String.Masked() extension method

StringMaskedExtension.cs

    public static class StringExtensions
    {
        /// <summary>
        /// Overwrite the right-most fraction 1/<paramref name="maskFractionOneOver"/> of <paramref name="str"/> with <paramref name="maskCharacter"/>
        /// </summary>
        /// <param name="str"></param>
        /// <param name="maskFractionOneOver"></param>
        /// <param name="maskCharacter"></param>
        /// <example>"1234".Masked() => "123*"</example>
        /// <returns>the masked string</returns>
        public static string Masked(this string str, int maskFractionOneOver = 5, char maskCharacter = '*')
        {
            if (string.IsNullOrEmpty(str)) return str;
            maskFractionOneOver = Math.Max(maskFractionOneOver, 1);
            int fraction = (maskFractionOneOver-1) * str.Length / maskFractionOneOver;
            return str.Substring(0,fraction)  .PadRight(str.Length, maskCharacter);
        }
    }