有谁知道为什么C#(。NET)的StartsWith函数比IsPrefix慢得多?
答案 0 :(得分:28)
我认为这主要取决于该线程目前的文化。
如果您更改Marc的测试以使用此String.StartsWith
形式:
Stopwatch watch = Stopwatch.StartNew();
CultureInfo cc = CultureInfo.CurrentCulture;
for (int i = 0; i < LOOP; i++)
{
if (s1.StartsWith(s2, false, cc)) chk++;
}
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds + "ms; chk: " + chk);
它更接近了。
如果您使用s1.StartsWith(s2, StringComparison.Ordinal)
,则比使用CompareInfo.IsPrefix
要快得多(当然,取决于CompareInfo
)。在我的方框上,结果是(不科学):
显然那是因为它实际上只是在每个点比较16位整数,这非常便宜。如果您不想要进行文化敏感检查,那么和对您来说特别重要,那就是我使用的重载。
答案 1 :(得分:6)
好问题;为了测试,我得到:
9156ms; chk: 50000000
6887ms; chk: 50000000
试验台:
using System;
using System.Diagnostics;
using System.Globalization;
class Program
{
static void Main()
{
string s1 = "abcdefghijklmnopqrstuvwxyz", s2 = "abcdefg";
const int LOOP = 50000000;
int chk = 0;
Stopwatch watch = Stopwatch.StartNew();
for (int i = 0; i < LOOP; i++)
{
if (s1.StartsWith(s2)) chk++;
}
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds + "ms; chk: " + chk);
chk = 0;
watch = Stopwatch.StartNew();
CompareInfo ci = CultureInfo.CurrentCulture.CompareInfo;
for (int i = 0; i < LOOP; i++)
{
if (ci.IsPrefix(s1, s2)) chk++;
}
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds + "ms; chk: " + chk);
}
}
答案 2 :(得分:5)
StartsWith内部调用IsPrefix。它在调用IsPrefix之前分配文化信息。
答案 3 :(得分:0)
查看IsPrefix的来源。问题是 - 在某些情况下,它会比StartsWith
慢,只是因为它实际上使用了StartsWith并且进行了更多的操作。
[System.Security.SecuritySafeCritical] // auto-generated
public unsafe virtual bool IsPrefix(String source, String prefix, CompareOptions options)
{
if (source == null || prefix == null) {
throw new ArgumentNullException((source == null ? "source" : "prefix"),
Environment.GetResourceString("ArgumentNull_String"));
}
Contract.EndContractBlock();
int prefixLen = prefix.Length;
if (prefixLen == 0)
{
return (true);
}
if (options == CompareOptions.OrdinalIgnoreCase)
{
return source.StartsWith(prefix, StringComparison.OrdinalIgnoreCase);
}
if (options == CompareOptions.Ordinal)
{
return source.StartsWith(prefix, StringComparison.Ordinal);
}
if ((options & ValidIndexMaskOffFlags) != 0) {
throw new ArgumentException(Environment.GetResourceString("Argument_InvalidFlag"), "options");
}
// to let the sorting DLL do the call optimization in case of Ascii strings, we check if the strings are in Ascii and then send the flag RESERVED_FIND_ASCII_STRING to
// the sorting DLL API SortFindString so sorting DLL don't have to check if the string is Ascii with every call to SortFindString.
return (InternalFindNLSStringEx(
m_dataHandle, m_handleOrigin, m_sortName,
GetNativeCompareFlags(options) | Win32Native.FIND_STARTSWITH | ((source.IsAscii() && prefix.IsAscii()) ? RESERVED_FIND_ASCII_STRING : 0),
source, source.Length, 0, prefix, prefix.Length) > -1);
}