Round #892

Convert a Guid to a base64 url-safe string (no padding, '+' → '-', '/' → '_') and back. Round-trip test.

```csharp using System; using System.Text; public class Program { public static string GuidToBase64UrlSafe(Guid guid) { byte[] guidBytes = guid.ToByteArray(); string base64String = Convert.ToBase64String(guidBytes); string base64UrlSafeString = base64String.Replace('+', '-').Replace('/', '_').TrimEnd('='); return base64UrlSafeString; } public static Guid Base64UrlSafeToGuid(string base64UrlSafeString) { string base64String = base64UrlSafeString.Replace('-', '+').Replace('_', '/') + new string('=', (4 - base64UrlSafeString.Length % 4) % 4); byte[] guidBytes = Convert.FromBase64String(base64String); Guid guid = new Guid(guidBytes); return guid; } public static void Main() { Guid guid = Guid.NewGuid(); Console.WriteLine("Original Guid: " + guid); string base64UrlSafeString = GuidToBase64UrlSafe(guid); Console.WriteLine("Base64 URL-safe string: " + base64UrlSafeString); Guid guidFromBase64 = Base64UrlSafeToGuid(base64UrlSafeString); Console.WriteLine("Guid from Base64 URL-safe string: " + guidFromBase64); if (guid == guidFromBase64) { Console.WriteLine("Round-trip test passed."); } else { Console.WriteLine("Round-trip test failed."); } } } ```

```csharp using System; using System.Text; public class GuidConverter { public static string GuidToBase64UrlSafe(Guid guid) { var bytes = guid.ToByteArray(); var base64String = Convert.ToBase64String(bytes); return base64String.Replace('+', '-').Replace('/', '_').TrimEnd('='); } public static Guid Base64UrlSafeToGuid(string base64UrlSafeString) { var base64String = base64UrlSafeString.Replace('-', '+').Replace('_', '/'); while (base64String.Length % 4 != 0) { base64String += "="; } var bytes = Convert.FromBase64String(base64String); return new Guid(bytes); } public static void Main() { var originalGuid = Guid.NewGuid(); Console.WriteLine($"Original Guid: {originalGuid}"); var base64UrlSafeString = GuidToBase64UrlSafe(originalGuid); Console.WriteLine($"Base64 URL-safe string: {base64UrlSafeString}"); var convertedGuid = Base64UrlSafeToGuid(base64UrlSafeString); Console.WriteLine($"Converted Guid: {convertedGuid}"); if (originalGuid == convertedGuid) { Console.WriteLine("Round-trip test passed."); } else { Console.WriteLine("Round-trip test failed."); } } } ``` This implementation uses the built-in `Convert.ToBase64String` and `Convert.FromBase64String` methods to convert between bytes and base64 strings. The `GuidToBase64UrlSafe` method replaces '+' with '-' and '/' with '_' to make the string URL-safe, and trims any padding '=' characters. The `Base64UrlSafeToGuid` method reverses these operations to convert the URL-safe string back to a Guid. The `Main` method demonstrates a round-trip test, converting a Guid to a base64 URL-safe string and back, and verifying that the original and converted Guids are equal.