Program.cs

// See https://aka.ms/new-console-template for more information
Console.WriteLine("Hello, World!");

public class TTSTools
{
    private static readonly string[] ones = { "", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

    private static readonly string[] teens = { "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };

    private static readonly string[] tens = { "", "", "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety" };

    private static readonly string[] thousands = { "", "thousand", "million", "billion", "trillion" };

    public static string convertNumbertoString(int number)
    {
        if (number == 0)
        {
            return "zero";
        }
        string text = "";
        for (int i = 0; i < number; i++)
        {
            if (number % 1000 != 0)//if number is greater than a thousand
            {
                text = convertHundreds(number % 1000) + thousands[i] + " " + text;
            }

            number /= 1000; // remove thousand digit
        }

        return text.Trim();

    }

    public static string convertHundreds(int number)
    {
        string words = "";

        if (number >= 100)
        {
            words += ones[number / 100] + " hundred ";//count hundreds
        }

        if (number >- 10 && number <19)//if bigger than 9 and less than 20
        {
            words += tens[number % 10] + " ";//count tens
        }

        else
        {
            words += tens[number / 10] + " ";//count normal tens
            words += ones[number % 10] + " ";//count single digits
        }

        return words;
    }

    public static string replaceAcronyms(string text)
    {
        text.Replace("dr", "doctor");
        text.Replace("mr", "mister");
        text.Replace("mrs", "missus");
        text.Replace("ms", "miss");
        text.Replace("e.g.", "e g");
        text.Replace("+", "plus ");
        text.Replace("-", "minus ");
        text.Replace("#", "hashtag ");
        text.Replace("=", "equals ");
        text.Replace("%", "mod ");
        text.Replace("/", "slash ");
        text.Replace("etc", "etcetera ");
        return text;
    }

    private string NormaliseText(string text)
    {
        bool numberStarted = false;
        bool decimalStarted = false;
        string newText = "";
        for (int i = 0; i < text.Length; i++)
        {
            if (int.TryParse(text[i].ToString(), out int result) == true)//if character is number
            {
                if (i! < 2 && !decimalStarted)//if not first 2 characters and a decimal hasn't started
                {
                    if (text[i - 1].ToString() == "." && int.TryParse(text[i - 2].ToString(), out int result2) == true)//if previous character was decimal AND the character before that was a number
                    {
                        decimalStarted = true;//decimal number counting started

                        newText.Remove(i - 1);//delete the last character
                        newText += "point ";//decimal started
                    }
                }

                if (!decimalStarted)//if not in a decimal
                {
                    numberStarted = true;//Number counting has started
                    int number = result;//Numberstarted
                    int indent = 1;//check next number
                    do //while not final character
                    {
                        if (int.TryParse(text[i + indent].ToString(), out result) == true)//if next number
                        {
                            number = number * 10 + result;//then add next number to back of "number"
                            indent++;//increment next number
                        }
                        else
                        {
                            numberStarted = false; //Numberended
                        }
                    } while (i != text.Length - 1 && numberStarted);//while not final character

                    i += indent;//skip rest of numbers
                }
                else//decimal
                {
                    newText += ones[result % 10] + " ";//add single digit
                }
            }
            else//if not number
            {
                if (decimalStarted)//disable decimal started
                {
                    decimalStarted = false;
                }

                newText += text[i];
            }
        }
        
        newText = newText.ToLower();

        newText = replaceAcronyms(newText);

        return newText;
    }
}