10000000; i++) {
            string x;
            if (d.ContainsKey("a")) {
                x = d["a"];
            } else {
                x = default(string);
            }
            if (d.ContainsKey("b")) {
                x = d["b"];
            } else {
                x = default(string);
            }
        }
   }

假设命中率和未命中率相等，使

ContainsKey+项的访问速度降低约40%

此外，当我将程序更改为“始终未命中”（即始终查找
“b”
）时，两个版本的速度相同：

00:00:00.2850000
00:00:00.2720000

然而，当我将其设置为“所有命中率”时，
TryGetValue
仍然是一个明显的赢家：

00:00:00.4930000
00:00:00.8110000

做一个快速测试程序，使用TryGetValue在字典中包含100万项肯定会有改进

结果：

Finished evaluation .... Time distribution:
Size: 000010: TryGetValue: 53,24%, ContainsKey: 1,74%, try-catch: 45,01% - Total: 2.006,00
Size: 000020: TryGetValue: 37,66%, ContainsKey: 0,53%, try-catch: 61,81% - Total: 2.443,00
Size: 000040: TryGetValue: 22,02%, ContainsKey: 0,73%, try-catch: 77,25% - Total: 7.147,00
Size: 000080: TryGetValue: 31,46%, ContainsKey: 0,42%, try-catch: 68,12% - Total: 17.793,00
Size: 000160: TryGetValue: 33,66%, ContainsKey: 0,37%, try-catch: 65,97% - Total: 36.840,00
Size: 000320: TryGetValue: 34,53%, ContainsKey: 0,39%, try-catch: 65,09% - Total: 71.059,00
Size: 000640: TryGetValue: 32,91%, ContainsKey: 0,32%, try-catch: 66,77% - Total: 141.789,00
Size: 001280: TryGetValue: 39,02%, ContainsKey: 0,35%, try-catch: 60,64% - Total: 244.657,00
Size: 002560: TryGetValue: 35,48%, ContainsKey: 0,19%, try-catch: 64,33% - Total: 420.121,00
Size: 005120: TryGetValue: 43,41%, ContainsKey: 0,24%, try-catch: 56,34% - Total: 625.969,00
Size: 010240: TryGetValue: 29,64%, ContainsKey: 0,61%, try-catch: 69,75% - Total: 1.197.242,00
Size: 020480: TryGetValue: 35,14%, ContainsKey: 0,53%, try-catch: 64,33% - Total: 2.405.821,00
Size: 040960: TryGetValue: 37,28%, ContainsKey: 0,24%, try-catch: 62,48% - Total: 4.200.839,00
Size: 081920: TryGetValue: 29,68%, ContainsKey: 0,54%, try-catch: 69,77% - Total: 8.980.230,00

    using System;
    using System.Collections.Generic;
    using System.Diagnostics;

    namespace ConsoleApplication1
    {
        class Program
        {
            static void Main(string[] args)
            {
                const int ENTRIES = 10000, MAXVAL = 15000, TRIALS = 100000, MULTIPLIER = 2;
                Dictionary<int, int> values = new Dictionary<int, int>();
                Random r = new Random();
                int[] lookups = new int[TRIALS];
                int val;
                List<Tuple<long, long, long>> durations = new List<Tuple<long, long, long>>(8);

                for (int i = 0;i < ENTRIES;++i) try
                    {
                        values.Add(r.Next(MAXVAL), r.Next());
                    }
                    catch { --i; }

                for (int i = 0;i < TRIALS;++i) lookups[i] = r.Next(MAXVAL);

                Stopwatch sw = new Stopwatch();
                ConsoleColor bu = Console.ForegroundColor;

                for (int size = 10;size <= TRIALS;size *= MULTIPLIER)
                {
                    long a, b, c;

                    Console.ForegroundColor = ConsoleColor.Yellow;
                    Console.WriteLine("Loop size: {0}", size);
                    Console.ForegroundColor = bu;

