Entity Framework context operations in perspective with their entry states

Posted on: 2013-05-27

When manipulating data with Entity Framework and the context, you are changing the state of the entity. For example, if you insert a new entity, the state will be to "Added". This article will show you all context operation that change the entity state and will show you that you can do what most of the operation do by simply changing manually the state of the entity.

Before going deeper with the operation, let see all states. You can get the list of states by going into EntityState class of System.Data.

 namespace System.Data { [BindableType(IsBindable = false)] [Flags] public enum EntityState { Detached = 1, Unchanged = 2, Added = 4, Deleted = 8, Modified = 16, } } 

The Detached state is when an object is not yet attached to Entity Framework's context. By default, if you create a new instance of a class, this one is not attached. Another way to be detached is when you delete an entity. This one will be deleted from the database by Entity Framework but the object reference remains in your code. This one will be back to detached. Having said that, moving back to detached won't delete the entity but won't make Entity Framework knows about it. We could also manually set the entry to detached to have this one not tracked.

 var category = context.Categories.Find(id); 
 context.Categories.Remove(category); 
 context.SaveChanges(); // Will delete and set category as detached 

 //Is the same as 
 var category = context.Categories.Find(id); 
 context.Entry(category).State = EntityState.Detached; 
 context.SaveChanges(); // Will do nothing in the database because it's detached and it would required to be "Deleted" to be deleted. 

The example above call Remove to delete the entity. When deleting the state goes from Added to Delete to Detached. So both code below are doing the same thing : delete the entity from the database.

 var category = context.Categories.Find(id); 
 context.Categories.Remove(category); 
 context.SaveChanges(); 

 //Is the same as 
 var category = context.Categories.Find(id); 
 context.Entry(category).State = EntityState.Deleted; 
 context.SaveChanges(); 

The Unchanged state occur when the entity is tracked by Entity Framework's context but has not changed yet. You can have this state if you use the method Attach() or if you change the state with Entry().

 var category = new Category{Id=123}; 
 context.Categories.Attach(category); 
 context.SaveChanges(); 

 //Is the same as 
 var category = new Category{Id=123}; 
 context.Entry(category).State = EntityState.Unchanged; 
 context.SaveChanges(); 

The Added state is synonym of insertion. When a new entity is added to the context, this one will be inserted into the database. In Entity Framework words, it's called Added. This can be done by using the Add method or by changing the state to EntityState.Added.

 var category = new Category{Id=123}; 
 context.Categories.Add(category); 
 context.SaveChanges(); 

 //Is the same as 
 var category = new Category{Id=123}; 
 context.Entry(category).State = EntityState.Added; 
 context.SaveChanges(); 

Finally, you can be Modified. This state will do an update to the property that has been changed. Once the update has been done by calling SaveChanges, the state come back to attached

 var category = context.Categories.Find(id); //State is to attached 
 context.Categories.Update(category); //State is now updated 
 context.SaveChanges(); //State is now attached 
 
 //Is the same as 
 var category = context.Categories.Find(id);  //State is to attached 
 context.Entry(category).State = EntityState.Modified; //State is now updated 
 context.SaveChanges(); //State is now attached 

Without going in to much details you can handle modified state by using "ApplyCurrentValues". This will check the object passed by parameter and if changes are found will mark those properties with the new value and with the Modified state.

var category = context.Categories.Attach(new Category { ID = categoryThatCameFromUserForm.ID }); 
context.Categories.ApplyCurrentValues(categoryThatCameFromUserForm); 
context.SaveChanges(); //State is now attached 

We could also specify manually which property that could have been changed with Entry method.

var category = context.Categories.Attach(new Category { ID = categoryThatCameFromUserForm.ID }); 
context.Entry(category).Property(d=>d.Name).IsModified = true; 
context.SaveChanges(); //State is now attached 

To conclude, it's possible to handle Entity Framework's entities with several approaches. The method one, with operation, is perfect for simple case when the method two, with state, is better to have further control over what is modified and to create abstract level over Entity Framework if requires. For example, you could easily add some code that check if the Id is Null or not, if it's null to change the state to Added, otherwise to set the state to Modified.