承认我是比较无聊。。。。

 

开个头吧

 

一个被人鄙视的小问题 -- C#中，如何判断字符串是空字符串呢。。。。？

 

string.IsNullOrEmpty?

string.Compare / string.CompareTo ?

string.Equals / string == "" ?

 

各有什么区别?

================================================================================

以下是CLR中的实现

 

        public static int CompareTo(string s1, string s2)
        {
            return System.Globalization.CultureInfo.CurrentCulture.CompareInfo.Compare(s1, s2, System.Globalization.CompareOptions.None);
        }

        public static bool IsNullOrEmpty(string value)
        {
            if (value != null)
            {
                return (value.Length == 0);
            }
            return true;
        }

 
        [ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
        private static unsafe bool EqualsHelper(string strA, string strB)
        {
            int length = strA.Length;
            if (length != strB.Length)
            {
                return false;
            }

            fixed (char* str = strA)
            {
                char* chPtr = str;
                fixed (char* str2 = strB)
                {
                    char* chPtr2 = str2;
                    char* chPtr3 = chPtr;
                    char* chPtr4 = chPtr2;
                    while (length >= 10)
                    {
                        if ((((*(((int*)chPtr3)) != *(((int*)chPtr4))) || (*(((int*)(chPtr3 + 2))) != *(((int*)(chPtr4 + 2))))) || ((*(((int*)(chPtr3 + 4))) != *(((int*)(chPtr4 + 4)))) || (*(((int*)(chPtr3 + 6))) != *(((int*)(chPtr4 + 6)))))) || (*(((int*)(chPtr3 + 8))) != *(((int*)(chPtr4 + 8)))))
                        {
                            break;
                        }
                        chPtr3 += 10;
                        chPtr4 += 10;
                        length -= 10;
                    }
                    while (length > 0)
                    {
                        if (*(((int*)chPtr3)) != *(((int*)chPtr4)))
                        {
                            break;
                        }
                        chPtr3 += 2;
                        chPtr4 += 2;
                        length -= 2;
                    }
                    return (length <= 0);
                }
            }
        }

====================================================================

IsNullOrEmpty是最直接的

==相当于调用重载了的Equals, 对于string来说是Equals(string)。从上面reflection出来的片断可以理解成以10字节为一大步，2字节为一小步的比较。（为什么这个组合performance最好？）

 

最本质的区别在于，CompareTo是culture sensitive的。它会根据当前的culture info，对字符串中的字符取权值，然后计算比较。（非顺序比较）