Entity Framework Code First DateTime field update on modification

Question

I have the following entity:

public class User
{
    public int UserId { get; set; }
    public string UserName { get;         


        

Answer 1

First, consider that DateTimeOffset is a better choice if you are going to be storing local times. Alternatively, you could use a UTC-based DateTime, but I will show DateTimeOffset in this example. Never store a local-based DateTime for this purpose, as you will have issues with time zones and daylight saving time. See also: The Case Against DateTime.Now.

Next, consider a common interface that you can use for entities that you wish to track created/updated values.

public interface IDatedEntity
{
    DateTimeOffset Created { get; set; }
    DateTimeOffset Updated { get; set; }
}

Apply that interface and its properties to the entities you are working with.

public class User : IDatedEntity
{
    // ...

    public DateTimeOffset Created { get; set; }
    public DateTimeOffset Updated { get; set; }
}

Now in your database context, you can attach to the SavingChanges event and apply the desired behavior:

public class MyContext : DbContext
{
    public DbSet Users { get; set; }

    public MyContext()
    {
        var objectContext = ((IObjectContextAdapter)this).ObjectContext;
        objectContext.SavingChanges += (sender, args) =>
        {
            var now = DateTimeOffset.Now;
            foreach (var entry in this.ChangeTracker.Entries())
            {
                var entity = entry.Entity;
                switch (entry.State)
                {
                    case EntityState.Added:
                        entity.Created = now;
                        entity.Updated = now;
                        break;
                    case EntityState.Modified:
                        entity.Updated = now;
                        break;
                }
            }
            this.ChangeTracker.DetectChanges();
        };
    }
}

Note the final call to DetectChanges, which will ensure that the new updated date values are applied to all modified entities.