Question

Is there such a method?

object GetPrimaryKeyValue(DbEntityEntry entry);

Or how should it be implemented?

Answer 1

You need to cast your DbContext to IObjectContextAdapter so you can access the underlying ObjectContext which gives you access to some more advanced features hidden by DbContext.

Inside your class which derives DbContext the following method will work.

object GetPrimaryKeyValue(DbEntityEntry entry)
{
    var objectStateEntry = ((IObjectContextAdapter)this).ObjectContext.ObjectStateManager.GetObjectStateEntry(entry.Entity);
    return objectStateEntry.EntityKey.EntityKeyValues[0].Value;
}

If there is more than one key then you should iterate over the EntityKeyValues property.