                    // ---------------------------------------------------------------------
                    sw.Start();
                    for (int i = 0;i < size;++i) values.TryGetValue(lookups[i], out val);
                    sw.Stop();
                    Console.WriteLine("TryGetValue: {0}", a = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    sw.Restart();
                    for (int i = 0;i < size;++i) val = values.ContainsKey(lookups[i]) ? values[lookups[i]] : default(int);
                    sw.Stop();
                    Console.WriteLine("ContainsKey: {0}", b = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    sw.Restart();
                    for (int i = 0;i < size;++i)
                        try { val = values[lookups[i]]; }
                        catch { }
                    sw.Stop();
                    Console.WriteLine("try-catch: {0}", c = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    Console.WriteLine();

                    durations.Add(new Tuple<long, long, long>(a, b, c));
                }

                Console.ForegroundColor = ConsoleColor.Yellow;
                Console.WriteLine("Finished evaluation .... Time distribution:");
                Console.ForegroundColor = bu;

                val = 10;
                foreach (Tuple<long, long, long> d in durations)
                {
                    long sum = d.Item1 + d.Item2 + d.Item3;

                    Console.WriteLine("Size: {0:D6}:", val);
                    Console.WriteLine("TryGetValue: {0:P2}, ContainsKey: {1:P2}, try-catch: {2:P2} - Total: {3:N}", (decimal)d.Item1 / sum, (decimal)d.Item2 / sum, (decimal)d.Item3 / sum, sum);
                    val *= MULTIPLIER;
                }

                Console.WriteLine();
            }
        }
    }

ContainsKey+项目点击1000000次：45毫秒

1000000次点击的TryGetValue:26ms

以下是测试应用程序：

static void Main(string[] args)
{
    const int size = 1000000;

    var dict = new Dictionary<int, string>();

    for (int i = 0; i < size; i++)
    {
        dict.Add(i, i.ToString());
    }

    var sw = new Stopwatch();
    string result;

    sw.Start();

    for (int i = 0; i < size; i++)
    {
        if (dict.ContainsKey(i))
            result = dict[i];
    }

    sw.Stop();
    Console.WriteLine("ContainsKey + Item for {0} hits: {1}ms", size, sw.ElapsedMilliseconds);

    sw.Reset();
    sw.Start();

    for (int i = 0; i < size; i++)
    {
        dict.TryGetValue(i, out result);
    }

    sw.Stop();
    Console.WriteLine("TryGetValue for {0} hits: {1}ms", size, sw.ElapsedMilliseconds);

}

static void Main（字符串[]args）
{
const int size=1000000；
var dict=新字典（）；
对于（int i=0；i
由于到目前为止，没有一个答案能够真正回答这个问题，因此，经过一些研究，我找到了一个可以接受的答案：

如果反编译TryGetValue，您会看到它正在执行以下操作：

public bool TryGetValue(TKey key, out TValue value)
{
  int index = this.FindEntry(key);
  if (index >= 0)
  {
    value = this.entries[index].value;
    return true;
  }
  value = default(TValue);
  return false;
}

鉴于ContainsKey方法是：

public bool ContainsKey(TKey key)
{
  return (this.FindEntry(key) >= 0);
}

因此TryGetValue只是ContainsKey加上数组查找（如果该项存在）


TryGetValue的速度似乎是ContainesKey+项组合的两倍。
如果您试图从字典中取出值，TryGetValue（key，out value）是最好的选择，但是如果您正在检查是否存在该键，是否需要新的插入，而不覆盖旧键，并且仅在该范围内，ContainsKey（键）是最好的选择，benchmark可以证实这一点：

using System;
using System.Threading;
using System.Diagnostics;
using System.Collections.Generic;
using System.Collections;

namespace benchmark
{
class Program
{
    public static Random m_Rand = new Random();
    public static Dictionary<int, int> testdict = new Dictionary<int, int>();
    public static Hashtable testhash = new Hashtable();

    public static void Main(string[] args)
    {
        Console.WriteLine("Adding elements into hashtable...");
        Stopwatch watch = Stopwatch.StartNew();
        for(int i=0; i<1000000; i++)
            testhash[i]=m_Rand.Next();
        watch.Stop();
        Console.WriteLine("Done in {0:F4} -- pause....", watch.Elapsed.TotalSeconds);
        Thread.Sleep(4000);
        Console.WriteLine("Adding elements into dictionary...");
        watch = Stopwatch.StartNew();
        for(int i=0; i<1000000; i++)
            testdict[i]=m_Rand.Next();
        watch.Stop();
        Console.WriteLine("Done in {0:F4} -- pause....", watch.Elapsed.TotalSeconds);
        Thread.Sleep(4000);

