Entity Framework Add if not exist without update

Have you tried to check if the entity exists and if not - add it? Like this :

UPDATE

using System.Linq.Expressions;
    public class ContextWithExtensionExample
    {
        public void DoSomeContextWork(DbContext context)
        {
            var uni = new Unicorn();
            context.Set<Unicorn>().AddIfNotExists(uni , x => x.Name == "James");
        }
    }

    public static class DbSetExtensions
    {
        public static T AddIfNotExists<T>(this DbSet<T> dbSet, T entity, Expression<Func<T, bool>> predicate = null) where T : class, new()
        {
            var exists = predicate != null ? dbSet.Any(predicate) : dbSet.Any();
            return !exists ? dbSet.Add(entity) : null;
        }
    }

You can use this method directly and remember to call DbContext.SaveChanges() after the call.

The solution is OK, when you have to add just one item, but it's very expensive in terms of performance in case you have to add multiple items. I think there is a better solution:

public static class DbSetExtensions
{
    public static EntityEntry<TEnt> AddIfNotExists<TEnt, TKey>(this DbSet<TEnt> dbSet, TEnt entity, Func<TEnt, TKey> predicate) where TEnt : class
    {
        var exists = dbSet.Any(c => predicate(entity).Equals(predicate(c)));
        return exists
            ? null
            : dbSet.Add(entity);
    }

    public static void AddRangeIfNotExists<TEnt, TKey>(this DbSet<TEnt> dbSet, IEnumerable<TEnt> entities, Func<TEnt, TKey> predicate) where TEnt : class
    {
        var entitiesExist = from ent in dbSet
            where entities.Any(add => predicate(ent).Equals(predicate(add)))
            select ent;

        dbSet.AddRange(entities.Except(entitiesExist));
    }
}

So later it can be used like this:

using (var context = new MyDbContext())
{
    var user1 = new User { Name = "Peter", Age = 32 };
    context.Users.AddIfNotExists(user1, u => u.Name);

    var user2 = new User { Name = "Joe", Age = 25 };
    context.Users.AddIfNotExists(user2, u => u.Age);

    // Adds user1 if there is no user with name "Peter"
    // Adds user2 if there is no user with age 25
    context.SaveChanges();
}

All the other answers are incorrect.

"Read before write" can violate data integrity without being put inside a transaction control.

In SQL Server, you can use merge statement. However merge statement is not available in EF.