I solve this problem with a "using" block

using (SqlConnection conn = new SqlConnection(connectionString))

    {

       // stuff to do with data base
    }

    // or if you are using entity framework 
    using (DataBaseEntity data = new DataBaseEntity)
{

    }

Here is where I get the idea https://social.msdn.microsoft.com/Forums/sqlserver/es-ES/b4b350ba-b0d5-464d-8656-8c117d55b2af/problema-al-modificar-en-entity-framework?forum=vcses is in spanish (look for the second answer)

I had a similar issue, after probing for 2-3 days found ".AsNoTracking" should be removed as EF doesn't track the changes and assumes there are no changes unless an object is attached. Also if we don't use .AsNoTracking, EF automatically knows which object to save/update so there is no need to use Attach/Added.

Try this:

var local = yourDbContext.Set<YourModel>()
                         .Local
                         .FirstOrDefault(f => f.Id == yourModel.Id);
if (local != null)
{
  yourDbContext.Entry(local).State = EntityState.Detached;
}
yourDbContext.Entry(applicationModel).State = EntityState.Modified;

i mange to fix the issue by updating state. when you trigger find or any other query operation on the same record sate has been updated with modified so we need to set status to Detached then you can fire your update change

     ActivityEntity activity = new ActivityEntity();
      activity.name="vv";
    activity.ID = 22 ; //sample id
   var savedActivity = context.Activities.Find(22);

            if (savedActivity!=null)
            {
                context.Entry(savedActivity).State = EntityState.Detached;
                context.SaveChanges();

                activity.age= savedActivity.age;
                activity.marks= savedActivity.marks; 

                context.Entry(activity).State = EntityState.Modified;
                context.SaveChanges();
                return activity.ID;
            }

I had this problem with local var and i just detach it like this:

if (ModelState.IsValid)
{
    var old = db.Channel.Find(channel.Id);
    if (Request.Files.Count > 0)
    {
        HttpPostedFileBase objFiles = Request.Files[0];
        using (var binaryReader = new BinaryReader(objFiles.InputStream))
        {
            channel.GateImage = binaryReader.ReadBytes(objFiles.ContentLength);
        }

    }
    else
        channel.GateImage = old.GateImage;
    var cat = db.Category.Find(CatID);
    if (cat != null)
        channel.Category = cat;
    db.Entry(old).State = EntityState.Detached; // just added this line
    db.Entry(channel).State = EntityState.Modified;
    await db.SaveChangesAsync();
    return RedirectToAction("Index");
}
return View(channel);

Problem causes of loaded objects with same Key, so first we will detach that object and do the the updating to avoid conflict between two object with the same Key

Problem SOLVED!

Attach method could potentially help somebody but it wouldn't help in this situation as the document was already being tracked while being loaded in Edit GET controller function. Attach would throw exactly the same error.

The issue I encounter here was caused by function canUserAccessA() which loads the A entity before updating the state of object a. This was screwing up the tracked entity and it was changing state of a object to Detached.

The solution was to amend canUserAccessA() so that the object I was loading wouldn't be tracked. Function AsNoTracking() should be called while querying the context.

// User -> Receipt validation
private bool canUserAccessA(int aID)
{
    int userID = WebSecurity.GetUserId(User.Identity.Name);
    int aFound = db.Model.AsNoTracking().Where(x => x.aID == aID && x.UserID==userID).Count();

    return (aFound > 0); //if aFound > 0, then return true, else return false.
}

For some reason I couldnt use .Find(aID) with AsNoTracking() but it doesn't really matter as I could achieve the same by changing the query.

Hope this will help anybody with similar problem!

Here what I did in the similar case.

That sitatuation means that same entity has already been existed in the context.So following can help

First check from ChangeTracker if the entity is in the context

var trackedEntries=GetContext().ChangeTracker.Entries<YourEntityType>().ToList();

var isAlreadyTracked =
                    trackedEntries.Any(trackedItem => trackedItem.Entity.Id ==myEntityToSave.Id);

If it exists

  if (isAlreadyTracked)
            {
                myEntityToSave= trackedEntries.First(trackedItem => trackedItem.Entity.Id == myEntityToSave.Id).Entity;
            } 

else
{
//Attach or Modify depending on your needs
}

This problem may also be seen during ViewModel to EntityModel mapping (by using AutoMapper, etc.) and trying to include context.Entry().State and context.SaveChanges() such a using block as shown below would solve the problem. Please keep in mind that context.SaveChanges() method is used two times instead of using just after if-block as it must be in using block also.

public void Save(YourEntity entity)
{
    if (entity.Id == 0)
    {
        context.YourEntity.Add(entity);
        context.SaveChanges();
    }
    else
    {
        using (var context = new YourDbContext())
        {
            context.Entry(entity).State = EntityState.Modified;
            context.SaveChanges(); //Must be in using block
        }
    }            
}

Hope this helps...

Similar to what Luke Puplett is saying, the problem can be caused by not properly disposing or creating your context.

