C#判断字符串中是否包含指定字符串及contains与indexof方法效率问题

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正文

#方法一：使用string.Contains方法

　　string.Contains是大小写敏感的，如果要用该方法来判断一个string是否包含某个关键字keyword，需要把这个string和这个keyword都转成小写或大写再调用Contains方法；

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string key = "bbb";
 string temp = "aaaBBBcccDDD";
 bool isContains= temp.ToLower().Contains(key.ToLower());//true

#方法二：使用sring.Index方法

　　使用string.Index方法，然后通过StringComparison.OrdinalIgnoreCase指定查找过程忽略大小写；

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string key = "bbb";
 string temp = "aaaBBBcccDDD";
 bool isContains = temp.IndexOf(key,StringComparison.OrdinalIgnoreCase)>=0;//true

#那什么时候使用Contains方法，什么时候使用Index方法，哪个效率高？

1、测试代码：

　　注：此测试针对的是拥有大量英文的情况下，并且指定的字符串为英文

　　每个方法测试1千万次，输出所用时间；

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class Program
  {
    private const int N = 10000000;
    private static Stopwatch watch = new Stopwatch();
    static void Main(string[] args)
    {
      string source = "aaabbbcccdddeeefffggghhhiiijjjkkklllmmmnnnooopppqqq";
      string target = "AAA";
      Console.WriteLine("目标在开头部分时：");
      Console.WriteLine("不区分大小写：");
      TestContains(source, target,true);
      TestIndexOf(source, target,true);
      Console.WriteLine("区分大小写：");
      target = "aaa";
      TestContains(source, target,false);
      TestIndexOf(source, target,false);
      Console.WriteLine();
      Console.WriteLine("目标在中部时：");
      Console.WriteLine("不区分大小写：");
      target = "HHH";
      TestContains(source, target, true);
      TestIndexOf(source, target, true);
      Console.WriteLine("区分大小写：");
      target = "hhh";
      TestContains(source, target, false);
      TestIndexOf(source, target, false);
      Console.WriteLine();
      Console.WriteLine("目标在结尾时：");
      Console.WriteLine("不区分大小写：");
      target = "QQQ";
      TestContains(source, target,true);
      TestIndexOf(source, target,true);
      Console.WriteLine("区分大小写：");
      target = "qqq";
      TestContains(source, target,false);
      TestIndexOf(source, target,false);
      Console.WriteLine("执行完毕，按任意键退出...");
      Console.ReadKey();
    }
    private static void TestIndexOf(string source, string target,bool isIgnoreCase)
    {
      watch.Reset();
      watch.Start();
      for (int i = 0; i < N; i++)
      {
        if (isIgnoreCase)
          source.IndexOf(target, StringComparison.OrdinalIgnoreCase);
        else
          source.IndexOf(target);
      }
      watch.Stop();
      Console.WriteLine("IndexOf: " + watch.ElapsedMilliseconds.ToString() + "ms");
      return;
    }
    private static void TestContains(string source, string target,bool isIgnoreCase)
    {
      watch.Reset();
      watch.Start();
      for (int i = 0; i < N; i++)
      {
        if (isIgnoreCase)
          source.ToLower().Contains(target.ToLower());
        else
          source.Contains(target);
      }
      watch.Stop();
      Console.WriteLine("Contains: " + watch.ElapsedMilliseconds.ToString() + "ms");
      return;
    }
  }

