Convert Infix to Postfix in a string in C# .Net

i am writing a program and it works compeletly 100% i take a txt file and read the files from it from infix notation and i transfer them into postfix it works for my first 3 expressions and then after my 3rd expression it goes into a infinite loop? it does not display the rest of my expressions i am sovery confused. i debugged it and i did nto understand why it solves everything before this with no problem at all but then this happens 

 public static string ChangePostfix(string infix)
        {
            StringBuilder postfix = new StringBuilder();
            char c;
            var stack = new Stack<char>();

            stack.Push('(');
            infix = infix + ")";

            int i = 0;
            while (stack.Count != 0)
            {
                if (infix[i] >= '0' && infix[i] <= '9')
                {
                    postfix.Append(infix[i]);
                }
                else if (infix[i] == '(')
                {
                    stack.Push(infix[i]);
                }
                else if (infix[i] == '*' || infix[i] == '/' || infix[i] == '%' || infix[i] == '+' || infix[i] == '-')
                {
                    c = stack.Peek();
                    while (stack.Count != 0 && (c == '*' || c == '/' || c == '%' || c == '+' || c == '-'))
                    {
                        if (Precedence(infix[i], c))
                            postfix.Append(stack.Pop());
                        c = stack.Peek();
                    }
                    stack.Push(infix[i]);
                }
                else if (infix[i] == ')')
                {
                    for (int j = 0; j < stack.Count && infix[i] != '('; j++)
                    {
                        postfix.Append(stack.Pop());
                    }
                    stack.Pop();
                }
                i++;
            }
            return postfix.ToString();
        }

        static bool Precedence(char one, char two)
        {
            if (two == '*' || two == '/' || two == '%')
                return true;
            if (one == '*' || one == '/' || one == '%')
                return false;
            return true;
        }


        static void Main(string[] args)
        {
            try
            {
                string[] expressions = File.ReadAllLines(@"expressions.txt");
                string infix = string.Empty;
                Console.WriteLine("Infix Expression \t\t Postfix Expression \t\t Value");
                string postfix = "";
                foreach (string str in expressions)
                {
                    infix = str;
                    Console.Write(str + "\t\t");

                    //postfix change
                    postfix = ChangePostfix(infix);
                    Console.Write(postfix);
                    Console.WriteLine();
                    //solve value
                }

            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }
            Console.ReadLine();
        }
    }
}

EXPRESSIONS TXT

1 + 2 - 3 + 4 - 5 1 * 2 * 3 / 2 / 3 1 + 2 * 3 - 4 / 2 1 * 2 + 3 - 4 / 5