C＃字符串的GetHashCode（）如何实现？

Z时代
2024-01-10
分类：问答

我只是好奇，因为我想它将对性能产生影响。是否考虑完整字符串？如果是，则在长字符串上会变慢。如果仅考虑字符串的一部分，则会导致性能下降（例如，如果仅考虑字符串的开头，则当HashSet主要包含相同的字符串时，它将导致性能下降。

回答：

遇到类似问题时，请确保获取参考源源代码。除了从反编译器中可以看到的内容之外，还有很多其他功能。选择与您的首选.NET目标相匹配的目标，该方法在版本之间已发生了很大变化。我将在此处重现它的.NET

4.5版本，该版本是从Source.NET 4.5 \ 4.6.0.0 \ net \ clr \ src \ BCL \ System \

String.cs \ 604718 \ String.cs检索的

        public override int GetHashCode() {
#if FEATURE_RANDOMIZED_STRING_HASHING
            if(HashHelpers.s_UseRandomizedStringHashing)
            { 
                return InternalMarvin32HashString(this, this.Length, 0);
            } 
#endif // FEATURE_RANDOMIZED_STRING_HASHING
            unsafe { 
                fixed (char *src = this) {
                    Contract.Assert(src[this.Length] == '\0', "src[this.Length] == '\\0'");
                    Contract.Assert( ((int)src)%4 == 0, "Managed string should start at 4 bytes boundary");
#if WIN32
                    int hash1 = (5381<<16) + 5381; 
#else 
                    int hash1 = 5381;
#endif 
                    int hash2 = hash1;
#if WIN32
                    // 32 bit machines. 
                    int* pint = (int *)src;
                    int len = this.Length; 
                    while (len > 2) 
                    {
                        hash1 = ((hash1 << 5) + hash1 + (hash1 >> 27)) ^ pint[0]; 
                        hash2 = ((hash2 << 5) + hash2 + (hash2 >> 27)) ^ pint[1];
                        pint += 2;
                        len  -= 4;
                    }
                    if (len > 0) 
                    { 
                        hash1 = ((hash1 << 5) + hash1 + (hash1 >> 27)) ^ pint[0];
                    } 
#else
                    int     c;
                    char *s = src;
                    while ((c = s[0]) != 0) { 
                        hash1 = ((hash1 << 5) + hash1) ^ c;
                        c = s[1]; 
                        if (c == 0) 
                            break;
                        hash2 = ((hash2 << 5) + hash2) ^ c; 
                        s += 2;
                    }
#endif
#if DEBUG 
                    // We want to ensure we can change our hash function daily.
                    // This is perfectly fine as long as you don't persist the 
                    // value from GetHashCode to disk or count on String A 
                    // hashing before string B.  Those are bugs in your code.
                    hash1 ^= ThisAssembly.DailyBuildNumber; 
#endif
                    return hash1 + (hash2 * 1566083941);
                }
            } 
        }