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class Program
  {
    private const int N = 10000000;
    private static Stopwatch watch = new Stopwatch();
    static void Main(string[] args)
    {
      string source = "aaabbbcccdddeeefffggghhhiiijjjkkklllmmmnnnooopppqqq";
      string target = "AAA";
      Console.WriteLine("目标在开头部分时：");
      Console.WriteLine("不区分大小写：");
      TestContains(source, target,true);
      TestIndexOf(source, target,true);
      Console.WriteLine("区分大小写：");
      target = "aaa";
      TestContains(source, target,false);
      TestIndexOf(source, target,false);
      Console.WriteLine();
      Console.WriteLine("目标在中部时：");
      Console.WriteLine("不区分大小写：");
      target = "HHH";
      TestContains(source, target, true);
      TestIndexOf(source, target, true);
      Console.WriteLine("区分大小写：");
      target = "hhh";
      TestContains(source, target, false);
      TestIndexOf(source, target, false);
      Console.WriteLine();
      Console.WriteLine("目标在结尾时：");
      Console.WriteLine("不区分大小写：");
      target = "QQQ";
      TestContains(source, target,true);
      TestIndexOf(source, target,true);
      Console.WriteLine("区分大小写：");
      target = "qqq";
      TestContains(source, target,false);
      TestIndexOf(source, target,false);
      Console.WriteLine("执行完毕，按任意键退出...");
      Console.ReadKey();
    }
    private static void TestIndexOf(string source, string target,bool isIgnoreCase)
    {
      watch.Reset();
      watch.Start();
      for (int i = 0; i < N; i++)
      {
        if (isIgnoreCase)
          source.IndexOf(target, StringComparison.OrdinalIgnoreCase);
        else
          source.IndexOf(target);
      }
      watch.Stop();
      Console.WriteLine("IndexOf: " + watch.ElapsedMilliseconds.ToString() + "ms");
      return;
    }
    private static void TestContains(string source, string target,bool isIgnoreCase)
    {
      watch.Reset();
      watch.Start();
      for (int i = 0; i < N; i++)
      {
        if (isIgnoreCase)
          source.ToLower().Contains(target.ToLower());
        else
          source.Contains(target);
      }
      watch.Stop();
      Console.WriteLine("Contains: " + watch.ElapsedMilliseconds.ToString() + "ms");
      return;
    }
  }

2、测试结果：

3、总结

　　1、从测试结果（大量测试）中能明显看出，当拥有大量英文的字符串中：

　　*当不区分大小写时，string.IndexOf方法的效率明显高于string.Contains方法；

　　*当区分大小写时，string.Contains方法的效率明显高于string.IndexOf方法；

　　*如果判断的是中文，没有大小写之分，还是string.Contains方法的效率高；

　　2、综合上述总结，定义了一个String扩展方法，该方法包含一个StringComparison参数，返回值为是否包含子字符串：

　　　　参考：https://docs.microsoft.com/zh-cn/dotnet/api/system.string.contains?redirectedfrom=MSDN&view=netframework-4.5#System_String_Contains_System_String_

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using System;
 public static class StringExtensions
 {
   public static bool Contains(this String str, String substring, 
                 StringComparison comp)
   {              
    if (substring == null)
      throw new ArgumentNullException("substring", 
                     "substring cannot be null.");
    else if (! Enum.IsDefined(typeof(StringComparison), comp))
     throw new ArgumentException("comp is not a member of StringComparison",
                   "comp");
    return str.IndexOf(substring, comp) >= ;           
  }
 }

using System;
public class Example
{
  public static void Main()
  {
   String s = "This is a string.";
   String sub1 = "this";
   Console.WriteLine("Does '{0}' contain '{1}'?", s, sub1);
   StringComparison comp = StringComparison.Ordinal;
   Console.WriteLine("  {0:G}: {1}", comp, s.Contains(sub1, comp));
   comp = StringComparison.OrdinalIgnoreCase;
   Console.WriteLine("  {0:G}: {1}", comp, s.Contains(sub1, comp));
  }
}
// The example displays the following output:
//    Does 'This is a string.' contain 'this'?
//     Ordinal: False
//     OrdinalIgnoreCase: True

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using System;
 public static class StringExtensions
 {
   public static bool Contains(this String str, String substring, 
                 StringComparison comp)
   {              
    if (substring == null)
      throw new ArgumentNullException("substring", 
                     "substring cannot be null.");
    else if (! Enum.IsDefined(typeof(StringComparison), comp))
     throw new ArgumentException("comp is not a member of StringComparison",
                   "comp");
    return str.IndexOf(substring, comp) >= ;           
  }
 }

using System;
public class Example
{
  public static void Main()
  {
   String s = "This is a string.";
   String sub1 = "this";
   Console.WriteLine("Does '{0}' contain '{1}'?", s, sub1);
   StringComparison comp = StringComparison.Ordinal;
   Console.WriteLine("  {0:G}: {1}", comp, s.Contains(sub1, comp));
   comp = StringComparison.OrdinalIgnoreCase;
   Console.WriteLine("  {0:G}: {1}", comp, s.Contains(sub1, comp));
  }
}
// The example displays the following output:
//    Does 'This is a string.' contain 'this'?
//     Ordinal: False
//     OrdinalIgnoreCase: True