Entity Framework Core 生成跟踪列-阴影属性

摘自：

Ef Core 官方文档： 

文章列举了三种：读写属性、只读属性、不定义属性(阴影属性 Shadow )。

下面是摘的一种阴影属性简单实现：

一、添加阴影属性：

　　1. 在 OnModelCreating 方法中声明阴影属性：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.AddAuditableShadowProperties();

    base.OnModelCreating(modelBuilder);
}

　　2. 这里是 AddAuditableShadowProperties 实现：

　　　　public static readonly string CreatedByUserId = nameof(CreatedByUserId);
　　　　public static readonly string ModifiedByUserId = nameof(ModifiedByUserId);
　　　　public static readonly string CreatedDateTime = nameof(CreatedDateTime);
　　　　public static readonly string ModifiedDateTime = nameof(ModifiedDateTime);

　　　　public static void AddAuditableShadowProperties(this ModelBuilder modelBuilder)
        {
            var models = modelBuilder.Model
                                     .GetEntityTypes()
                                     .Where(e => typeof(IEntityBase).IsAssignableFrom(e.ClrType));
            foreach (var entityType in models)
            {
                modelBuilder.Entity(entityType.ClrType)
                            .Property<long?>(CreatedByUserId);
                modelBuilder.Entity(entityType.ClrType)
                            .Property<long?>(ModifiedByUserId);

                modelBuilder.Entity(entityType.ClrType)
                            .Property<DateTimeOffset?>(CreatedDateTime);
                modelBuilder.Entity(entityType.ClrType)
                            .Property<DateTimeOffset?>(ModifiedDateTime);
            }
        }

二、修改阴影属性：

　　1. 重写 SaveChange 方法：

　　　　public override int SaveChanges()
        {
            ChangeTracker.DetectChanges();
            
            //保存之前修改阴影属性
            SetShadowProperties();  

            // for performance reasons, to avoid calling DetectChanges() again.
            ChangeTracker.AutoDetectChangesEnabled = false;
            var result = base.SaveChanges();
            ChangeTracker.AutoDetectChangesEnabled = true;
            return result;
        }
　　　　private void SetShadowProperties()
        {
            // we can't use constructor injection anymore, because we are using the `AddDbContextPool<>`
            var httpContextAccessor = this.GetService<IHttpContextAccessor>();
            ChangeTracker.SetAuditableEntityPropertyValues(httpContextAccessor);
        }

　　2. 修改阴影属性：

　　　　public static void SetAuditableEntityPropertyValues(this ChangeTracker changeTracker, IHttpContextAccessor httpContextAccessor)
        {　
            var httpContext = httpContextAccessor?.HttpContext;
            // var userAgent = httpContext?.Request?.Headers["User-Agent"].ToString();
            // var userIp = httpContext?.Connection?.RemoteIpAddress?.ToString();
            var now = DateTimeOffset.Now;

            var modifiedEntries = changeTracker.Entries<IEntityBase>().Where(x => x.State == EntityState.Modified);
            foreach (var modifiedEntry in modifiedEntries)
            {
                modifiedEntry.Property(ModifiedDateTime).CurrentValue = now;
                modifiedEntry.Property(ModifiedByUserId).CurrentValue = 953833173853548544;
            }

            var addedEntries = changeTracker.Entries<IEntityBase>().Where(x => x.State == EntityState.Added);
            foreach (var addedEntry in addedEntries)
            {
                addedEntry.Property(CreatedDateTime).CurrentValue = now;
                addedEntry.Property(CreatedByUserId).CurrentValue = 953833173853548544;
            }
        }