>First Time try to use Entity Framework and when build, found the following error.
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>'Northwind.ProductsEntity' does not contain a definition for 'ProductName' and no extension method 'ProductName' accepting a first argument of type 'Northwind.ProductsEntity' could be found (are you missing a using directive or an assembly reference?) D:\Code\Northwind\Northwind\Products\ProductsRules.cs 49 39 Northwind

I suspect that you have somehow changed the EntityType from 'Products' to 'Product'. Try deleting the '.tt' files and re-building.