        Console.WriteLine("Finding the first free number for insertion");
        Console.WriteLine("First method: ContainsKey");
        watch = Stopwatch.StartNew();
        int intero=0;
        while (testdict.ContainsKey(intero))
        {
            intero++;
        }
        testdict.Add(intero, m_Rand.Next());
        watch.Stop();
        Console.WriteLine("Done in {0:F4} -- added value {1} in dictionary -- pause....", watch.Elapsed.TotalSeconds, intero);
        Thread.Sleep(4000);
        Console.WriteLine("Second method: TryGetValue");
        watch = Stopwatch.StartNew();
        intero=0;
        int result=0;
        while(testdict.TryGetValue(intero, out result))
        {
            intero++;
        }
        testdict.Add(intero, m_Rand.Next());
        watch.Stop();
        Console.WriteLine("Done in {0:F4} -- added value {1} in dictionary -- pause....", watch.Elapsed.TotalSeconds, intero);
        Thread.Sleep(4000);
        Console.WriteLine("Test hashtable");
        watch = Stopwatch.StartNew();
        intero=0;
        while(testhash.Contains(intero))
        {
            intero++;
        }
        testhash.Add(intero, m_Rand.Next());
        watch.Stop();
        Console.WriteLine("Done in {0:F4} -- added value {1} into hashtable -- pause....", watch.Elapsed.TotalSeconds, intero);
        Console.Write("Press any key to continue . . . ");
        Console.ReadKey(true);
    }
}
}

使用系统；
使用系统线程；
使用系统诊断；
使用System.Collections.Generic；
使用系统集合；
名称空间基准
{
班级计划
{
公共静态随机m_Rand=新随机（）；
公共静态字典testdict=新字典（）；
公共静态哈希表testhash=新哈希表（）；
公共静态void Main（字符串[]args）
{
WriteLine（“将元素添加到哈希表…”）；
秒表=Stopwatch.StartNew（）；
对于（int i=0；i在我的机器上，RAM加载，在释放模式（非调试）下运行时，
ContainsKey
等于
TryGetValue
/
如果找到
字典中的所有条目，请尝试catch

ContainsKey
在只有几个字典条目未找到时，其性能远远优于它们（在我下面的示例中，将
MAXVAL
设置为大于
条目的任何值，以丢失一些条目）：

结果：

Finished evaluation .... Time distribution:
Size: 000010: TryGetValue: 53,24%, ContainsKey: 1,74%, try-catch: 45,01% - Total: 2.006,00
Size: 000020: TryGetValue: 37,66%, ContainsKey: 0,53%, try-catch: 61,81% - Total: 2.443,00
Size: 000040: TryGetValue: 22,02%, ContainsKey: 0,73%, try-catch: 77,25% - Total: 7.147,00
Size: 000080: TryGetValue: 31,46%, ContainsKey: 0,42%, try-catch: 68,12% - Total: 17.793,00
Size: 000160: TryGetValue: 33,66%, ContainsKey: 0,37%, try-catch: 65,97% - Total: 36.840,00
Size: 000320: TryGetValue: 34,53%, ContainsKey: 0,39%, try-catch: 65,09% - Total: 71.059,00
Size: 000640: TryGetValue: 32,91%, ContainsKey: 0,32%, try-catch: 66,77% - Total: 141.789,00
Size: 001280: TryGetValue: 39,02%, ContainsKey: 0,35%, try-catch: 60,64% - Total: 244.657,00
Size: 002560: TryGetValue: 35,48%, ContainsKey: 0,19%, try-catch: 64,33% - Total: 420.121,00
Size: 005120: TryGetValue: 43,41%, ContainsKey: 0,24%, try-catch: 56,34% - Total: 625.969,00
Size: 010240: TryGetValue: 29,64%, ContainsKey: 0,61%, try-catch: 69,75% - Total: 1.197.242,00
Size: 020480: TryGetValue: 35,14%, ContainsKey: 0,53%, try-catch: 64,33% - Total: 2.405.821,00
Size: 040960: TryGetValue: 37,28%, ContainsKey: 0,24%, try-catch: 62,48% - Total: 4.200.839,00
Size: 081920: TryGetValue: 29,68%, ContainsKey: 0,54%, try-catch: 69,77% - Total: 8.980.230,00

    using System;
    using System.Collections.Generic;
    using System.Diagnostics;

    namespace ConsoleApplication1
    {
        class Program
        {
            static void Main(string[] args)
            {
                const int ENTRIES = 10000, MAXVAL = 15000, TRIALS = 100000, MULTIPLIER = 2;
                Dictionary<int, int> values = new Dictionary<int, int>();
                Random r = new Random();
                int[] lookups = new int[TRIALS];
                int val;
                List<Tuple<long, long, long>> durations = new List<Tuple<long, long, long>>(8);

                for (int i = 0;i < ENTRIES;++i) try
                    {
                        values.Add(r.Next(MAXVAL), r.Next());
                    }
                    catch { --i; }

                for (int i = 0;i < TRIALS;++i) lookups[i] = r.Next(MAXVAL);

