C#中的OR运算符

Question

我能实现吗？

if (a == "b" || "c")

代替

if (a == "b" || a== "c")

？

Answer 1

嗯,最接近你可以得到的是:

switch (a) {
   case "b":
   case "c":
      // variable a is either "b" or "c"
      break;
}

Answer 2

不,你可以这样做:

if (new[] { "b", "c" }.Contains(a))

如果你有LINQ扩展可用,但这几乎没有改进.

在回应有关性能的评论时,这里有一些基本的时序代码.请注意,必须以批判的眼光来看待代码,我可能在这里做了偏差时间的事情.

结果首先:

||, not found: 26 ms
||, found: 8 ms
array.Contains, not found: 1407 ms
array.Contains, found: 1388 ms
array.Contains, inline array, not found: 1456 ms
array.Contains, inline array, found: 1427 ms
switch-statement, not interned, not found: 26 ms
switch-statement, not interned, found: 14 ms
switch-statement, interned, not found: 25 ms
switch-statement, interned, found: 8 ms

所有代码都执行了两次,并且只传递了nr.报告了2,从等式中删除JITting开销.两个传递执行每种类型的检查一百万次,并执行它,其中要查找的元素是找到它的元素之一(即,if语句将执行其块),并且一次元素不在(该块不会执行).报告每个的时间.我测试了一个预构建的数组和每次构建的数组,这部分我不确定编译器推断和优化了多少,这里可能存在缺陷.

在任何情况下,看起来使用switch语句,无论是否首先实现字符串,都会给出与简单的or-statement大致相同的结果,这是预期的,而数组查找的成本要高得多,对我来说也是预料之中的.

请修改代码,并在出现问题时更正(或评论).

这里是源代码,相当长:

using System;
using System.Linq;
using System.Diagnostics;
namespace StackOverflow826081
{
    class Program
    {
        private const Int32 ITERATIONS = 1000000;
        static void Main()
        {
            String a;
            String[] ops = CreateArray();
            Int32 count;
            Stopwatch sw = new Stopwatch();
            Int32 pass = 0;
            Action<String, Int32> report = delegate(String title, Int32 i)
            {
                if (pass == 2)
                    Console.Out.WriteLine(title + ": " + sw.ElapsedMilliseconds + " ms");
            };

            for (pass = 1; pass <= 2; pass++)
            {
                #region || operator

                a = "a";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    if (a == "b" || a == "c")
                    {
                        count++;
                    }
                }
                sw.Stop();
                report("||, not found", count);
                sw.Reset();

                a = "b";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    if (a == "b" || a == "c")
                    {
                        count++;
                    }
                }
                sw.Stop();
                report("||, found", count);
                sw.Reset();

                #endregion

                #region array.Contains

                a = "a";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    if (ops.Contains(a))
                    {
                        count++;
                    }
                }
                sw.Stop();
                report("array.Contains, not found", count);
                sw.Reset();

                a = "b";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    if (ops.Contains(a))
                    {
                        count++;
                    }
                }
                sw.Stop();
                report("array.Contains, found", count);
                sw.Reset();

                #endregion           

                #region array.Contains

                a = "a";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    if (CreateArray().Contains(a))
                    {
                        count++;
                    }
                }
                sw.Stop();
                report("array.Contains, inline array, not found", count);
                sw.Reset();

                a = "b";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    if (CreateArray().Contains(a))
                    {
                        count++;
                    }
                }
                sw.Stop();
                report("array.Contains, inline array, found", count);
                sw.Reset();

                #endregion

                #region switch-statement

                a = GetString().Substring(0, 1); // avoid interned string
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    switch (a)
                    {
                        case "b":
                        case "c":
                            count++;
                            break;
                    }
                }
                sw.Stop();
                report("switch-statement, not interned, not found", count);
                sw.Reset();

                a = GetString().Substring(1, 1); // avoid interned string
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    switch (a)
                    {
                        case "b":
                        case "c":
                            count++;
                            break;
                    }
                }
                sw.Stop();
                report("switch-statement, not interned, found", count);
                sw.Reset();

                #endregion                      

                #region switch-statement

                a = "a";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    switch (a)
                    {
                        case "b":
                        case "c":
                            count++;
                            break;
                    }
                }
                sw.Stop();
                report("switch-statement, interned, not found", count);
                sw.Reset();

                a = "b";
                sw.Start();

                count = 0;
                for (Int32 index = 0; index < ITERATIONS; index++)
                {
                    switch (a)
                    {
                        case "b":
                        case "c":
                            count++;
                            break;
                    }
                }
                sw.Stop();
                report("switch-statement, interned, found", count);
                sw.Reset();

                #endregion
            }
        }

        private static String GetString()
        {
            return "ab";
        }

        private static String[] CreateArray()
        {
            return new String[] { "b", "c" };
        }
    }
}