In my case, I had a class which accepted a context called ContextService:

public class ContextService : IDisposable
{
    private Context _context;

    public void Dispose()
    {
        _context.Dispose();
    }
    public ContextService(Context context)
    {
        _context = context;
    }
//... do stuff with the context

My context service had a function which updates an entity using an instantiated entity object:

        public void UpdateEntity(MyEntity myEntity, ICollection<int> ids)
        {
            var item = _context.Entry(myEntity);
            item.State = EntityState.Modified;
            item.Collection(x => x.RelatedEntities).Load();
            myEntity.RelatedEntities.Clear();
            foreach (var id in ids)
            {
                myEntity.RelatedEntities.Add(_context.RelatedEntities.Find(id));
            }
            _context.SaveChanges();
        }

All of this was fine, my controller where I initialized the service was the problem. My controller originally looked like this:

    private static NotificationService _service = 
        new NotificationService(new NotificationContext());
    public void Dispose()
    {
    }

I changed it to this and the error went away:

    private static NotificationService _service;
    public TemplateController()
    {
        _service = new NotificationService(new NotificationContext());
    }
    public void Dispose()
    {
        _service.Dispose();
    }

for me the local copy was the source of the problem. this solved it

var local = context.Set<Contact>().Local.FirstOrDefault(c => c.ContactId == contact.ContactId);
                if (local != null)
                {
                    context.Entry(local).State = EntityState.Detached;
                }

Interestingly:

_dbContext.Set<T>().AddOrUpdate(entityToBeUpdatedWithId);

Or if you still is not generic:

_dbContext.Set<UserEntity>().AddOrUpdate(entityToBeUpdatedWithId);

seems to solved my problem smoothly.

My case was that I did not have direct access to EF context from my MVC app.

So if you are using some kind of repository for entity persistence it could be appropiate to simply detach explicitly loaded entity and then set binded EntityState to Modified.

Sample (abstract) code:

MVC

public ActionResult(A a)
{
  A aa = repo.Find(...);
  // some logic
  repo.Detach(aa);
  repo.Update(a);
}

Repository

void Update(A a)
{
   context.Entry(a).EntityState = EntityState.Modified;
   context.SaveChanges();
}

void Detach(A a)
{
   context.Entry(a).EntityState = EntityState.Detached;
}

I have added this answer only because the problem is explained based on more complex data pattern and I found it hard to understand here.

I created a fairly simple application. This error occurred inside Edit POST action. The action accepted ViewModel as an input parameter. The reason for using the ViewModel was to make some calculation before the record was saved.

Once the action passed through validation such as if(ModelState.IsValid), my wrongdoing was to project values from ViewModel into a completely new instance of Entity. I thought I'd have to create a new instance to store updated data and then saved such instance.

What I had realised later was that I had to read the record from database:

Student student = db.Students.Find(s => s.StudentID == ViewModel.StudentID);

and updated this object. Everything works now.

I thought I'd share my experience on this one, even though I feel a bit silly for not realising sooner.

I am using the repository pattern with the repo instances injected into my controllers. The concrete repositories instantiate my ModelContext (DbContext) which lasts the lifetime of the repository, which is IDisposable and disposed by the controller.

The issue for me was that I have a modified stamp and row version on my entities, so I was getting them first in order to compare with the inbound headers. Of course, this loaded and tracked the entity that was subsequently being updated.

The fix was simply to change the repository from newing-up a context once in the constructor to having the following methods:

    private DbContext GetDbContext()
    {
        return this.GetDbContext(false);
    }


    protected virtual DbContext GetDbContext(bool canUseCachedContext)
    {
        if (_dbContext != null)
        {
            if (canUseCachedContext)
            {
                return _dbContext;
            }
            else
            {
                _dbContext.Dispose();
            }
        }

        _dbContext = new ModelContext();

        return _dbContext;
    }

    #region IDisposable Members

    public void Dispose()
    {
        this.Dispose(true);
    }

    protected virtual void Dispose(bool isDisposing)
    {
        if (!_isDisposed)
        {
            if (isDisposing)
            {
                // Clear down managed resources.

                if (_dbContext != null)
                    _dbContext.Dispose();
            }

            _isDisposed = true;
        }
    }

    #endregion

This allows the repository methods to re-new their context instance upon each use by calling GetDbContext, or use a previous instance if they so desire by specifying true.

It seems that entity you are trying to modify is not being tracked correctly and therefore is not recognized as edited, but added instead.

Instead of directly setting state, try to do the following:

//db.Entry(aViewModel.a).State = EntityState.Modified;
db.As.Attach(aViewModel.a); 
db.SaveChanges();

Also, I would like to warn you that your code contains potential security vulnerability. If you are using entity directly in your view model, then you risk that somebody could modify contents of entity by adding correctly named fields in submitted form. For example, if user added input box with name "A.FirstName" and the entity contained such field, then the value would be bound to viewmodel and saved to database even if the user would not be allowed to change that in normal operation of application.

Update:

To get over security vulnerability mentioned previously, you should never expose your domain model as your viewmodel but use separate viewmodel instead. Then your action would receive viewmodel which you could map back to domain model using some mapping tool like AutoMapper. This would keep you safe from user modifying sensitive data.

Here is extended explanation:

http://www.stevefenton.co.uk/Content/Blog/Date/201303/Blog/Why-You-Never-Expose-Your-Domain-Model-As-Your-MVC-Model/