How can I convert an integer into its verbal representation?

Currently the best, most robust, library for this is definitely Humanizer. It's open sourced and available as a nuget:

Console.WriteLine(4567788.ToWords()); // => four million five hundred and sixty-seven thousand seven hundred and eighty-eight

It also has a wide range of tools solving the small problems every application has with strings, enums, DateTimes, TimeSpans and so forth, and supports many different languages.

Console.WriteLine(4567788.ToOrdinalWords().Underscore().Hyphenate().ApplyCase(LetterCasing.AllCaps)); // => FOUR-MILLION-FIVE-HUNDRED-AND-SIXTY-SEVEN-THOUSAND-SEVEN-HUNDRED-AND-EIGHTY-EIGHTH

Fully recursive version:

private static string[] ones = {
    "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", 
    "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen",
};

private static string[] tens = { "zero", "ten", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety" };

private static string[] thous = { "hundred", "thousand", "million", "billion", "trillion", "quadrillion" };

private static string fmt_negative = "negative {0}";
private static string fmt_dollars_and_cents = "{0} dollars and {1} cents";
private static string fmt_tens_ones = "{0}-{1}"; // e.g. for twenty-one, thirty-two etc. You might want to use an en-dash or em-dash instead of a hyphen.
private static string fmt_large_small = "{0} {1}"; // stitches together the large and small part of a number, like "{three thousand} {five hundred forty two}"
private static string fmt_amount_scale = "{0} {1}"; // adds the scale to the number, e.g. "{three} {million}";

public static string ToWords(decimal number) {
    if (number < 0)
        return string.format(fmt_negative, ToWords(Math.Abs(number)));

    int intPortion = (int)number;
    int decPortion = (int)((number - intPortion) * (decimal) 100);

    return string.Format(fmt_dollars_and_cents, ToWords(intPortion), ToWords(decPortion));
}

private static string ToWords(int number, string appendScale = "") {
    string numString = "";
    // if the number is less than one hundred, then we're mostly just pulling out constants from the ones and tens dictionaries
    if (number < 100) {
        if (number < 20)
            numString = ones[number];
        else {
            numString = tens[number / 10];
            if ((number % 10) > 0)
                numString = string.Format(fmt_tens_ones, numString, ones[number % 10]);
        }
    } else {
        int pow = 0; // we'll divide the number by pow to figure out the next chunk
        string powStr = ""; // powStr will be the scale that we append to the string e.g. "hundred", "thousand", etc.

        if (number < 1000) { // number is between 100 and 1000
            pow = 100; // so we'll be dividing by one hundred
            powStr = thous[0]; // and appending the string "hundred"
        } else { // find the scale of the number
            // log will be 1, 2, 3 for 1_000, 1_000_000, 1_000_000_000, etc.
            int log = (int)Math.Log(number, 1000);
            // pow will be 1_000, 1_000_000, 1_000_000_000 etc.
            pow = (int)Math.Pow(1000, log);
            // powStr will be thousand, million, billion etc.
            powStr = thous[log];
        }

        // we take the quotient and the remainder after dividing by pow, and call ToWords on each to handle cases like "{five thousand} {thirty two}" (curly brackets added for emphasis)
        numString = string.Format(fmt_large_small, ToWords(number / pow, powStr), ToWords(number % pow)).Trim();
    }

    // and after all of this, if we were passed in a scale from above, we append it to the current number "{five} {thousand}"
    return string.Format(fmt_amount_scale, numString, appendScale).Trim();
}

Current works up to the (short scale) quadrillions. Additional support (for larger numbers, or for the long scale) can be added simply by changing the thous variable.

if you use the code found in: converting numbers in to words C# and you need it for decimal numbers, here is how to do it:

public string DecimalToWords(decimal number)
{
    if (number == 0)
        return "zero";

    if (number < 0)
        return "minus " + DecimalToWords(Math.Abs(number));

    string words = "";

    int intPortion = (int)number;
    decimal fraction = (number - intPortion)*100;
    int decPortion = (int)fraction;

    words = NumericToWords(intPortion);
    if (decPortion > 0)
    {
        words += " and ";
        words += NumericToWords(decPortion);
    }
    return words;
}

In case anyone wants a JavaScript version

Number.prototype.numberToWords = function () {
    var unitsMap = ["zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"];
    var tensMap = ["zero", "ten", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"];

    var num = this.valueOf();
    if (Math.round(num == 0)) {
        return "zero";
    }
    if (num < 0) {
        var positivenum = Math.abs(num);
        return "minus " + Number(positivenum).numberToWords();
    }
    var words = "";
    if (Math.floor(num / 1000000) > 0) {
        words += Math.floor(num / 1000000).numberToWords() + " million ";
        num = Math.floor(num % 1000000);
    }
    if (Math.floor(num / 1000) > 0) {
        words += Math.floor(num / 1000).numberToWords() + " thousand ";
        num = Math.floor(num % 1000);
    }
    if (Math.floor(num / 100) > 0) {
        words += Math.floor(num / 100).numberToWords() + " hundred ";
        num = Math.floor(num % 100);
    }
    if (Math.floor(num > 0)) {
        if (words != "") {
            words += "and ";
        }
        if (num < 20) {
        words += unitsMap[num];
        }
        else {
            words += tensMap[Math.floor(num / 10)];
            if ((num % 10) > 0) {
                words += "-" + unitsMap[Math.round(num % 10)];
            }
        }
    }
    return words.trim();
}