ToCharArray变为字符数组后在每个元素后添加空格，请问如何修改，谢谢

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "this is a test";
            char[] c = s.ToCharArray();
            for (int i = 0; i < c.Length; i++)
            {
                c[i] = c[i] + ' ';      //此句出错
                Console.WriteLine("{0}", c[i]);
            }
            Console.ReadKey();
        }
    }
}