Reverse word in a string

Reverse word in a string without using inbuilt function in C#

class ResverseWord

    {

        public static void Main()

        {

            string s = "Hi , how are you ?";

            //use for store string into list of word

            List<string> lst = new List<string>();

            //use for populate reverse word string

            string finalString = string.Empty;

            string str = string.Empty;

            for (int i=0;i<s.Length;i++)

            {

                if(s[i]==' ')

                {

                  

                    lst.Add(str);

                    str = "";

                }

                else

                {

                    str += s[i];

                }

                if(i==s.Length-1)

                    lst.Add(str);

            }

            for(int i=lst.Count-1;i>=0;i--)

            {

                finalString += lst[i]+" ";

            }

            Console.WriteLine(finalString);

            Console.ReadLine();

        }

    }