Is is possible to check if an object is already attached to a data context in Entity Framework?

Question

I am getting the following error when trying to attach an object that is already attached to a given context via context.AttachTo(...):

An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

Is there a way of achieving something along the lines of:

context.IsAttachedTo(...)

Cheers!

Edit:

The extension method Jason outlined is close, but it doesn't work for my situation.

I am trying to do some work using the method outlined in the answer to another question:

How do I delete one or more rows from my table using Linq to Entities *without* retrieving the rows first?

My code looks a bit like this:

var user = new User() { Id = 1 };
context.AttachTo("Users", user);
comment.User = user;
context.SaveChanges();

This works fine, except when I do something else for that user where I use the same method and try to attach a dummy User object. This fails because I have previously attached that dummy user object. How can I check for this?

Answer 1

Here's what I ended up with, which works very nicely:

public static void AttachToOrGet<T>(this ObjectContext context, string entitySetName, ref T entity)
    where T : IEntityWithKey
{
    ObjectStateEntry entry;
    // Track whether we need to perform an attach
    bool attach = false;
    if (
        context.ObjectStateManager.TryGetObjectStateEntry
            (
                context.CreateEntityKey(entitySetName, entity),
                out entry
            )
        )
    {
        // Re-attach if necessary
        attach = entry.State == EntityState.Detached;
        // Get the discovered entity to the ref
        entity = (T)entry.Entity;
    }
    else
    {
        // Attach for the first time
        attach = true;
    }
    if (attach)
        context.AttachTo(entitySetName, entity);
}

You can call it as follows:

User user = new User() { Id = 1 };
II.AttachToOrGet<Users>("Users", ref user);

This works very nicely because it's just like context.AttachTo(...) except you can use the ID trick I cited above each time. You end up with either the object previously attached or your own object being attached. Calling CreateEntityKey on the context makes sure it's nice and generic and will work even with composite keys with no further coding (because EF can already do that for us!).

Edit, twelve years later (Dec 2021)... oof!

Here's what I use in EF Core:

public static class EfExtensions
{
    public static T AttachToOrGet<T>(this DbContext context, Func<T,bool> predicate, Func<T> factory)
        where T : class, new()
    {
        var match = context.Set<T>().Local.FirstOrDefault(predicate);
        if (match == null)
        {
            match = factory();
            context.Attach(match);
        }

        return match;
    }
}

Usage:

var item = db.AttachToOrGet(_ => _.Id == someId, () => new MyItem { Id = someId });

You could refactor this to work with the entity key but this is enough to get anyone going!

Answer 2

A simpler approach is:

 bool isDetached = context.Entry(user).State == EntityState.Detached;
 if (isDetached)
     context.Users.Attach(user);

Answer 3

Try this extension method (this is untested and off-the-cuff):

public static bool IsAttachedTo(this ObjectContext context, object entity) {
    if(entity == null) {
        throw new ArgumentNullException("entity");
    }
    ObjectStateEntry entry;
    if(context.ObjectStateManager.TryGetObjectStateEntry(entity, out entry)) {
        return (entry.State != EntityState.Detached);
    }
    return false;
}

Given the situation that you describe in your edit, you might need to use the following overload that accepts an EntityKey instead of an object:

public static bool IsAttachedTo(this ObjectContext, EntityKey key) {
    if(key == null) {
        throw new ArgumentNullException("key");
    }
    ObjectStateEntry entry;
    if(context.ObjectStateManager.TryGetObjectStateEntry(key, out entry)) {
        return (entry.State != EntityState.Detached);
    }
    return false;
}

To build an EntityKey in your situation, use the following as a guideline:

EntityKey key = new EntityKey("MyEntities.User", "Id", 1);

You can get the EntityKey from an existing instance of User by using the property User.EntityKey (from interface IEntityWithKey).

Answer 4

Using the entity key of the object you are trying to check:

var entry = context.ObjectStateManager.GetObjectStateEntry("EntityKey");
if (entry.State == EntityState.Detached)
{
  // Do Something
}

Kindness,

Dan

Answer 5

This does not directly answer OPs question but this is how I solved mine.

This is for those who are using DbContext instead of ObjectContext.

    public TEntity Retrieve(object primaryKey)
    {
        return DbSet.Find(primaryKey);
    }

DbSet.Find Method:

Finds an entity with the given primary key values. If an entity with the given primary key values exists in the context, then it is returned immediately without making a request to the store. Otherwise, a request is made to the store for an entity with the given primary key values and this entity, if found, is attached to the context and returned. If no entity is found in the context or the store, then null is returned.

Basically, it returns the attached object of the given primaryKey so you just need to apply the changes on the returned object to keep the right instance.