The type 'System.Data.Objects.DataClasses.EntityObject' is defined in an assembly that is not referenced

I got this error today when working on a migrated ASP .NET MVC 4 project. It was especially confusing for me because I already had a reference to System.Data.Entity in my web project.

The type ‘System.Data.Objects.DataClasses.EntityObject’ is defined in an assembly that is not referenced. You must add a reference to assembly ‘System.Data.Entity, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089’. c:\tfs\trunk\Site\Views\Customers\Details.cshtml 19

It turns out that for some reason the assembly information for System.Data.Entity was not added to my Web.config after I added the reference. I tried removing and adding back the reference in my project with no success. In order to get everything working again I had to manually add the assembly to my Web.config.

Update: Before trying to update your Web.config manually you should install the EntityFramework NuGet package in your project. If that doesn’t work, then try the solution below.

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What do you think?