Spaces.cs

using System;

public class Spaces {
    public static void Main(string[] args) {
        var s = "12345678";
        var r = InjectSpaces(s); // r == "1 2 3 4"
        for (int i = 0; i < 200000000; ++i) {
            r = InjectSpaces(s);
        }
        Console.WriteLine("'{0}' -> '{1}'", s, r);
    }

    static unsafe string InjectSpaces(string s) {
        fixed (char* pS = s) {
            var pLength = (int*)pS - 1;
            var length = *pLength & 0x3fffffff;
            var result = new string(' ', 2*length - 1);
            fixed (char* pResult = result) {
                for (int i = 0; i < length; ++i) {
                    pResult[2*i] = pS[i];
                }
            }
            return result;
        }
    }
    
    static string InjectSpaces2(string s) {
        var ra = new char[s.Length * 2 - 1];
        ra[0] = s[0];
        for (int i = 1; i < s.Length; ++i) {
            ra[2*i - 1] = ' ';
            ra[2*i] = s[i];
        }

        return new string (ra);
    }
}