Why is EF Core trying to insert nulls when using custom ValueGenerator?

Question

Question updated to reflect more detail and a simplified reproduction case as I have dug deeper with the help of comments.

I have a custom ValueGenerator as follows:

public class UtcDateTimeGenerator : ValueGenerator<DateTime>
{
    public override DateTime Next(EntityEntry entry)
    {
        // This method never seems to be called
        return DateTime.UtcNow;
    }

    protected override object NextValue(EntityEntry entry)
    {
        // This one is called.
        return DateTime.UtcNow;
    }

    public override bool GeneratesTemporaryValues => false;
}

My Entity:

public abstract class AggregateRoot<TId>
    where TId : IComparable
{
    public TId Id { get; set; }
}

public abstract class AuditedAggregateRoot<TId> : AggregateRoot<TId>
    where TId : IComparable
{
    public DateTime Created { get; internal set; }
    public DateTime LastModified { get; internal set; }
}

public class Player : AuditedAggregateRoot<Guid>
{
    public string ExternalId { get; internal set; }
    public string DisplayName { get; internal set; }
    public string Email { get; internal set; }
    public DateTime LastLogin { get; internal set; }

    /// <summary>
    /// Required for Entity Framework Core
    /// </summary>
    private Player()
    {
    }

    public Player([NotNull]string externalId, [NotNull]string displayName, [NotNull]string email)
    {
        ExternalId = externalId;
        DisplayName = displayName;
        Email = email;
    }
}

My entity's IEntityTypeConfiguration:

public abstract class AggregateRootTypeConfiguration<TEntity, TKey> : IEntityTypeConfiguration<TEntity>
    where TEntity : AggregateRoot<TKey>
    where TKey : IComparable
{
    public virtual void Configure(EntityTypeBuilder<TEntity> builder)
    {
        builder.HasKey(x => x.Id);
        builder.Property(x => x.Id)
            .HasValueGenerator<SequentialGuidValueGenerator>()
            .ValueGeneratedOnAdd()
            .IsRequired();
    }
}

public class AuditedAggregateRootTypeConfiguration<TEntity, TKey> : AggregateRootTypeConfiguration<TEntity, TKey>
    where TEntity : AuditedAggregateRoot<TKey>
    where TKey : IComparable
{
    public override void Configure(EntityTypeBuilder<TEntity> builder)
    {
        base.Configure(builder);
        builder.Property(x => x.Created)
            .HasValueGenerator<UtcDateTimeGenerator>()
            .ValueGeneratedOnAdd()
            .IsRequired();

        builder.Property(x => x.LastModified)
            .HasValueGenerator<UtcDateTimeGenerator>()
            .ValueGeneratedOnAddOrUpdate()
            .IsRequired();
    }
}

public class PlayerEntityTypeConfiguration : AuditedAggregateRootTypeConfiguration<Player, Guid>
{
    public override void Configure(EntityTypeBuilder<Player> builder)
    {
        base.Configure(builder);
        builder.Property(x => x.Email)
            .HasMaxLength(254)
            .IsRequired();

        builder.Property(x => x.ExternalId)
            .HasMaxLength(200)
            .IsRequired();

        builder.Property(x => x.DisplayName)
            .HasMaxLength(200)
            .IsRequired();

        builder.HasIndex(x => x.ExternalId).IsUnique();
        builder.HasIndex(x => x.Email).IsUnique();
        builder.HasIndex(x => x.DisplayName).IsUnique();
    }
}

DbContext:

public class MyDbContext : DbContext
{
    public EmpiresDbContext(DbContextOptions<EmpiresDbContext> dbContextOptions) : base(dbContextOptions)
    {
    }

    public DbSet<Player> Players { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfigurationsFromAssembly(typeof(EmpiresDbContext).Assembly);
        base.OnModelCreating(modelBuilder);
    }
    
