public string swapChar(string s)
        {
            char[] newStr = s.ToCharArray();
            newStr[0] = s[10];
            newStr[1] = s[4];
            newStr[10] = s[5];
            newStr[4] = s[7];
            newStr[8] = s[2];
            newStr[10] = s[3];
            return new string(newStr);
        }

using System.Linq;

var newStr = new string(str.ToCharArray().OrderBy(e => Guid.NewGuid()));