SequentialGuidValueGenerator.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Runtime.InteropServices;

namespace Microsoft.EntityFrameworkCore.ValueGeneration;

/// <summary>
///     Generates sequential <see cref="Guid" /> values optimized for use in Microsoft SQL server clustered
///     keys or indexes, yielding better performance than random values. This is the default generator for
///     SQL Server <see cref="Guid" /> columns which are set to be generated on add.
/// </summary>
/// <remarks>
///     <para>
///         Although this generator achieves the same goals as SQL Server's NEWSEQUENTIALID, the algorithm used
///         to generate the GUIDs is different. See
///         <see href="https://docs.microsoft.com/sql/t-sql/functions/newsequentialid-transact-sql">NEWSEQUENTIALID</see>
///         for more information on the advantages of sequential GUIDs.
///     </para>
///     <para>
///         The generated values are non-temporary, meaning they will be saved to the database.
///     </para>
///     <para>
///         See <see href="https://aka.ms/efcore-docs-value-generation">EF Core value generation</see> for more information and examples.
///     </para>
/// </remarks>
public class SequentialGuidValueGenerator : ValueGenerator<Guid>
{
    private long _counter = DateTime.UtcNow.Ticks;

    /// <summary>
    ///     Gets a value to be assigned to a property.
    /// </summary>
    /// <param name="entry">The change tracking entry of the entity for which the value is being generated.</param>
    /// <returns>The value to be assigned to a property.</returns>
    public override Guid Next(EntityEntry entry)
    {
        Span<byte> guidBytes = stackalloc byte[16];
        var succeeded = Guid.NewGuid().TryWriteBytes(guidBytes);
        Check.DebugAssert(succeeded, "Could not write Guid to Span");
        var incrementedCounter = Interlocked.Increment(ref _counter);
        Span<byte> counterBytes = stackalloc byte[sizeof(long)];
        MemoryMarshal.Write(counterBytes, in incrementedCounter);

        if (!BitConverter.IsLittleEndian)
        {
            counterBytes.Reverse();
        }

        guidBytes[08] = counterBytes[1];
        guidBytes[09] = counterBytes[0];
        guidBytes[10] = counterBytes[7];
        guidBytes[11] = counterBytes[6];
        guidBytes[12] = counterBytes[5];
        guidBytes[13] = counterBytes[4];
        guidBytes[14] = counterBytes[3];
        guidBytes[15] = counterBytes[2];

        return new Guid(guidBytes);
    }

    /// <summary>
    ///     Gets a value indicating whether the values generated are temporary or permanent. This implementation
    ///     always returns false, meaning the generated values will be saved to the database.
    /// </summary>
    public override bool GeneratesTemporaryValues
        => false;
}