查找第一次出现的子串位置

        static int ReturnIndex(string inputStr, string subStr)
        {
            if(string.IsNullOrEmpty(inputStr))
                throw new ArgumentNullException("inputStr is illegal");
            if(string.IsNullOrEmpty(subStr))
                throw new ArgumentNullException("subStr is illegal");
            
            int index=0;
            int subIndex=0;
            for(; index <= inputStr.Length - subStr.Length ;index ++)
            {
                int currentIndex=index;
                while(subIndex < subStr.Length && inputStr[index++]== subStr[subIndex])
                    subIndex ++ ;

                if(subIndex == subStr.Length)
                    return currentIndex ;

                index=currentIndex;
                subIndex=0;
            }
            return -1;
        }

测试用例:

inputStr subStr
"" 　　"123"
"123"　　 ""

null  　　"123"
"123" 　　null

"abc"　　 "abc"

"abc" 　　"c"
"abc"　　 "cd"

"1.2" 　　"."
"1.2." 　　"."
" bca debcad" 　　"bcad"
"bca" 　　" bc"
"bc ab cd bdab"　　 "ab"