String相等比较测试

       String类字符串比较大概有4种方法:Compare(),CompareTo(), CompareOrdinal()和Equals(). Compare()方法是CompareTo()的静态版本.而Equals()与"=="是等价的,只要使用"=="运算符,就会调用Equals()方法.CompareOrdinal()对两个字符串进行比较,不考虑本地化语言和文化.

看测试比较代码:

            const int Num = 10000000;
            string s1 = "abcdefgh";
            string s2 = "abcdefgh";

            //string s1 = "abcdefgh";
            //string s2 = "amklklkl";
            int end;
            //Compare
            int start = Environment.TickCount;
            for (int i = 0; i < Num; i++)
            {
                string.Compare(s1, s2);
            }
            end=Environment.TickCount;
            Console.WriteLine("Compare : " + (end - start));

            //CompareTo
            start = Environment.TickCount;
            for (int i = 0; i < Num; i++)
            {
                s1.CompareTo(s2);
            }
            end = Environment.TickCount;
            Console.WriteLine("CompareTo : " + (end - start));

            //CompareOrdinal
            start = Environment.TickCount;
            for (int i = 0; i < Num; i++)
            {
                string.CompareOrdinal(s1, s2);
            }
            end = Environment.TickCount;
            Console.WriteLine("CompareOrdinal : " + (end - start));

            //静态Equals
            start = Environment.TickCount;
            for (int i = 0; i < Num; i++)
            {
                string.Equals(s1, s2);
            }
            end = Environment.TickCount;
            Console.WriteLine("静态Equals : " + (end - start));

            //实例Equals
            start = Environment.TickCount;
            for (int i = 0; i < Num; i++)
            {
                s1.Equals(s2);
            }
            end = Environment.TickCount;
            Console.WriteLine("实例Equals : " + (end - start));

　　其中,s1和s2分相等和不相等两种情况. 看结果:

1.Compare方法可能会比CompareTo快一些,因为它是静态的,调用它的开销要比实例的小一些

2.CompareOrdinal要不以上两个方法夸得多,因为它不用考虑文化方面的信息,减少了系统调用开销(当然Compare有忽略文化,区域信息比较的重载,这里没做比较)

3.Equals是一个比较迅速的操作,因为比较散列码很快.

看静态Equals:

public static bool Equals(string a, string b)
{
    return ((a == b) || (((a != null) && (b != null)) &&      EqualsHelper(a, b)));
}

实例Equals:

public bool Equals(string value)
{
    if (this == null)
    {
        throw new NullReferenceException();
    }
    if (value == null)
    {
        return false;
    }
    return (object.ReferenceEquals(this, value) || EqualsHelper(this, value));
}

　　

其中:EqualsHelper

private static unsafe bool EqualsHelper(string strA, string strB)
{
    int length = strA.Length;
    if (length != strB.Length)
    {
       return false;
    }
    fixed (char* chRef = &strA.m_firstChar)
    {
        fixed (char* chRef2 = &strB.m_firstChar)
        {
            char* chPtr = chRef;
            char* chPtr2 = chRef2;
            while (length >= 10)
            {
                if ((((*(((int*) chPtr)) != *(((int*) chPtr2))) || (*(((int*) (chPtr + 2))) != *(((int*) (chPtr2 + 2))))) || ((*(((int*) (chPtr + 4))) != *(((int*) (chPtr2 + 4)))) || (*(((int*) (chPtr + 6))) != *(((int*) (chPtr2 + 6)))))) || (*(((int*) (chPtr + 8))) != *(((int*) (chPtr2 + 8)))))
                {
                    break;
                }
                chPtr += 10;
                chPtr2 += 10;
                length -= 10;
            }

           while (length > 0)
            {
                if (*(((int*) chPtr)) != *(((int*) chPtr2)))
                {
                    break;
                }
                chPtr += 2;
                chPtr2 += 2;
                length -= 2;
            }
            return (length <= 0);
        }
    }
}