title: The name 'EntityState' does not exist in the current context
body: <p>In <em>Entity Framework</em>, this sometimes occurs when the <code>System.data.entity</code> assembly is not added into the Project. But, why I didn't have this error before in other MVC project.</p>
  
  <p>it occurs sometimes but frequently and I have to add it manually in <em>Add References</em>. What can I do?</p>