How to replace a string without using REPLACE function using C#?

This code snippet is used to replace the string without using the replace function using C#.Most of the times we get a requirement to replace the String with a certain string.

class StringReplaceWithOutUsingFunction
    {
        public static void Main(string[] args)
        {
            string sentence = "Welcome to English Club Grammar for English learners.";
            string[] parts = sentence.Split(' ');
            string matchString = "English";
            string replaceString = "engLISh";
            for (int i = 0; i < parts.Count(); i++)
            {
                parts[i] = parts[i].Length == 7 == true && parts[i] == matchString == true ? parts[i] = replaceString : parts[i];
            }
            foreach (string item in parts)
            {
                Console.Write(item + " ");
            }
            Console.ReadKey();
        }

    }