CSUlidRng.cs

using System;
using System.Security.Cryptography;

namespace NUlid.Rng;

/// <summary>
/// A cryptographically secure RNG for the random part of ulid's.
/// </summary>
public class CSUlidRng : BaseUlidRng
{
#if !NET6_0_OR_GREATER
    // We only need one, single, instance of an RNG so we keep it around.
    private static readonly RNGCryptoServiceProvider _rng = new();
#endif

    /// <summary>
    /// Creates and returns cryptographically secure random bytes.
    /// </summary>
    /// <param name="dateTime">DateTime for which the random bytes need to be generated; is ignored.</param>
    /// <returns>Random bytes.</returns>
    public override byte[] GetRandomBytes(DateTimeOffset dateTime)
#if NET6_0_OR_GREATER
        => RandomNumberGenerator.GetBytes(RANDLEN);
#else
    {
        var buffer = new byte[RANDLEN];
        _rng.GetBytes(buffer);
        return buffer;
    }
#endif

    /// <summary>
    /// Fills the <paramref name="buffer"/> with returns cryptographically secure random bytes.
    /// </summary>
    /// <param name="buffer">The buffer to fill with cryptographically secure random bytes.</param>
    /// <param name="dateTime">>DateTime for which the random bytes need to be generated; is ignored.</param>
    /// <exception cref="ArgumentException">The buffer is too small.</exception>
    public override void GetRandomBytes(Span<byte> buffer, DateTimeOffset dateTime)
    {
#if NET6_0_OR_GREATER
        if (buffer.Length < RANDLEN)
        {
            Throw(buffer.Length);
            static void Throw(int len) => throw new ArgumentException($"The given buffer must be at least {RANDLEN} bytes long, actual: {len}");
        }

        RandomNumberGenerator.Fill(buffer);
#else
        var tmp = new byte[RANDLEN];
        _rng.GetBytes(tmp);
        tmp.AsSpan().CopyTo(buffer);
#endif
    }
}