DbSet.Attach Method (Object)
Attaches the given entity to the context underlying the set. That is, the entity is placed into the context in the Unchanged state, just as if it had been read from the database.
Assembly: EntityFramework (in EntityFramework.dll)
Attach is used to repopulate a context with an entity that is known to already exist in the database. SaveChanges will therefore not attempt to insert an attached entity into the database because it is assumed to already be there. Note that entities that are already in the context in some other state will have their state set to Unchanged. Attach is a no-op if the entity is already in the context in the Unchanged state.