    /*
     * Overrides below to fix EF Core not calling ValueGenerator's on update
     * See: https://github.com/dotnet/efcore/issues/19765#issuecomment-617679987
     */
    
    public override int SaveChanges(bool acceptAllChangesOnSuccess)
    {
        GenerateOnUpdate();
        return base.SaveChanges(acceptAllChangesOnSuccess);
    }

    public override Task<int> SaveChangesAsync(
        bool acceptAllChangesOnSuccess, CancellationToken cancellationToken = default)
    {
        GenerateOnUpdate();
        return base.SaveChangesAsync(acceptAllChangesOnSuccess, cancellationToken);
    }

    private void GenerateOnUpdate()
    {
        foreach (var entityEntry in ChangeTracker.Entries())
        {
            foreach (var propertyEntry in entityEntry.Properties)
            {
                var property = propertyEntry.Metadata;
                var valueGeneratorFactory =
                    property.GetValueGeneratorFactory();
                var generatedOnUpdate = (property.ValueGenerated & ValueGenerated.OnUpdate)
                                        == ValueGenerated.OnUpdate;
                if (!generatedOnUpdate || valueGeneratorFactory == null)
                {
                    continue;
                }

                var valueGenerator = valueGeneratorFactory.Invoke(
                    property,
                    entityEntry.Metadata);
                propertyEntry.CurrentValue = valueGenerator.Next(entityEntry);
            }
        }
    }
}

I'm injecting MyDbContext into the service and then, in the CreatePlayer method:

var player = new Player(externalId, displayName, email);
_dbContext.Players.Add(player);
await _dbContext.SaveChangesAsync();

However, when await _dbContext.SaveChangesAsync(); is called, I get an exception:

SqlException: Cannot insert the value NULL into column 'LastModified'

I have checked with a break-point that the propertyEntry.CurrentValue = valueGenerator.Next(entityEntry); line in the Dbcontext is being hit and that it is assigning a correct value to the LastModified property.

With a SQL trace I can see the SQL that is generated is as follows:

exec sp_executesql N'SET NOCOUNT ON;
INSERT INTO [Players] ([Id], [Created], [DisplayName], [Email], [ExternalId], [LastLogin])
VALUES (@p0, @p1, @p2, @p3, @p4, @p5);
SELECT [LastModified]
FROM [Players]
WHERE @@ROWCOUNT = 1 AND [Id] = @p0;

',N'@p0 uniqueidentifier,@p1 datetime2(7),@p2 nvarchar(200),@p3 nvarchar(254),@p4 nvarchar(4000),@p5 datetime2(7)',@p0='C17E4EC8-CDD6-458A-8CEF-08D8C5D6F63A',@p1='2021-01-31 11:00:27.1068666',@p2=N'Kyr',@p3=N'test@test.test',@p4=N'my-iDp-id-removed-for-security',@p5='0001-01-01 00:00:00'

As you can see above, the LastModified column isn't even referenced.

Answer 1

Your ValueGenerator works perfectly fine with me.

To narrow down the cause of the issue, take a dependency of your DbContext directly in the service class, temporarily -

public class PlayerService : IPlayerService
{
    private EmpiresDbContext _dbContext;
    
    public PlayerService(EmpiresDbContext context)
    {
        _dbContext = context;
    }
}

and see if the following code works -

var player = new Player(externalId, displayName, email);
try
{
    _dbContext.Players.Add(player);
    await _dbContext.SaveChangesAsync();
}
catch(DbUpdateException ex)
{
    var message = ex.Message;
    throw;
}

Two suggestions (not directly related to your issue) -

1. Remove all the .ConfigureAwait(false) from your code. Check this answer to see why.
2. Do not use transaction for a single insert operation like this; it'll cost performance for no benefit.

EDIT :
Going through the code in your updated post, I see you are using the .ValueGeneratedOnAddOrUpdate() configuration with LastModified field, and as far as I can remember, this configuration has a known issue. You might even find issues posted on EF Core's GitHub repo on this.

I'd suggest to use the .ValueGeneratedOnAdd() configuration, as with Created field, and then handle the update scenarios manually.