Reverse string using Recursion and loop in c#

Method I. 

       ///// Call- ReverseString('abcdefg');

        //Recursion method; simple, for small strings 

        public string ReverseString(string str)

        {

            if (str.Length <= 1) return str;

            else return ReverseString(str.Substring(1)) + str[0];

        }

     ///// Result- ReverseString - gfedcba

Method II. 

       ///// Call- ReverseString('gfedcba', 0);

       //Recursion method; Need to starting index as 0,  for long strings

        public string ReverseString(string str, int Chrindex)

        {

            char[] chr = str.ToCharArray();

            int len = chr.Length;

            if (Chrindex < len / 2)

            {

                char c = chr[Chrindex];

                chr[Chrindex] = chr[len - Chrindex - 1];

                chr[len - Chrindex - 1] = c;

                Chrindex++;

                return ReverseString(new string(chr), Chrindex);

            }

            else

            {

                return new string(chr);

            }

        }

     ///// Result- ReverseString - gfedcba

Method III.

       ///// Call- ReverseString('abcdefg');

        //Using Another array, need to traverse full array. 

        public string ReverseString(string str)

        {

            char[] chr = str.ToCharArray();

            char[] result = new char[chr.Length];

            for (int i = 0, j = str.Length - 1; i < str.Length; i++, j--)

            {

                result[i] = chr[j];

            }

            return new string(result);

        }

     ///// Result- ReverseString - gfedcba

Method IV.

       ///// Call- ReverseString('abcdefg');

      //Using swap method; need to traverse only half of the array. 

        public string ReverseString(string str)

        {

            char[] chr = str.ToCharArray();

            for (int i = 0, j = str.Length - 1; i < j; i++, j--)

            {

                char c = chr[i];

                chr[i] = chr[j];

                chr[j] = c;

            }

            return new string(chr);

        }

     ///// Result- ReverseString - gfedcba