在C＃中轉義字符串中字符的最快方法

Question

我想找到一種最快的方法，用轉義的版本替換字符串中的所有保留字符。

我自然想到兩種天真的方法（請注意，保留字符集只是一個例子）：

答：使用查找字典和String.Replace

private Dictionary<string, string> _someEscapeTokens = new Dictionary<string, string>()
{
    {"\t", @"\t"},
    {"\n", @"\n"},
    {"\r", @"\r"}
};

public string GetEscapedStringByNaiveLookUp(string s)
{
    foreach (KeyValuePair<string, string> escapeToken in _someEscapeTokens.Where(kvp => s.Contains(kvp.Key)))
    {
        s = s.Replace(escapeToken.Key, escapeToken.Value);
    }
    return s;
}

B：遍歷字符串中的每個字符

public string GetEscapedStringByTraversingCharArray(string s)
{
    char[] chars = s.ToCharArray();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < chars.Length; i++)
    {
        switch (chars[i])
        {
            case '\t':
                sb.Append(@"\t"); break;
            case '\n':
                sb.Append(@"\n"); break;
            case '\r':
                sb.Append(@"\r"); break;
            default:
                sb.Append(chars[i]); break;
         }
    }
    return sb.ToString();
}

正如我已經測試過的，版本B輕松勝過第一個。

注意：我已經考慮過Regex.Escape，但是由於字符集與我的字符集不匹配，因此不合適。

但是，還有其他方法可以解決此問題（考慮性能）？

更新：測試

我做了更多測試，並希望分享結果。 參見下面的代碼。

在針對.Net Framework 4.0的 兩個不同系統上進行了測試。 無論如何，結果幾乎是相同的：

Char Array (short string) average time: 38600 ns
Foreach (short string) average time: 26680 ns
Char Array (long string) average time: 48,1 ms
Foreach (long string) average time: 64,2 ms
Char Array (escaping only) average time: 13,6 ms
Foreach (escaping only) average time: 17,3 ms

這使我得出以下結論：對於短字符串， foreach版本似乎稍快一些，但對於較長的字符串則以某種方式“掉線”。 但是，我們在這里談論的是很小的差異。

測試代碼：

private static void Main(string[] args)
{
    //around 700 characters
    string shortString = new StackTrace().ToString();
    string longString;
    string pureEscape;
    //loading from a file with 1000000 words http://loremipsum.de/
    using (StreamReader sr = new StreamReader(@"C:\users\ekrueger\desktop\LoremIpsum.txt"))
    {
        longString = sr.ReadToEnd();
    }
    //text file containing only escapable characters (length ~1000000)
    using (StreamReader sr = new StreamReader(@"C:\users\ekrueger\desktop\PureEscape.txt"))
    {
        pureEscape = sr.ReadToEnd();
    }
    List<double> timesCharArrayShortString = new List<double>();
    List<double> timesForeachShortString = new List<double>();
    List<long> timesCharArrayLongString = new List<long>();
    List<long> timesForeachLongString = new List<long>();
    List<long> timesCharArrayPureEscape = new List<long>();
    List<long> timesForeachPureEscape = new List<long>();
    Stopwatch sw = new Stopwatch();

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringByTraversingCharArray(shortString);
        sw.Stop();
        timesCharArrayShortString.Add(sw.Elapsed.TotalMilliseconds * 1000000);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringForeach(shortString);
        sw.Stop();
        timesForeachShortString.Add(sw.Elapsed.TotalMilliseconds * 1000000);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringByTraversingCharArray(longString);
        sw.Stop();
        timesCharArrayLongString.Add(sw.ElapsedMilliseconds);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringForeach(longString);
        sw.Stop();
        timesForeachLongString.Add(sw.ElapsedMilliseconds);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringByTraversingCharArray(pureEscape);
        sw.Stop();
        timesCharArrayPureEscape.Add(sw.ElapsedMilliseconds);
    }

    for (int i = 0; i < 10; i++)
    {
        sw.Restart();
        GetEscapedStringForeach(pureEscape);
        sw.Stop();
        timesForeachPureEscape.Add(sw.ElapsedMilliseconds);
    }

    Console.WriteLine("Char Array (short string) average time: {0} ns", timesCharArrayShortString.Average());
    Console.WriteLine("Foreach (short string) average time: {0} ns", timesForeachShortString.Average());
    Console.WriteLine("Char Array (long string) average time: {0} ms", timesCharArrayLongString.Average());
    Console.WriteLine("Foreach (long string) average time: {0} ms", timesForeachLongString.Average());
    Console.WriteLine("Char Array (escaping only) average time: {0} ms", timesCharArrayPureEscape.Average());
    Console.WriteLine("Foreach (escaping only) average time: {0} ms", timesForeachPureEscape.Average());

    Console.Read();
}

private static string GetEscapedStringByTraversingCharArray(string s)
{
    if (String.IsNullOrEmpty(s))
        return s;

    char[] chars = s.ToCharArray();
    StringBuilder sb = new StringBuilder(s.Length);
    for (int i = 0; i < chars.Length; i++)
    {
        switch (chars[i])
        {
            case '\t':
                sb.Append(@"\t"); break;
            case '\n':
                sb.Append(@"\n"); break;
            case '\r':
                sb.Append(@"\r"); break;
            case '\f':
                sb.Append(@"\f"); break;
            default:
                sb.Append(chars[i]); break;
        }
    }
    return sb.ToString();
}

public static string GetEscapedStringForeach(string s)
{
    if (String.IsNullOrEmpty(s))
        return s;

    StringBuilder sb = new StringBuilder(s.Length);
    foreach (Char ch in s)
    {
        switch (ch)
        {
            case '\t':
                sb.Append(@"\t"); break;
            case '\n':
                sb.Append(@"\n"); break;
            case '\r':
                sb.Append(@"\r"); break;
            default:
                sb.Append(ch); break;
        }
    }
    return sb.ToString();
}

Answer 1

第一個選擇的速度較慢是有道理的，因為您使用以下方法創建了許多字符串對象：

s = s.Replace(escapeToken.Key, escapeToken.Value);

在第二種方法中，無需創建char []，因為string也具有索引器。 可能唯一可以提高性能的方法就是使用容量初始化StringBuilder，因此不需要調整大小。 您仍然可以在第二種方法中使用“詞典”。

Answer 2

您無需將string轉換為Char[] ，因此可以稍微改善解決方案：

public string GetEscapedStringByTraversingCharArray(string s) {
  // do not forget about null...
  if (String.IsNullOrEmpty(s))
    return s;

  // we can be sure, that it requires at least s.Length symbols, let's allocate then
  // in case we want to trade memory for speed we can even put
  //   StringBuilder sb = new StringBuilder(2 * s.Length);
  // for the worst case when all symbols should be escaped
  StringBuilder sb = new StringBuilder(s.Length);

  foreach(Char ch in s) {
    switch (ch) {
      case '\t':
        sb.Append(@"\t"); 

        break;
      case '\n':
        sb.Append(@"\n"); 

        break;
      case '\r':
        sb.Append(@"\r"); 

        break;
      default:
        sb.Append(ch); 

        break;
    }
  }

  return sb.ToString();
}