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Our last look at LINQ demonstrates the ability to create calculated columns. We know that each Country has an Area and a Population. The Density would be Population divided by Area to get people per square mile. This match could be done client side in JavaScript or server side with some C# syntax. Instead, we will make a new data model that inherits from Country.cs called CountryStat.cs. It will have an additional member called Density.

In the Pages/Countries/Detail.cshtml.cs page, we will retrieve a single CountryStat object instead of a Country. The final step will be the addition of a new field in the page markup.

Instructions

The new file Models/CountryStat.cs is open. Make this class inherit from Country.

Add a new member for population Density as a nullable float. Add a [Display] annotation with a Name of "Ppl/SqMi".

Open the file Pages/Countries/Detail.cshtml.cs. Add a using statement for LINQ:

System.Linq

Replace the Country member type with CountryStat.

Add a LINQ query at the top of the OnGetAsync() method:

var country = from c in _context.Countries
  where c.ID == id
  select new CountryStat {
    ID = c.ID,
    ContinentID = c.ContinentID,
    Name = c.Name,
    Population = c.Population,
    Area = c.Area,
    UnitedNationsDate = c.UnitedNationsDate,
    Density = c.Population / c.Area
  };

Assign the result of the query to the Country member.

Open the file Pages/Countries/Detail.cshtml. Add a new <div> at the bottom of the markup for the Density field. You can copy one of the other fields as a template:

<div class="d-flex">
  <div class="p-2 bg-primary text-white text-right" style="flex:0 0 15%">
    @Html.DisplayNameFor(model => model.Country.Density)
  </div>
  <div class="p-2 border-right border-bottom border-primary" style="flex:1 0 auto">
    @Html.DisplayFor(model => model.Country.Density)
  </div>
</div>

Browse the list of Countries and select Detail on a few. You should see the calculated population density.

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