C# formula from string

public static double Evaluate(string expr)
    {
        expr = expr.ToLower();
        expr = expr.Replace(" ", "");
        expr = expr.Replace("true", "1");
        expr = expr.Replace("false", "0");

        Stack<String> stack = new Stack<String>();

        string value = "";
        for (int i = 0; i < expr.Length; i++)
        {
            String s = expr.Substring(i, 1);
            // pick up any doublelogical operators first.
            if (i < expr.Length - 1)
            {
                String op = expr.Substring(i, 2);
                if (op == "<=" || op == ">=" || op == "==")
                {
                    stack.Push(value);
                    value = "";
                    stack.Push(op);
                    i++;
                    continue;
                }
            }

            char chr = s.ToCharArray()[0];

            if (!char.IsDigit(chr) && chr != '.' && value != "")
            {
                stack.Push(value);
                value = "";
            }
            if (s.Equals("("))
            {
                string innerExp = "";
                i++; //Fetch Next Character
                int bracketCount = 0;
                for (; i < expr.Length; i++)
                {
                    s = expr.Substring(i, 1);

                    if (s.Equals("(")) bracketCount++;

                    if (s.Equals(")"))
                    {
                        if (bracketCount == 0) break;
                        bracketCount--;
                    }
                    innerExp += s;
                }
                stack.Push(Evaluate(innerExp).ToString());
            }
            else if (s.Equals("+") ||
                     s.Equals("-") ||
                     s.Equals("*") ||
                     s.Equals("/") ||
                     s.Equals("<") ||
                     s.Equals(">"))
            {
                stack.Push(s);
            }
            else if (char.IsDigit(chr) || chr == '.')
            {
                value += s;

                if (value.Split('.').Length > 2)
                    throw new Exception("Invalid decimal.");

                if (i == (expr.Length - 1))
                    stack.Push(value);

            }
            else
            {
                throw new Exception("Invalid character.");
            }

        }
        double result = 0;
        List<String> list = stack.ToList<String>();
        for (int i = list.Count - 2; i >= 0; i--)
        {
            if (list[i] == "/")
            {
                list[i] = (Convert.ToDouble(list[i - 1]) / Convert.ToDouble(list[i + 1])).ToString();
                list.RemoveAt(i + 1);
                list.RemoveAt(i - 1);
                i -= 2;
            }
        }

        for (int i = list.Count - 2; i >= 0; i--)
        {
            if (list[i] == "*")
            {
                list[i] = (Convert.ToDouble(list[i - 1]) * Convert.ToDouble(list[i + 1])).ToString();
                list.RemoveAt(i + 1);
                list.RemoveAt(i - 1);
                i -= 2;
            }
        }
        for (int i = list.Count - 2; i >= 0; i--)
        {
            if (list[i] == "+")
            {
                list[i] = (Convert.ToDouble(list[i - 1]) + Convert.ToDouble(list[i + 1])).ToString();
                list.RemoveAt(i + 1);
                list.RemoveAt(i - 1);
                i -= 2;
            }
        }
        for (int i = list.Count - 2; i >= 0; i--)
        {
            if (list[i] == "-")
            {
                list[i] = (Convert.ToDouble(list[i - 1]) - Convert.ToDouble(list[i + 1])).ToString();
                list.RemoveAt(i + 1);
                list.RemoveAt(i - 1);
                i -= 2;
            }
        }
        stack.Clear();
        for (int i = 0; i < list.Count; i++)
        {
            stack.Push(list[i]);
        }
        while (stack.Count >= 3)
        {
            double right = Convert.ToDouble(stack.Pop());
            string op = stack.Pop();
            double left = Convert.ToDouble(stack.Pop());

            if (op == "<") result = (left < right) ? 1 : 0;
            else if (op == ">") result = (left > right) ? 1 : 0;
            else if (op == "<=") result = (left <= right) ? 1 : 0;
            else if (op == ">=") result = (left >= right) ? 1 : 0;
            else if (op == "==") result = (left == right) ? 1 : 0;

            stack.Push(result.ToString());
        }
        return Convert.ToDouble(stack.Pop());
    }

Source: stackoverflow.com

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