Product Entities

Category.cs

    public class Category
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public List<Field> Fields { get; set; }
    }

Field.cs

   // Fields
   public class Field
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public int CategoryId { get; set; }

        public Category Category { get; set; }

        public IEnumerable<ValidationRule> ValidationRules { get; set; }
    }

    public class IntegerField : Field { }
    public class StringField : Field { }
    // etc.

FieldValue.cs

   // Field Values
    public class FieldValue
    {
        public int FieldId { get; set; }

        public Field Field { get; set; }
    }

    public class IntegerFieldValue : FieldValue
    {
        public int IntegerValue { get; set; }
    }

    public class StringFieldValue : FieldValue
    {
        public string StringValue { get; set; }
    }
    // etc.

Products.cs

    public class Product
    {
        public int Id { get; set; } 

        public string Name { get; set; }

        public int CategoryId { get; set; }

        public Category Category { get; set; }

        public List<FieldValue> FieldValues { get; set; }

        public DateTime CreationDate { get; set; }
    }