Method not found: 'void system.data.entity.infrastructure.dbentityentry' while updating entry

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C#

generics

repository

I'm Trying to update entry using EF 6.0, and generic repository. here is my base class code:

<pre>public virtual void Edit(T entity)
    {
         _entities.Entry(entity).State = EntityState.Modified;
        _entities.SaveChanges();

    }
public virtual  T GetById(int id)
    {

        return _entities.Set<T>().Find(id);
    }

Here is code for updating data

int id = Convert.ToInt32(HiddenField1.Value);

            tblamt = amtdll.GetById(id);
            tblamt.Amt_Type = txt_amt_type.Text;
            tblamt.UserId = User.Identity.GetUserId();
                if (CheckBox1.Checked == true)
                {
                    tblamt.IsActive = true;
                }
                else
                {
                    tblamt.IsActive = false;
                }
                amtdll.Edit(tblamt);

But when i hit the update button it shows the following error. Method not found: 'Void System.Data.Entity.Infrastructure.DbEntityEntry`1.set_State(System.Data.EntityState)'.

please help me thanks

What I have tried:

i have updated EF in my current project to 6.0.2