common/AMWD.Common/Extensions/StringExtensions.cs

using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Net;
using System.Net.Mail;
using System.Text;
using System.Text.RegularExpressions;
using DnsClient;

namespace System
{
	/// <summary>
	/// String extensions.
	/// </summary>
	public static class StringExtensions
	{
		/// <summary>
		/// Converts a hex string into a byte array.
		/// </summary>
		/// <param name="hexString">The hex encoded string.</param>
		/// <param name="delimiter">A delimiter between the bytes (e.g. for MAC address).</param>
		/// <returns>The bytes.</returns>
		public static IEnumerable<byte> HexToBytes(this string hexString, string delimiter = "")
		{
			if (string.IsNullOrWhiteSpace(hexString))
				yield break;

			string str = string.IsNullOrEmpty(delimiter) ? hexString : hexString.Replace(delimiter, "");
			if (str.Length % 2 == 1)
				yield break;

			if (Regex.IsMatch(str, "[^0-9a-fA-F]"))
				yield break;

			for (int i = 0; i < str.Length; i += 2)
				yield return Convert.ToByte(str.Substring(i, 2), 16);
		}

		/// <summary>
		/// Converts a byte collection into a hex string.
		/// </summary>
		/// <param name="bytes">The bytes.</param>
		/// <param name="delimiter">A delimiter to set between the bytes (e.g. for MAC address).</param>
		/// <returns>The hex encoded string.</returns>
		public static string BytesToHex(this IEnumerable<byte> bytes, string delimiter = "")
		{
			if (bytes?.Any() != true)
				return null;

			return string.Join(delimiter, bytes.Select(b => b.ToString("x2")));
		}

		/// <summary>
		/// Encodes a string to the hexadecimal system (base 16).
		/// </summary>
		/// <param name="str">The string to encode hexadecimal.</param>
		/// <param name="encoding">The text encoding to use (default: <see cref="Encoding.Default"/>)</param>
		/// <returns></returns>
		public static string HexEncode(this string str, Encoding encoding = null)
		{
			if (string.IsNullOrEmpty(str))
				return str;

			return (encoding ?? Encoding.Default).GetBytes(str).BytesToHex();
		}

		/// <summary>
		/// Decodes a string from the hexadecimal system (base 16).
		/// </summary>
		/// <param name="str">The hexadecimal encoded string to decode.</param>
		/// <param name="encoding">The text encoding to use (default: <see cref="Encoding.Default"/>)</param>
		/// <returns></returns>
		public static string HexDecode(this string str, Encoding encoding = null)
		{
			if (string.IsNullOrEmpty(str))
				return str;

			return (encoding ?? Encoding.Default).GetString(str.HexToBytes().ToArray());
		}

		/// <summary>
		/// Encodes a string to base64.
		/// </summary>
		/// <param name="str">The string to encode with base64.</param>
		/// <param name="encoding">The text encoding to use (default: <see cref="Encoding.Default"/>)</param>
		/// <returns></returns>
		public static string Base64Encode(this string str, Encoding encoding = null)
			=> Convert.ToBase64String((encoding ?? Encoding.Default).GetBytes(str));

		/// <summary>
		/// Decodes a string from base64.
		/// </summary>
		/// <param name="str">The base64 encoded string to decode.</param>
		/// <param name="encoding">The text encoding to use (default: <see cref="Encoding.Default"/>)</param>
		/// <returns></returns>
		public static string Base64Decode(this string str, Encoding encoding = null)
			=> (encoding ?? Encoding.Default).GetString(Convert.FromBase64String(str));

		/// <summary>
		/// Replaces the search substring with the replacement when it was found at the beginning of the string.
		/// </summary>
		/// <param name="str">The string.</param>
		/// <param name="search">The searched substring.</param>
		/// <param name="replacement">The replacement.</param>
		/// <returns></returns>
		public static string ReplaceStart(this string str, string search, string replacement)
		{
			if (str.StartsWith(search))
				return replacement + str.Substring(search.Length);

			return str;
		}

		/// <summary>
		/// Replaces the search substring with the replacement when it was found at the end of the string.
		/// </summary>
		/// <param name="str">The string.</param>
		/// <param name="search">The searched substring.</param>
		/// <param name="replacement">The replacement.</param>
		/// <returns></returns>
		public static string ReplaceEnd(this string str, string search, string replacement)
		{
			if (str.EndsWith(search))
				return str.Substring(0, str.Length - search.Length) + replacement;

			return str;
		}

		/// <summary>
		/// Parses a <see cref="string"/> to a <see cref="decimal"/> using culture "de" or invariant.
		/// </summary>
		/// <param name="decString">The string to parse.</param>
		/// <returns></returns>
		public static decimal ParseDecimal(this string decString)
		{
			int dotIndex = decString.LastIndexOf('.');
			int commaIndex = decString.LastIndexOf(',');

			var culture = dotIndex < commaIndex ? new CultureInfo("de-DE") : CultureInfo.InvariantCulture;

			return decimal.Parse(decString, culture);
		}

		/// <summary>
		/// Checks whether the given <see cref="string"/> is a valid <see cref="MailAddress"/>.
		/// </summary>
		/// <remarks>
		/// You can enhance the check by requesting the MX record of the domain.
		/// </remarks>
		/// <param name="email">The email address as string to validate.</param>
		/// <param name="checkRecordExists">A value indicating whether to resolve a MX record.</param>
		/// <returns><c>true</c> when the email address is valid, other wise <c>false</c>.</returns>
		public static bool IsValidEmailAddress(this string email, bool checkRecordExists = false)
			=> email.IsValidEmailAddress(checkRecordExists ? new LookupClient() : null);

		/// <summary>
		/// Checks whether the given <see cref="string"/> is a valid <see cref="MailAddress"/>.
		/// </summary>
		/// <remarks>
		/// The check is enhanced by a request for MX records on the defined <paramref name="nameservers"/>.
		/// </remarks>
		/// <param name="email">The email address as string to validate.</param>
		/// <param name="nameservers">A list of <see cref="IPEndPoint"/>s of nameservers.</param>
		/// <returns><c>true</c> when the email address is valid, other wise <c>false</c>.</returns>
		public static bool IsValidEmailAddress(this string email, IEnumerable<IPEndPoint> nameservers)
			=> email.IsValidEmailAddress(new LookupClient(nameservers.ToArray()));

		private static bool IsValidEmailAddress(this string email, LookupClient lookupClient)
		{
			try
			{
				var mailAddress = new MailAddress(email);
				bool isValid = mailAddress.Address == email;

				if (isValid && lookupClient != null)
				{
					var result = lookupClient.Query(mailAddress.Host, QueryType.MX);
					isValid &= result.Answers.MxRecords().Any();
				}

				return isValid;
			}
			catch
			{
				return false;
			}
		}
	}
}