将方法添加到类 DbSet<TEntity>

Question

I'm working on asp.net mvc 5 and want to implement 3 methods (Add / Update / Remove) to class Topic :

class Topic : IDisposable
{
        public Topic() { }
        private TopicViewModels Model { get; set; }
        private string Flag { get; set; }

        public void Add(TopicViewModels model)
        {
            Model = model;
            Flag = "Add";
        }

        public void Update(TopicViewModels model)
        {
            Model = model;
            Flag = "Update";
        }

        public void Remove(TopicViewModels model)
        {
            Model = model;
            Flag = "Remove";
        }

        public async Task<int> SaveChangesAsync()
        {
            using (var db = new MyDbContext())
            {
                switch (Flag)
                {
                    case "Add":
                        db.Topics.Add(Model);
                        break;
                    case "Update":
                        var topic = await db.Topics.SingleAsync(x => x.Id == Model.Id);    
                        topic = Model;
                        break;
                    case "Remove":
                        db.Topics.Remove(Model);
                        break;
                }
                return await db.SaveChangesAsync();
            }
        }

        public void Dispose()
        {
            throw new NotImplementedException();
        }
}

My question: how can I declare method

public virtual TEntity Update(TEntity entity); 

in class DbSet<TEntity> ?

What I want to achieve in case "Update":

switch (Flag)
{
    case "Update":
        db.Topics.Update(Model);
        break;
}

And my sub-question: is there another way to call method SaveChangesAsync exactly without using a flag?

Answer 1

You can do something like;

public void Update<T>(T entity) where T:class
{
        db.Entry<T>(entity).State = EntityState.Modified;
        db.SaveChanges();
}