                Stopwatch sw = new Stopwatch();
                ConsoleColor bu = Console.ForegroundColor;

                for (int size = 10;size <= TRIALS;size *= MULTIPLIER)
                {
                    long a, b, c;

                    Console.ForegroundColor = ConsoleColor.Yellow;
                    Console.WriteLine("Loop size: {0}", size);
                    Console.ForegroundColor = bu;

                    // ---------------------------------------------------------------------
                    sw.Start();
                    for (int i = 0;i < size;++i) values.TryGetValue(lookups[i], out val);
                    sw.Stop();
                    Console.WriteLine("TryGetValue: {0}", a = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    sw.Restart();
                    for (int i = 0;i < size;++i) val = values.ContainsKey(lookups[i]) ? values[lookups[i]] : default(int);
                    sw.Stop();
                    Console.WriteLine("ContainsKey: {0}", b = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    sw.Restart();
                    for (int i = 0;i < size;++i)
                        try { val = values[lookups[i]]; }
                        catch { }
                    sw.Stop();
                    Console.WriteLine("try-catch: {0}", c = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    Console.WriteLine();

                    durations.Add(new Tuple<long, long, long>(a, b, c));
                }

                Console.ForegroundColor = ConsoleColor.Yellow;
                Console.WriteLine("Finished evaluation .... Time distribution:");
                Console.ForegroundColor = bu;

                val = 10;
                foreach (Tuple<long, long, long> d in durations)
                {
                    long sum = d.Item1 + d.Item2 + d.Item3;

                    Console.WriteLine("Size: {0:D6}:", val);
                    Console.WriteLine("TryGetValue: {0:P2}, ContainsKey: {1:P2}, try-catch: {2:P2} - Total: {3:N}", (decimal)d.Item1 / sum, (decimal)d.Item2 / sum, (decimal)d.Item3 / sum, sum);
                    val *= MULTIPLIER;
                }

                Console.WriteLine();
            }
        }
    }

这是我的代码：

Finished evaluation .... Time distribution:
Size: 000010: TryGetValue: 53,24%, ContainsKey: 1,74%, try-catch: 45,01% - Total: 2.006,00
Size: 000020: TryGetValue: 37,66%, ContainsKey: 0,53%, try-catch: 61,81% - Total: 2.443,00
Size: 000040: TryGetValue: 22,02%, ContainsKey: 0,73%, try-catch: 77,25% - Total: 7.147,00
Size: 000080: TryGetValue: 31,46%, ContainsKey: 0,42%, try-catch: 68,12% - Total: 17.793,00
Size: 000160: TryGetValue: 33,66%, ContainsKey: 0,37%, try-catch: 65,97% - Total: 36.840,00
Size: 000320: TryGetValue: 34,53%, ContainsKey: 0,39%, try-catch: 65,09% - Total: 71.059,00
Size: 000640: TryGetValue: 32,91%, ContainsKey: 0,32%, try-catch: 66,77% - Total: 141.789,00
Size: 001280: TryGetValue: 39,02%, ContainsKey: 0,35%, try-catch: 60,64% - Total: 244.657,00
Size: 002560: TryGetValue: 35,48%, ContainsKey: 0,19%, try-catch: 64,33% - Total: 420.121,00
Size: 005120: TryGetValue: 43,41%, ContainsKey: 0,24%, try-catch: 56,34% - Total: 625.969,00
Size: 010240: TryGetValue: 29,64%, ContainsKey: 0,61%, try-catch: 69,75% - Total: 1.197.242,00
Size: 020480: TryGetValue: 35,14%, ContainsKey: 0,53%, try-catch: 64,33% - Total: 2.405.821,00
Size: 040960: TryGetValue: 37,28%, ContainsKey: 0,24%, try-catch: 62,48% - Total: 4.200.839,00
Size: 081920: TryGetValue: 29,68%, ContainsKey: 0,54%, try-catch: 69,77% - Total: 8.980.230,00

