Windows Forms Currency and Thousand Format (NAIVE) Based on => https://github.com/igorescobar/jQuery-Mask-Plugin/blob/d4b031bdcfe367d4372fc13de0dff024e2bef768/src/jquery.mask.js#L271

Formatar.cs

protected void MaskedOnKeyUp(object sender, KeyEventArgs e)
{   
    textBox.Text = Mascara(textBox.Text, "#.##0,00");
    textBox.Select(textBox.Text.Length, 0);
}

static string Mascara(string valor, string mascara)
{
    var translations = new Dictionary<char, Tuple<string, bool>>
        {
            { '0', new Tuple<string, bool>("\\d", false) },
            { '#', new Tuple<string, bool>("\\d", true) },
        };

    var buffer = new List<char>();
    var value = valor == null ? "" : valor + "";
    value = value.Replace(",", string.Empty).Replace(".", string.Empty);
    var lengthMask = mascara.Length;
    var legthValue = value.Length;
    var maskIndex = lengthMask - 1;
    var valueIndex = legthValue - 1;
    var offset = -1;
    var resetPos = -1;
    var maskDigitCount = 0;
    var lastMaskChar = 0;
    var maskDigitPosArr = new List<int>();
    Func<int, int, bool> verificarSeContinua = (mm, vv) => mm > -1 && vv > -1;

    char? lastUntranslatedMaskChar = null;
    while (verificarSeContinua(maskIndex, valueIndex))
    {
        var maskDigit = mascara[maskIndex];
        var valDigit = value[valueIndex];

        if (translations.TryGetValue(maskDigit, out Tuple<string, bool> translation))
        {
            if (Regex.IsMatch(valDigit.ToString(), translation.Item1))
            {
                buffer.Insert(0, valDigit);

                if (translation.Item2)
                {
                    if (resetPos == -1)
                    {
                        resetPos = maskIndex;
                    }
                    else if (maskIndex == lastMaskChar && maskIndex != resetPos)
                    {
                        maskIndex = resetPos - offset;
                    }

                    if (lastMaskChar == resetPos)
                    {
                        maskIndex -= offset;
                    }
                }

                maskIndex += offset;
            }
            else if (valDigit == lastUntranslatedMaskChar)
            {
                maskDigitCount--;
                lastUntranslatedMaskChar = null;
            }
            else { }

            valueIndex += offset;
        }
        else
        {

            buffer.Insert(0, maskDigit);

            if (valDigit == maskDigit)
            {
                maskDigitPosArr.Add(valueIndex);
                valueIndex += offset;
            }
            else
            {
                lastUntranslatedMaskChar = maskDigit;
                maskDigitPosArr.Add(valueIndex + maskDigitCount);
                maskDigitCount++;
            }

            maskIndex += offset;
        }
    }

    return string.Join("", buffer);

}