נושא: שאלה בסיסית בC#

string str = "gioiGag";
            char ch = 'g';
            Console.WriteLine(str);
            string res = checkandreplace(str, ch);
        }
        static string checkandreplace(string str, char ch)
        {
            ch = ch.ToUpper();
            string letter = ch.ToString();//for comparing it to other string
            
            for (int i = 0; i < str.Length; i++)
            {
                if (str == ch)
                {
                     str = str.ToUpper();
                }

                else if (str != ch)
                {
                     str = str.ToLower();
                     if (str != ch)
                     {
                          str = str.ToUppper();


                     }

                }
                return str;
            }