    using System;
    using System.Collections.Generic;
    using System.Diagnostics;

    namespace ConsoleApplication1
    {
        class Program
        {
            static void Main(string[] args)
            {
                const int ENTRIES = 10000, MAXVAL = 15000, TRIALS = 100000, MULTIPLIER = 2;
                Dictionary<int, int> values = new Dictionary<int, int>();
                Random r = new Random();
                int[] lookups = new int[TRIALS];
                int val;
                List<Tuple<long, long, long>> durations = new List<Tuple<long, long, long>>(8);

                for (int i = 0;i < ENTRIES;++i) try
                    {
                        values.Add(r.Next(MAXVAL), r.Next());
                    }
                    catch { --i; }

                for (int i = 0;i < TRIALS;++i) lookups[i] = r.Next(MAXVAL);

                Stopwatch sw = new Stopwatch();
                ConsoleColor bu = Console.ForegroundColor;

                for (int size = 10;size <= TRIALS;size *= MULTIPLIER)
                {
                    long a, b, c;

                    Console.ForegroundColor = ConsoleColor.Yellow;
                    Console.WriteLine("Loop size: {0}", size);
                    Console.ForegroundColor = bu;

                    // ---------------------------------------------------------------------
                    sw.Start();
                    for (int i = 0;i < size;++i) values.TryGetValue(lookups[i], out val);
                    sw.Stop();
                    Console.WriteLine("TryGetValue: {0}", a = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    sw.Restart();
                    for (int i = 0;i < size;++i) val = values.ContainsKey(lookups[i]) ? values[lookups[i]] : default(int);
                    sw.Stop();
                    Console.WriteLine("ContainsKey: {0}", b = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    sw.Restart();
                    for (int i = 0;i < size;++i)
                        try { val = values[lookups[i]]; }
                        catch { }
                    sw.Stop();
                    Console.WriteLine("try-catch: {0}", c = sw.ElapsedTicks);

                    // ---------------------------------------------------------------------
                    Console.WriteLine();

                    durations.Add(new Tuple<long, long, long>(a, b, c));
                }

                Console.ForegroundColor = ConsoleColor.Yellow;
                Console.WriteLine("Finished evaluation .... Time distribution:");
                Console.ForegroundColor = bu;

                val = 10;
                foreach (Tuple<long, long, long> d in durations)
                {
                    long sum = d.Item1 + d.Item2 + d.Item3;

                    Console.WriteLine("Size: {0:D6}:", val);
                    Console.WriteLine("TryGetValue: {0:P2}, ContainsKey: {1:P2}, try-catch: {2:P2} - Total: {3:N}", (decimal)d.Item1 / sum, (decimal)d.Item2 / sum, (decimal)d.Item3 / sum, sum);
                    val *= MULTIPLIER;
                }

                Console.WriteLine();
            }
        }
    }

使用系统；
使用System.Collections.Generic；
使用系统诊断；
命名空间控制台应用程序1
{
班级计划
{
静态void Main（字符串[]参数）
{
const int ENTRIES=10000，MAXVAL=15000，TRIALS=100000，乘数=2；
字典值=新字典（）；
随机r=新随机（）；
int[]查找=新的int[试验]；
int-val；
dict.GetValueOrDefault("keyname")

(dict.GetValueOrDefault("keyname") ?? fallbackValue) 

using System.Collections.Generic;

namespace Project.Common.Extensions
{
    public static class DictionaryExtensions
    {
        public static TValue GetValueOrDefault<TKey, TValue>(
            this IDictionary<TKey, TValue> dictionary,
            TKey key,
            TValue defaultValue = default(TValue))
        {
            if (dictionary.TryGetValue(key, out TValue value))
            {
                return value;
            }
            return defaultValue;
        }
    }
}

var dict = new Dictionary<string, string>();
string val = dict.GetValueOrDefault("theKey", "value used if theKey is not found in dict");

var dictObj = new Dictionary<string, object>();
object valObj = dictObj.GetValueOrDefault("nonexistent");
Debug.Assert(valObj == null);

val dictInt = new Dictionary<string, int>();
int valInt = dictInt.GetValueOrDefault("nonexistent");
Debug.Assert(valInt == 0);