Programatically get next value for primary key when IDENTITY is set to OFF

Although I experienced a similar issue, I did not require a second id field. Although the client requested a number field for the contactId to appear in the application, my primary id was already a guid. It appears to work well because there is no concern about repeated numbers because we simply mark all entities as isActive = false instead of deleting them.

Entity base class

public virtual void PrepareSave(EntityState state)
{
    var identityName = Thread.CurrentPrincipal.Identity.Name;
    var now = DateTime.UtcNow;

    if (state == EntityState.Added)
    {
        CreateBy = identityName ?? "unknown";
        CreateDate = now;
    }

    LastModifedBy = identityName ?? "unknown";
    LastModifed = now;
}

I included this in the list of the sole entity in need of that condition.

public override void PrepareSave(EntityState state)
{
    var context = new DbContext();
    var maxId = context.Contacts.Max(model => model.ContactId);
    ContactId = maxId + 1;
    context.Dispose();

    var identityName = Thread.CurrentPrincipal.Identity.Name;
    var now = DateTime.UtcNow;

    if (state == EntityState.Added)
    {
        CreateBy = identityName ?? "unknown";
        CreateDate = now;
    }

    LastModifedBy = identityName ?? "unknown";
    LastModifed = now;
}

I put this to the Datacontext as well.

public override async Task<int> SaveChangesAsync()
    {
        var modifiedEntries = ChangeTracker.Entries()
            .Where(x => x.State == EntityState.Added || x.State == EntityState.Modified);

        foreach (var entry in modifiedEntries)
        {
            // try and convert to an Auditable Entity
            var entity = entry.Entity as BaseEntity;
            // call PrepareSave on the entity, telling it the state it is in
            entity?.PrepareSave(entry.State);
        }

        var result = await base.SaveChangesAsync();
        return result;
    }

This was based on SpiderCode's response that was marked as correct above. I appreciate your suggestion; it was quite helpful.